RAID-Z expansion feature

This feature allows disks to be added one at a time to a RAID-Z group,
expanding its capacity incrementally.  This feature is especially useful
for small pools (typically with only one RAID-Z group), where there
isn't sufficient hardware to add capacity by adding a whole new RAID-Z
group (typically doubling the number of disks).

== Initiating expansion ==

A new device (disk) can be attached to an existing RAIDZ vdev, by
running `zpool attach POOL raidzP-N NEW_DEVICE`, e.g. `zpool attach tank
raidz2-0 sda`.  The new device will become part of the RAIDZ group.  A
"raidz expansion" will be initiated, and the new device will contribute
additional space to the RAIDZ group once the expansion completes.

The `feature@raidz_expansion` on-disk feature flag must be `enabled` to
initiate an expansion, and it remains `active` for the life of the pool.
In other words, pools with expanded RAIDZ vdevs can not be imported by
older releases of the ZFS software.

== During expansion ==

The expansion entails reading all allocated space from existing disks in
the RAIDZ group, and rewriting it to the new disks in the RAIDZ group
(including the newly added device).

The expansion progress can be monitored with `zpool status`.

Data redundancy is maintained during (and after) the expansion.  If a
disk fails while the expansion is in progress, the expansion pauses
until the health of the RAIDZ vdev is restored (e.g. by replacing the
failed disk and waiting for reconstruction to complete).

The pool remains accessible during expansion.  Following a reboot or
export/import, the expansion resumes where it left off.

== After expansion ==

When the expansion completes, the additional space is available for use,
and is reflected in the `available` zfs property (as seen in `zfs list`,
`df`, etc).

Expansion does not change the number of failures that can be tolerated
without data loss (e.g. a RAIDZ2 is still a RAIDZ2 even after
expansion).

A RAIDZ vdev can be expanded multiple times.

After the expansion completes, old blocks remain with their old
data-to-parity ratio (e.g. 5-wide RAIDZ2, has 3 data to 2 parity), but
distributed among the larger set of disks.  New blocks will be written
with the new data-to-parity ratio (e.g. a 5-wide RAIDZ2 which has been
expanded once to 6-wide, has 4 data to 2 parity).  However, the RAIDZ
vdev's "assumed parity ratio" does not change, so slightly less space
than is expected may be reported for newly-written blocks, according to
`zfs list`, `df`, `ls -s`, and similar tools.

Sponsored-by: The FreeBSD Foundation
Sponsored-by: iXsystems, Inc.
Sponsored-by: vStack
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Mark Maybee <[email protected]>
Authored-by: Matthew Ahrens <[email protected]>
Contributions-by: Fedor Uporov <[email protected]>
Contributions-by: Stuart Maybee <[email protected]>
Contributions-by: Thorsten Behrens <[email protected]>
Contributions-by: Fmstrat <[email protected]>
Contributions-by: Don Brady <[email protected]>
Signed-off-by: Don Brady <[email protected]>
Closes #15022 

17 files changed, 2930 insertions, 228 deletions

diff --git a/module/Kbuild.in b/module/Kbuild.in
index c13217159..6408fb106 100644
--- a/module/Kbuild.in
+++ b/module/Kbuild.in
@@ -445,6 +445,7 @@ ZFS_OBJS_OS := \
 	trace.o \
 	vdev_disk.o \
 	vdev_file.o \
+	vdev_label_os.o \
 	zfs_acl.o \
 	zfs_ctldir.o \
 	zfs_debug.o \
diff --git a/module/os/freebsd/zfs/vdev_label_os.c b/module/os/freebsd/zfs/vdev_label_os.c
index bc856b930..338982ff6 100644
--- a/module/os/freebsd/zfs/vdev_label_os.c
+++ b/module/os/freebsd/zfs/vdev_label_os.c
@@ -72,3 +72,62 @@ retry:
 	abd_free(pad2);
 	return (error);
 }
+
+static void
+vdev_child_done(zio_t *zio)
+{
+	zio_t *pio = zio->io_private;
+
+	mutex_enter(&pio->io_lock);
+	pio->io_error = zio_worst_error(pio->io_error, zio->io_error);
+	mutex_exit(&pio->io_lock);
+}
+
+/*
+ * Check if the reserved boot area is in-use.
+ *
+ * When booting FreeBSD with an MBR partition with ZFS, the zfsboot file
+ * (which understands the ZFS file system) is written to the ZFS BOOT
+ * reserve area (at offset 512K). We check for that here before attaching
+ * a disk to raidz which would then corrupt this boot data.
+ */
+int
+vdev_check_boot_reserve(spa_t *spa, vdev_t *childvd)
+{
+	ASSERT(childvd->vdev_ops->vdev_op_leaf);
+
+	size_t size = SPA_MINBLOCKSIZE;
+	abd_t *abd = abd_alloc_linear(size, B_FALSE);
+
+	zio_t *pio = zio_root(spa, NULL, NULL, 0);
+	/*
+	 * Note: zio_vdev_child_io() adds VDEV_LABEL_START_SIZE to the offset
+	 * to calculate the physical offset to write to.  Passing in a negative
+	 * offset lets us access the boot area.
+	 */
+	zio_nowait(zio_vdev_child_io(pio, NULL, childvd,
+	    VDEV_BOOT_OFFSET - VDEV_LABEL_START_SIZE, abd, size, ZIO_TYPE_READ,
+	    ZIO_PRIORITY_ASYNC_READ, 0, vdev_child_done, pio));
+	zio_wait(pio);
+
+	unsigned char *buf = abd_to_buf(abd);
+
+	/*
+	 * The BTX server has a special header at the begining.
+	 *
+	 * btx_hdr:	.byte 0xeb		# Machine ID
+	 *		.byte 0xe		# Header size
+	 *		.ascii "BTX"		# Magic
+	 *		.byte 0x1		# Major version
+	 *		.byte 0x2		# Minor version
+	 *		.byte BTX_FLAGS		# Flags
+	 */
+	if (buf[0] == 0xeb && buf[1] == 0x0e &&
+	    buf[2] == 'B' && buf[3] == 'T' && buf[4] == 'X') {
+		abd_free(abd);
+		return (EBUSY);
+	}
+
+	abd_free(abd);
+	return (0);
+}
diff --git a/module/os/linux/zfs/vdev_label_os.c b/module/os/linux/zfs/vdev_label_os.c
new file mode 100644
index 000000000..3d965b89a
--- /dev/null
+++ b/module/os/linux/zfs/vdev_label_os.c
@@ -0,0 +1,45 @@
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or https://opensource.org/licenses/CDDL-1.0.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+
+/*
+ * Copyright (c) 2023 by iXsystems, Inc.
+ */
+
+#include <sys/zfs_context.h>
+#include <sys/spa.h>
+#include <sys/spa_impl.h>
+#include <sys/vdev.h>
+#include <sys/vdev_impl.h>
+
+/*
+ * Check if the reserved boot area is in-use.
+ *
+ * This function always returns 0, as there are no known external uses
+ * of the reserved area on Linux.
+ */
+int
+vdev_check_boot_reserve(spa_t *spa, vdev_t *childvd)
+{
+	(void) spa;
+	(void) childvd;
+
+	return (0);
+}
diff --git a/module/os/linux/zfs/zfs_debug.c b/module/os/linux/zfs/zfs_debug.c
index b090ec684..f707959c9 100644
--- a/module/os/linux/zfs/zfs_debug.c
+++ b/module/os/linux/zfs/zfs_debug.c
@@ -175,7 +175,8 @@ __dprintf(boolean_t dprint, const char *file, const char *func,
 		newfile = file;
 	}
 
-	i = snprintf(buf, size, "%s%s:%d:%s(): ", prefix, newfile, line, func);
+	i = snprintf(buf, size, "%px %s%s:%d:%s(): ",
+	    curthread, prefix, newfile, line, func);
 
 	if (i < size) {
 		va_start(adx, fmt);
diff --git a/module/zcommon/zfeature_common.c b/module/zcommon/zfeature_common.c
index 2c74d10f4..309d9bf14 100644
--- a/module/zcommon/zfeature_common.c
+++ b/module/zcommon/zfeature_common.c
@@ -749,6 +749,11 @@ zpool_feature_init(void)
 		    redact_list_spill_deps, sfeatures);
 	}
 
+	zfeature_register(SPA_FEATURE_RAIDZ_EXPANSION,
+	    "org.openzfs:raidz_expansion", "raidz_expansion",
+	    "Support for raidz expansion",
+	    ZFEATURE_FLAG_MOS, ZFEATURE_TYPE_BOOLEAN, NULL, sfeatures);
+
 	zfs_mod_list_supported_free(sfeatures);
 }
 
diff --git a/module/zcommon/zpool_prop.c b/module/zcommon/zpool_prop.c
index c4aca04a9..e98063e8b 100644
--- a/module/zcommon/zpool_prop.c
+++ b/module/zcommon/zpool_prop.c
@@ -439,6 +439,9 @@ vdev_prop_init(void)
 	zprop_register_index(VDEV_PROP_ALLOCATING, "allocating", 1,
 	    PROP_DEFAULT, ZFS_TYPE_VDEV, "on | off", "ALLOCATING",
 	    boolean_na_table, sfeatures);
+	zprop_register_index(VDEV_PROP_RAIDZ_EXPANDING, "raidz_expanding", 0,
+	    PROP_READONLY, ZFS_TYPE_VDEV, "on | off", "RAIDZ_EXPANDING",
+	    boolean_table, sfeatures);
 
 	/* default index properties */
 	zprop_register_index(VDEV_PROP_FAILFAST, "failfast", B_TRUE,
diff --git a/module/zfs/arc.c b/module/zfs/arc.c
index dfea15b74..dc2dd63b3 100644
--- a/module/zfs/arc.c
+++ b/module/zfs/arc.c
@@ -4518,7 +4518,7 @@ arc_evict_cb_check(void *arg, zthr_t *zthr)
 static void
 arc_evict_cb(void *arg, zthr_t *zthr)
 {
-	(void) arg, (void) zthr;
+	(void) arg;
 
 	uint64_t evicted = 0;
 	fstrans_cookie_t cookie = spl_fstrans_mark();
@@ -4542,9 +4542,13 @@ arc_evict_cb(void *arg, zthr_t *zthr)
 	 * infinite loop.  Additionally, zthr_iscancelled() is
 	 * checked here so that if the arc is shutting down, the
 	 * broadcast will wake any remaining arc evict waiters.
+	 *
+	 * Note we cancel using zthr instead of arc_evict_zthr
+	 * because the latter may not yet be initializd when the
+	 * callback is first invoked.
 	 */
 	mutex_enter(&arc_evict_lock);
-	arc_evict_needed = !zthr_iscancelled(arc_evict_zthr) &&
+	arc_evict_needed = !zthr_iscancelled(zthr) &&
 	    evicted > 0 && aggsum_compare(&arc_sums.arcstat_size, arc_c) > 0;
 	if (!arc_evict_needed) {
 		/*
diff --git a/module/zfs/dsl_scan.c b/module/zfs/dsl_scan.c
index 34012db82..e16128fdf 100644
--- a/module/zfs/dsl_scan.c
+++ b/module/zfs/dsl_scan.c
@@ -3066,7 +3066,6 @@ dsl_scan_visit(dsl_scan_t *scn, dmu_tx_t *tx)
 		scn->scn_phys.scn_cur_max_txg = scn->scn_phys.scn_max_txg;
 		dsl_scan_visit_rootbp(scn, NULL,
 		    &dp->dp_meta_rootbp, tx);
-		spa_set_rootblkptr(dp->dp_spa, &dp->dp_meta_rootbp);
 		if (scn->scn_suspending)
 			return;
 
diff --git a/module/zfs/metaslab.c b/module/zfs/metaslab.c
index e0d4a6a63..0983ba143 100644
--- a/module/zfs/metaslab.c
+++ b/module/zfs/metaslab.c
@@ -4342,7 +4342,8 @@ metaslab_sync_done(metaslab_t *msp, uint64_t txg)
 
 	uint64_t free_space = metaslab_class_get_space(spa_normal_class(spa)) -
 	    metaslab_class_get_alloc(spa_normal_class(spa));
-	if (free_space <= spa_get_slop_space(spa) || vd->vdev_removing) {
+	if (free_space <= spa_get_slop_space(spa) || vd->vdev_removing ||
+	    vd->vdev_rz_expanding) {
 		defer_allowed = B_FALSE;
 	}
 
@@ -4650,6 +4651,7 @@ metaslab_block_alloc(metaslab_t *msp, uint64_t size, uint64_t txg)
 	ASSERT(MUTEX_HELD(&msp->ms_lock));
 	VERIFY(!msp->ms_condensing);
 	VERIFY0(msp->ms_disabled);
+	VERIFY0(msp->ms_new);
 
 	start = mc->mc_ops->msop_alloc(msp, size);
 	if (start != -1ULL) {
@@ -4721,10 +4723,10 @@ find_valid_metaslab(metaslab_group_t *mg, uint64_t activation_weight,
 		}
 
 		/*
-		 * If the selected metaslab is condensing or disabled,
-		 * skip it.
+		 * If the selected metaslab is condensing or disabled, or
+		 * hasn't gone through a metaslab_sync_done(), then skip it.
 		 */
-		if (msp->ms_condensing || msp->ms_disabled > 0)
+		if (msp->ms_condensing || msp->ms_disabled > 0 || msp->ms_new)
 			continue;
 
 		*was_active = msp->ms_allocator != -1;
@@ -5270,7 +5272,7 @@ top:
 
 		ASSERT(mg->mg_class == mc);
 
-		uint64_t asize = vdev_psize_to_asize(vd, psize);
+		uint64_t asize = vdev_psize_to_asize_txg(vd, psize, txg);
 		ASSERT(P2PHASE(asize, 1ULL << vd->vdev_ashift) == 0);
 
 		/*
diff --git a/module/zfs/spa.c b/module/zfs/spa.c
index 68f367c1c..20225640f 100644
--- a/module/zfs/spa.c
+++ b/module/zfs/spa.c
@@ -63,6 +63,7 @@
 #include <sys/vdev_rebuild.h>
 #include <sys/vdev_trim.h>
 #include <sys/vdev_disk.h>
+#include <sys/vdev_raidz.h>
 #include <sys/vdev_draid.h>
 #include <sys/metaslab.h>
 #include <sys/metaslab_impl.h>
@@ -1709,6 +1710,10 @@ spa_destroy_aux_threads(spa_t *spa)
 		zthr_destroy(spa->spa_livelist_condense_zthr);
 		spa->spa_livelist_condense_zthr = NULL;
 	}
+	if (spa->spa_raidz_expand_zthr != NULL) {
+		zthr_destroy(spa->spa_raidz_expand_zthr);
+		spa->spa_raidz_expand_zthr = NULL;
+	}
 }
 
 /*
@@ -1861,6 +1866,8 @@ spa_unload(spa_t *spa)
 		spa->spa_compatibility = NULL;
 	}
 
+	spa->spa_raidz_expand = NULL;
+
 	spa_config_exit(spa, SCL_ALL, spa);
 }
 
@@ -2999,6 +3006,7 @@ spa_spawn_aux_threads(spa_t *spa)
 
 	ASSERT(MUTEX_HELD(&spa_namespace_lock));
 
+	spa_start_raidz_expansion_thread(spa);
 	spa_start_indirect_condensing_thread(spa);
 	spa_start_livelist_destroy_thread(spa);
 	spa_start_livelist_condensing_thread(spa);
@@ -3753,6 +3761,12 @@ spa_ld_select_uberblock(spa_t *spa, spa_import_type_t type)
 	}
 	spa_load_note(spa, "using uberblock with txg=%llu",
 	    (u_longlong_t)ub->ub_txg);
+	if (ub->ub_raidz_reflow_info != 0) {
+		spa_load_note(spa, "uberblock raidz_reflow_info: "
+		    "state=%u offset=%llu",
+		    (int)RRSS_GET_STATE(ub),
+		    (u_longlong_t)RRSS_GET_OFFSET(ub));
+	}
 
 
 	/*
@@ -5092,6 +5106,13 @@ spa_load_impl(spa_t *spa, spa_import_type_t type, const char **ereport)
 		ASSERT(spa->spa_load_state != SPA_LOAD_TRYIMPORT);
 
 		/*
+		 * Before we do any zio_write's, complete the raidz expansion
+		 * scratch space copying, if necessary.
+		 */
+		if (RRSS_GET_STATE(&spa->spa_uberblock) == RRSS_SCRATCH_VALID)
+			vdev_raidz_reflow_copy_scratch(spa);
+
+		/*
 		 * In case of a checkpoint rewind, log the original txg
 		 * of the checkpointed uberblock.
 		 */
@@ -6905,9 +6926,10 @@ spa_vdev_add(spa_t *spa, nvlist_t *nvroot)
 }
 
 /*
- * Attach a device to a mirror.  The arguments are the path to any device
- * in the mirror, and the nvroot for the new device.  If the path specifies
- * a device that is not mirrored, we automatically insert the mirror vdev.
+ * Attach a device to a vdev specified by its guid.  The vdev type can be
+ * a mirror, a raidz, or a leaf device that is also a top-level (e.g. a
+ * single device). When the vdev is a single device, a mirror vdev will be
+ * automatically inserted.
  *
  * If 'replacing' is specified, the new device is intended to replace the
  * existing device; in this case the two devices are made into their own
@@ -6930,7 +6952,7 @@ spa_vdev_attach(spa_t *spa, uint64_t guid, nvlist_t *nvroot, int replacing,
 	vdev_t *oldvd, *newvd, *newrootvd, *pvd, *tvd;
 	vdev_ops_t *pvops;
 	char *oldvdpath, *newvdpath;
-	int newvd_isspare;
+	int newvd_isspare = B_FALSE;
 	int error;
 
 	ASSERT(spa_writeable(spa));
@@ -6961,16 +6983,35 @@ spa_vdev_attach(spa_t *spa, uint64_t guid, nvlist_t *nvroot, int replacing,
 			    ZFS_ERR_REBUILD_IN_PROGRESS));
 	}
 
-	if (spa->spa_vdev_removal != NULL)
-		return (spa_vdev_exit(spa, NULL, txg, EBUSY));
+	if (spa->spa_vdev_removal != NULL) {
+		return (spa_vdev_exit(spa, NULL, txg,
+		    ZFS_ERR_DEVRM_IN_PROGRESS));
+	}
 
 	if (oldvd == NULL)
 		return (spa_vdev_exit(spa, NULL, txg, ENODEV));
 
-	if (!oldvd->vdev_ops->vdev_op_leaf)
+	boolean_t raidz = oldvd->vdev_ops == &vdev_raidz_ops;
+
+	if (raidz) {
+		if (!spa_feature_is_enabled(spa, SPA_FEATURE_RAIDZ_EXPANSION))
+			return (spa_vdev_exit(spa, NULL, txg, ENOTSUP));
+
+		/*
+		 * Can't expand a raidz while prior expand is in progress.
+		 */
+		if (spa->spa_raidz_expand != NULL) {
+			return (spa_vdev_exit(spa, NULL, txg,
+			    ZFS_ERR_RAIDZ_EXPAND_IN_PROGRESS));
+		}
+	} else if (!oldvd->vdev_ops->vdev_op_leaf) {
 		return (spa_vdev_exit(spa, NULL, txg, ENOTSUP));
+	}
 
-	pvd = oldvd->vdev_parent;
+	if (raidz)
+		pvd = oldvd;
+	else
+		pvd = oldvd->vdev_parent;
 
 	if (spa_config_parse(spa, &newrootvd, nvroot, NULL, 0,
 	    VDEV_ALLOC_ATTACH) != 0)
@@ -7026,6 +7067,7 @@ spa_vdev_attach(spa_t *spa, uint64_t guid, nvlist_t *nvroot, int replacing,
 		 * vdev.
 		 */
 		if (pvd->vdev_ops != &vdev_mirror_ops &&
+		    pvd->vdev_ops != &vdev_raidz_ops &&
 		    pvd->vdev_ops != &vdev_root_ops)
 			return (spa_vdev_exit(spa, newrootvd, txg, ENOTSUP));
 
@@ -7065,7 +7107,8 @@ spa_vdev_attach(spa_t *spa, uint64_t guid, nvlist_t *nvroot, int replacing,
 	/*
 	 * Make sure the new device is big enough.
 	 */
-	if (newvd->vdev_asize < vdev_get_min_asize(oldvd))
+	vdev_t *min_vdev = raidz ? oldvd->vdev_child[0] : oldvd;
+	if (newvd->vdev_asize < vdev_get_min_asize(min_vdev))
 		return (spa_vdev_exit(spa, newrootvd, txg, EOVERFLOW));
 
 	/*
@@ -7076,31 +7119,74 @@ spa_vdev_attach(spa_t *spa, uint64_t guid, nvlist_t *nvroot, int replacing,
 		return (spa_vdev_exit(spa, newrootvd, txg, ENOTSUP));
 
 	/*
+	 * RAIDZ-expansion-specific checks.
+	 */
+	if (raidz) {
+		if (vdev_raidz_attach_check(newvd) != 0)
+			return (spa_vdev_exit(spa, newrootvd, txg, ENOTSUP));
+
+		/*
+		 * Fail early if a child is not healthy or being replaced
+		 */
+		for (int i = 0; i < oldvd->vdev_children; i++) {
+			if (vdev_is_dead(oldvd->vdev_child[i]) ||
+			    !oldvd->vdev_child[i]->vdev_ops->vdev_op_leaf) {
+				return (spa_vdev_exit(spa, newrootvd, txg,
+				    ENXIO));
+			}
+			/* Also fail if reserved boot area is in-use */
+			if (vdev_check_boot_reserve(spa, oldvd->vdev_child[i])
+			    != 0) {
+				return (spa_vdev_exit(spa, newrootvd, txg,
+				    EADDRINUSE));
+			}
+		}
+	}
+
+	if (raidz) {
+		/*
+		 * Note: oldvdpath is freed by spa_strfree(),  but
+		 * kmem_asprintf() is freed by kmem_strfree(), so we have to
+		 * move it to a spa_strdup-ed string.
+		 */
+		char *tmp = kmem_asprintf("raidz%u-%u",
+		    (uint_t)vdev_get_nparity(oldvd), (uint_t)oldvd->vdev_id);
+		oldvdpath = spa_strdup(tmp);
+		kmem_strfree(tmp);
+	} else {
+		oldvdpath = spa_strdup(oldvd->vdev_path);
+	}
+	newvdpath = spa_strdup(newvd->vdev_path);
+
+	/*
 	 * If this is an in-place replacement, update oldvd's path and devid
 	 * to make it distinguishable from newvd, and unopenable from now on.
 	 */
-	if (strcmp(oldvd->vdev_path, newvd->vdev_path) == 0) {
+	if (strcmp(oldvdpath, newvdpath) == 0) {
 		spa_strfree(oldvd->vdev_path);
-		oldvd->vdev_path = kmem_alloc(strlen(newvd->vdev_path) + 5,
+		oldvd->vdev_path = kmem_alloc(strlen(newvdpath) + 5,
 		    KM_SLEEP);
-		(void) snprintf(oldvd->vdev_path, strlen(newvd->vdev_path) + 5,
-		    "%s/%s", newvd->vdev_path, "old");
+		(void) sprintf(oldvd->vdev_path, "%s/old",
+		    newvdpath);
 		if (oldvd->vdev_devid != NULL) {
 			spa_strfree(oldvd->vdev_devid);
 			oldvd->vdev_devid = NULL;
 		}
+		spa_strfree(oldvdpath);
+		oldvdpath = spa_strdup(oldvd->vdev_path);
 	}
 
 	/*
 	 * If the parent is not a mirror, or if we're replacing, insert the new
 	 * mirror/replacing/spare vdev above oldvd.
 	 */
-	if (pvd->vdev_ops != pvops)
+	if (!raidz && pvd->vdev_ops != pvops) {
 		pvd = vdev_add_parent(oldvd, pvops);
+		ASSERT(pvd->vdev_ops == pvops);
+		ASSERT(oldvd->vdev_parent == pvd);
+	}
 
 	ASSERT(pvd->vdev_top->vdev_parent == rvd);
-	ASSERT(pvd->vdev_ops == pvops);
-	ASSERT(oldvd->vdev_parent == pvd);
 
 	/*
 	 * Extract the new device from its root and add it to pvd.
@@ -7128,41 +7214,66 @@ spa_vdev_attach(spa_t *spa, uint64_t guid, nvlist_t *nvroot, int replacing,
 	 */
 	dtl_max_txg = txg + TXG_CONCURRENT_STATES;
 
-	vdev_dtl_dirty(newvd, DTL_MISSING,
-	    TXG_INITIAL, dtl_max_txg - TXG_INITIAL);
+	if (raidz) {
+		/*
+		 * Wait for the youngest allocations and frees to sync,
+		 * and then wait for the deferral of those frees to finish.
+		 */
+		spa_vdev_config_exit(spa, NULL,
+		    txg + TXG_CONCURRENT_STATES + TXG_DEFER_SIZE, 0, FTAG);
 
-	if (newvd->vdev_isspare) {
-		spa_spare_activate(newvd);
-		spa_event_notify(spa, newvd, NULL, ESC_ZFS_VDEV_SPARE);
-	}
+		vdev_initialize_stop_all(tvd, VDEV_INITIALIZE_ACTIVE);
+		vdev_trim_stop_all(tvd, VDEV_TRIM_ACTIVE);
+		vdev_autotrim_stop_wait(tvd);
 
-	oldvdpath = spa_strdup(oldvd->vdev_path);
-	newvdpath = spa_strdup(newvd->vdev_path);
-	newvd_isspare = newvd->vdev_isspare;
+		dtl_max_txg = spa_vdev_config_enter(spa);
 
-	/*
-	 * Mark newvd's DTL dirty in this txg.
-	 */
-	vdev_dirty(tvd, VDD_DTL, newvd, txg);
+		tvd->vdev_rz_expanding = B_TRUE;
 
-	/*
-	 * Schedule the resilver or rebuild to restart in the future. We do
-	 * this to ensure that dmu_sync-ed blocks have been stitched into the
-	 * respective datasets.
-	 */
-	if (rebuild) {
-		newvd->vdev_rebuild_txg = txg;
+		vdev_dirty_leaves(tvd, VDD_DTL, dtl_max_txg);
+		vdev_config_dirty(tvd);
 
-		vdev_rebuild(tvd);
+		dmu_tx_t *tx = dmu_tx_create_assigned(spa->spa_dsl_pool,
+		    dtl_max_txg);
+		dsl_sync_task_nowait(spa->spa_dsl_pool, vdev_raidz_attach_sync,
+		    newvd, tx);
+		dmu_tx_commit(tx);
 	} else {
-		newvd->vdev_resilver_txg = txg;
+		vdev_dtl_dirty(newvd, DTL_MISSING, TXG_INITIAL,
+		    dtl_max_txg - TXG_INITIAL);
+
+		if (newvd->vdev_isspare) {
+			spa_spare_activate(newvd);
+			spa_event_notify(spa, newvd, NULL, ESC_ZFS_VDEV_SPARE);
+		}
+
+		newvd_isspare = newvd->vdev_isspare;
+
+		/*
+		 * Mark newvd's DTL dirty in this txg.
+		 */
+		vdev_dirty(tvd, VDD_DTL, newvd, txg);
 
-		if (dsl_scan_resilvering(spa_get_dsl(spa)) &&
-		    spa_feature_is_enabled(spa, SPA_FEATURE_RESILVER_DEFER)) {
-			vdev_defer_resilver(newvd);
+		/*
+		 * Schedule the resilver or rebuild to restart in the future.
+		 * We do this to ensure that dmu_sync-ed blocks have been
+		 * stitched into the respective datasets.
+		 */
+		if (rebuild) {
+			newvd->vdev_rebuild_txg = txg;
+
+			vdev_rebuild(tvd);
 		} else {
-			dsl_scan_restart_resilver(spa->spa_dsl_pool,
-			    dtl_max_txg);
+			newvd->vdev_resilver_txg = txg;
+
+			if (dsl_scan_resilvering(spa_get_dsl(spa)) &&
+			    spa_feature_is_enabled(spa,
+			    SPA_FEATURE_RESILVER_DEFER)) {
+				vdev_defer_resilver(newvd);
+			} else {
+				dsl_scan_restart_resilver(spa->spa_dsl_pool,
+				    dtl_max_txg);
+			}
 		}
 	}
 
@@ -7487,7 +7598,7 @@ spa_vdev_initialize_impl(spa_t *spa, uint64_t guid, uint64_t cmd_type,
 	 */
 	if (cmd_type == POOL_INITIALIZE_START &&
 	    (vd->vdev_initialize_thread != NULL ||
-	    vd->vdev_top->vdev_removing)) {
+	    vd->vdev_top->vdev_removing || vd->vdev_top->vdev_rz_expanding)) {
 		mutex_exit(&vd->vdev_initialize_lock);
 		return (SET_ERROR(EBUSY));
 	} else if (cmd_type == POOL_INITIALIZE_CANCEL &&
@@ -7609,7 +7720,8 @@ spa_vdev_trim_impl(spa_t *spa, uint64_t guid, uint64_t cmd_type,
 	 * which has completed but the thread is not exited.
 	 */
 	if (cmd_type == POOL_TRIM_START &&
-	    (vd->vdev_trim_thread != NULL || vd->vdev_top->vdev_removing)) {
+	    (vd->vdev_trim_thread != NULL || vd->vdev_top->vdev_removing ||
+	    vd->vdev_top->vdev_rz_expanding)) {
 		mutex_exit(&vd->vdev_trim_lock);
 		return (SET_ERROR(EBUSY));
 	} else if (cmd_type == POOL_TRIM_CANCEL &&
@@ -8512,6 +8624,10 @@ spa_async_suspend(spa_t *spa)
 	if (condense_thread != NULL)
 		zthr_cancel(condense_thread);
 
+	zthr_t *raidz_expand_thread = spa->spa_raidz_expand_zthr;
+	if (raidz_expand_thread != NULL)
+		zthr_cancel(raidz_expand_thread);
+
 	zthr_t *discard_thread = spa->spa_checkpoint_discard_zthr;
 	if (discard_thread != NULL)
 		zthr_cancel(discard_thread);
@@ -8538,6 +8654,10 @@ spa_async_resume(spa_t *spa)
 	if (condense_thread != NULL)
 		zthr_resume(condense_thread);
 
+	zthr_t *raidz_expand_thread = spa->spa_raidz_expand_zthr;
+	if (raidz_expand_thread != NULL)
+		zthr_resume(raidz_expand_thread);
+
 	zthr_t *discard_thread = spa->spa_checkpoint_discard_zthr;
 	if (discard_thread != NULL)
 		zthr_resume(discard_thread);
@@ -9343,6 +9463,27 @@ spa_sync_iterate_to_convergence(spa_t *spa, dmu_tx_t *tx)
 		    != NULL)
 			vdev_sync(vd, txg);
 
+		if (pass == 1) {
+			/*
+			 * dsl_pool_sync() -> dp_sync_tasks may have dirtied
+			 * the config. If that happens, this txg should not
+			 * be a no-op. So we must sync the config to the MOS
+			 * before checking for no-op.
+			 *
+			 * Note that when the config is dirty, it will
+			 * be written to the MOS (i.e. the MOS will be
+			 * dirtied) every time we call spa_sync_config_object()
+			 * in this txg.  Therefore we can't call this after
+			 * dsl_pool_sync() every pass, because it would
+			 * prevent us from converging, since we'd dirty
+			 * the MOS every pass.
+			 *
+			 * Sync tasks can only be processed in pass 1, so
+			 * there's no need to do this in later passes.
+			 */
+			spa_sync_config_object(spa, tx);
+		}
+
 		/*
 		 * Note: We need to check if the MOS is dirty because we could
 		 * have marked the MOS dirty without updating the uberblock
@@ -10100,7 +10241,8 @@ spa_activity_in_progress(spa_t *spa, zpool_wait_activity_t activity,
 		    DSS_SCANNING);
 		break;
 	case ZPOOL_WAIT_RESILVER:
-		if ((*in_progress = vdev_rebuild_active(spa->spa_root_vdev)))
+		*in_progress = vdev_rebuild_active(spa->spa_root_vdev);
+		if (*in_progress)
 			break;
 		zfs_fallthrough;
 	case ZPOOL_WAIT_SCRUB:
@@ -10115,6 +10257,12 @@ spa_activity_in_progress(spa_t *spa, zpool_wait_activity_t activity,
 		    is_scrub == (activity == ZPOOL_WAIT_SCRUB));
 		break;
 	}
+	case ZPOOL_WAIT_RAIDZ_EXPAND:
+	{
+		vdev_raidz_expand_t *vre = spa->spa_raidz_expand;
+		*in_progress = (vre != NULL && vre->vre_state == DSS_SCANNING);
+		break;
+	}
 	default:
 		panic("unrecognized value for activity %d", activity);
 	}
diff --git a/module/zfs/spa_checkpoint.c b/module/zfs/spa_checkpoint.c
index b588f7041..1efff47f8 100644
--- a/module/zfs/spa_checkpoint.c
+++ b/module/zfs/spa_checkpoint.c
@@ -465,6 +465,9 @@ spa_checkpoint_check(void *arg, dmu_tx_t *tx)
 	if (spa->spa_removing_phys.sr_state == DSS_SCANNING)
 		return (SET_ERROR(ZFS_ERR_DEVRM_IN_PROGRESS));
 
+	if (spa->spa_raidz_expand != NULL)
+		return (SET_ERROR(ZFS_ERR_RAIDZ_EXPAND_IN_PROGRESS));
+
 	if (spa->spa_checkpoint_txg != 0)
 		return (SET_ERROR(ZFS_ERR_CHECKPOINT_EXISTS));
 
diff --git a/module/zfs/vdev.c b/module/zfs/vdev.c
index afb01c0ef..c10c78ebf 100644
--- a/module/zfs/vdev.c
+++ b/module/zfs/vdev.c
@@ -58,6 +58,7 @@
 #include <sys/abd.h>
 #include <sys/vdev_initialize.h>
 #include <sys/vdev_trim.h>
+#include <sys/vdev_raidz.h>
 #include <sys/zvol.h>
 #include <sys/zfs_ratelimit.h>
 #include "zfs_prop.h"
@@ -305,13 +306,13 @@ vdev_derive_alloc_bias(const char *bias)
  * all children.  This is what's used by anything other than RAID-Z.
  */
 uint64_t
-vdev_default_asize(vdev_t *vd, uint64_t psize)
+vdev_default_asize(vdev_t *vd, uint64_t psize, uint64_t txg)
 {
 	uint64_t asize = P2ROUNDUP(psize, 1ULL << vd->vdev_top->vdev_ashift);
 	uint64_t csize;
 
 	for (int c = 0; c < vd->vdev_children; c++) {
-		csize = vdev_psize_to_asize(vd->vdev_child[c], psize);
+		csize = vdev_psize_to_asize_txg(vd->vdev_child[c], psize, txg);
 		asize = MAX(asize, csize);
 	}
 
@@ -930,6 +931,8 @@ vdev_alloc(spa_t *spa, vdev_t **vdp, nvlist_t *nv, vdev_t *parent, uint_t id,
 		    &vd->vdev_removing);
 		(void) nvlist_lookup_uint64(nv, ZPOOL_CONFIG_VDEV_TOP_ZAP,
 		    &vd->vdev_top_zap);
+		vd->vdev_rz_expanding = nvlist_exists(nv,
+		    ZPOOL_CONFIG_RAIDZ_EXPANDING);
 	} else {
 		ASSERT0(vd->vdev_top_zap);
 	}
@@ -1692,6 +1695,8 @@ vdev_probe_done(zio_t *zio)
 
 		vd->vdev_cant_read |= !vps->vps_readable;
 		vd->vdev_cant_write |= !vps->vps_writeable;
+		vdev_dbgmsg(vd, "probe done, cant_read=%u cant_write=%u",
+		    vd->vdev_cant_read, vd->vdev_cant_write);
 
 		if (vdev_readable(vd) &&
 		    (vdev_writeable(vd) || !spa_writeable(spa))) {
@@ -1913,17 +1918,20 @@ vdev_open_children_subset(vdev_t *vd, vdev_open_children_func_t *open_func)
 }
 
 /*
- * Compute the raidz-deflation ratio.  Note, we hard-code
- * in 128k (1 << 17) because it is the "typical" blocksize.
- * Even though SPA_MAXBLOCKSIZE changed, this algorithm can not change,
- * otherwise it would inconsistently account for existing bp's.
+ * Compute the raidz-deflation ratio.  Note, we hard-code 128k (1 << 17)
+ * because it is the "typical" blocksize.  Even though SPA_MAXBLOCKSIZE
+ * changed, this algorithm can not change, otherwise it would inconsistently
+ * account for existing bp's.  We also hard-code txg 0 for the same reason
+ * since expanded RAIDZ vdevs can use a different asize for different birth
+ * txg's.
  */
 static void
 vdev_set_deflate_ratio(vdev_t *vd)
 {
 	if (vd == vd->vdev_top && !vd->vdev_ishole && vd->vdev_ashift != 0) {
 		vd->vdev_deflate_ratio = (1 << 17) /
-		    (vdev_psize_to_asize(vd, 1 << 17) >> SPA_MINBLOCKSHIFT);
+		    (vdev_psize_to_asize_txg(vd, 1 << 17, 0) >>
+		    SPA_MINBLOCKSHIFT);
 	}
 }
 
@@ -3228,32 +3236,43 @@ vdev_dtl_reassess(vdev_t *vd, uint64_t txg, uint64_t scrub_txg,
 
 		if (txg != 0)
 			vdev_dirty(vd->vdev_top, VDD_DTL, vd, txg);
-		return;
+	} else {
+		mutex_enter(&vd->vdev_dtl_lock);
+		for (int t = 0; t < DTL_TYPES; t++) {
+			/* account for child's outage in parent's missing map */
+			int s = (t == DTL_MISSING) ? DTL_OUTAGE: t;
+			if (t == DTL_SCRUB) {
+				/* leaf vdevs only */
+				continue;
+			}
+			if (t == DTL_PARTIAL) {
+				/* i.e. non-zero */
+				minref = 1;
+			} else if (vdev_get_nparity(vd) != 0) {
+				/* RAIDZ, DRAID */
+				minref = vdev_get_nparity(vd) + 1;
+			} else {
+				/* any kind of mirror */
+				minref = vd->vdev_children;
+			}
+			space_reftree_create(&reftree);
+			for (int c = 0; c < vd->vdev_children; c++) {
+				vdev_t *cvd = vd->vdev_child[c];
+				mutex_enter(&cvd->vdev_dtl_lock);
+				space_reftree_add_map(&reftree,
+				    cvd->vdev_dtl[s], 1);
+				mutex_exit(&cvd->vdev_dtl_lock);
+			}
+			space_reftree_generate_map(&reftree,
+			    vd->vdev_dtl[t], minref);
+			space_reftree_destroy(&reftree);
+		}
+		mutex_exit(&vd->vdev_dtl_lock);
 	}
 
-	mutex_enter(&vd->vdev_dtl_lock);
-	for (int t = 0; t < DTL_TYPES; t++) {
-		/* account for child's outage in parent's missing map */
-		int s = (t == DTL_MISSING) ? DTL_OUTAGE: t;
-		if (t == DTL_SCRUB)
-			continue;			/* leaf vdevs only */
-		if (t == DTL_PARTIAL)
-			minref = 1;			/* i.e. non-zero */
-		else if (vdev_get_nparity(vd) != 0)
-			minref = vdev_get_nparity(vd) + 1; /* RAID-Z, dRAID */
-		else
-			minref = vd->vdev_children;	/* any kind of mirror */
-		space_reftree_create(&reftree);
-		for (int c = 0; c < vd->vdev_children; c++) {
-			vdev_t *cvd = vd->vdev_child[c];
-			mutex_enter(&cvd->vdev_dtl_lock);
-			space_reftree_add_map(&reftree, cvd->vdev_dtl[s], 1);
-			mutex_exit(&cvd->vdev_dtl_lock);
-		}
-		space_reftree_generate_map(&reftree, vd->vdev_dtl[t], minref);
-		space_reftree_destroy(&reftree);
+	if (vd->vdev_top->vdev_ops == &vdev_raidz_ops) {
+		raidz_dtl_reassessed(vd);
 	}
-	mutex_exit(&vd->vdev_dtl_lock);
 }
 
 /*
@@ -3628,6 +3647,12 @@ vdev_load(vdev_t *vd)
 
 	vdev_set_deflate_ratio(vd);
 
+	if (vd->vdev_ops == &vdev_raidz_ops) {
+		error = vdev_raidz_load(vd);
+		if (error != 0)
+			return (error);
+	}
+
 	/*
 	 * On spa_load path, grab the allocation bias from our zap
 	 */
@@ -4005,10 +4030,22 @@ vdev_sync(vdev_t *vd, uint64_t txg)
 	dmu_tx_commit(tx);
 }
 
+/*
+ * Return the amount of space that should be (or was) allocated for the given
+ * psize (compressed block size) in the given TXG. Note that for expanded
+ * RAIDZ vdevs, the size allocated for older BP's may be larger. See
+ * vdev_raidz_asize().
+ */
+uint64_t
+vdev_psize_to_asize_txg(vdev_t *vd, uint64_t psize, uint64_t txg)
+{
+	return (vd->vdev_ops->vdev_op_asize(vd, psize, txg));
+}
+
 uint64_t
 vdev_psize_to_asize(vdev_t *vd, uint64_t psize)
 {
-	return (vd->vdev_ops->vdev_op_asize(vd, psize));
+	return (vdev_psize_to_asize_txg(vd, psize, 0));
 }
 
 /*
@@ -4174,9 +4211,6 @@ vdev_online(spa_t *spa, uint64_t guid, uint64_t flags, vdev_state_t *newstate)
 	if ((vd = spa_lookup_by_guid(spa, guid, B_TRUE)) == NULL)
 		return (spa_vdev_state_exit(spa, NULL, SET_ERROR(ENODEV)));
 
-	if (!vd->vdev_ops->vdev_op_leaf)
-		return (spa_vdev_state_exit(spa, NULL, SET_ERROR(ENOTSUP)));
-
 	wasoffline = (vd->vdev_offline || vd->vdev_tmpoffline);
 	oldstate = vd->vdev_state;
 
@@ -5457,7 +5491,9 @@ vdev_expand(vdev_t *vd, uint64_t txg)
 
 	vdev_set_deflate_ratio(vd);
 
-	if ((vd->vdev_asize >> vd->vdev_ms_shift) > vd->vdev_ms_count &&
+	if ((vd->vdev_spa->spa_raidz_expand == NULL ||
+	    vd->vdev_spa->spa_raidz_expand->vre_vdev_id != vd->vdev_id) &&
+	    (vd->vdev_asize >> vd->vdev_ms_shift) > vd->vdev_ms_count &&
 	    vdev_is_concrete(vd)) {
 		vdev_metaslab_group_create(vd);
 		VERIFY(vdev_metaslab_init(vd, txg) == 0);
@@ -6209,6 +6245,14 @@ vdev_prop_get(vdev_t *vd, nvlist_t *innvl, nvlist_t *outnvl)
 				vdev_prop_add_list(outnvl, propname, NULL,
 				    vd->vdev_removing, ZPROP_SRC_NONE);
 				continue;
+			case VDEV_PROP_RAIDZ_EXPANDING:
+				/* Only expose this for raidz */
+				if (vd->vdev_ops == &vdev_raidz_ops) {
+					vdev_prop_add_list(outnvl, propname,
+					    NULL, vd->vdev_rz_expanding,
+					    ZPROP_SRC_NONE);
+				}
+				continue;
 			/* Numeric Properites */
 			case VDEV_PROP_ALLOCATING:
 				/* Leaf vdevs cannot have this property */
diff --git a/module/zfs/vdev_draid.c b/module/zfs/vdev_draid.c
index 307e2353d..ec961255f 100644
--- a/module/zfs/vdev_draid.c
+++ b/module/zfs/vdev_draid.c
@@ -577,8 +577,9 @@ vdev_draid_permute_id(vdev_draid_config_t *vdc,
  * i.e. vdev_draid_psize_to_asize().
  */
 static uint64_t
-vdev_draid_asize(vdev_t *vd, uint64_t psize)
+vdev_draid_asize(vdev_t *vd, uint64_t psize, uint64_t txg)
 {
+	(void) txg;
 	vdev_draid_config_t *vdc = vd->vdev_tsd;
 	uint64_t ashift = vd->vdev_ashift;
 
@@ -960,7 +961,7 @@ vdev_draid_map_alloc_row(zio_t *zio, raidz_row_t **rrp, uint64_t io_offset,
 	vdev_draid_config_t *vdc = vd->vdev_tsd;
 	uint64_t ashift = vd->vdev_top->vdev_ashift;
 	uint64_t io_size = abd_size;
-	uint64_t io_asize = vdev_draid_asize(vd, io_size);
+	uint64_t io_asize = vdev_draid_asize(vd, io_size, 0);
 	uint64_t group = vdev_draid_offset_to_group(vd, io_offset);
 	uint64_t start_offset = vdev_draid_group_to_offset(vd, group + 1);
 
@@ -1025,15 +1026,9 @@ vdev_draid_map_alloc_row(zio_t *zio, raidz_row_t **rrp, uint64_t io_offset,
 
 	ASSERT3U(vdc->vdc_nparity, >, 0);
 
-	raidz_row_t *rr;
-	rr = kmem_alloc(offsetof(raidz_row_t, rr_col[groupwidth]), KM_SLEEP);
-	rr->rr_cols = groupwidth;
-	rr->rr_scols = groupwidth;
+	raidz_row_t *rr = vdev_raidz_row_alloc(groupwidth);
 	rr->rr_bigcols = bc;
-	rr->rr_missingdata = 0;
-	rr->rr_missingparity = 0;
 	rr->rr_firstdatacol = vdc->vdc_nparity;
-	rr->rr_abd_empty = NULL;
 #ifdef ZFS_DEBUG
 	rr->rr_offset = io_offset;
 	rr->rr_size = io_size;
@@ -1053,14 +1048,6 @@ vdev_draid_map_alloc_row(zio_t *zio, raidz_row_t **rrp, uint64_t io_offset,
 
 		rc->rc_devidx = vdev_draid_permute_id(vdc, base, iter, c);
 		rc->rc_offset = physical_offset;
-		rc->rc_abd = NULL;
-		rc->rc_orig_data = NULL;
-		rc->rc_error = 0;
-		rc->rc_tried = 0;
-		rc->rc_skipped = 0;
-		rc->rc_force_repair = 0;
-		rc->rc_allow_repair = 1;
-		rc->rc_need_orig_restore = B_FALSE;
 
 		if (q == 0 && i >= bc)
 			rc->rc_size = 0;
@@ -1129,7 +1116,7 @@ vdev_draid_map_alloc(zio_t *zio)
 	if (size < abd_size) {
 		vdev_t *vd = zio->io_vd;
 
-		io_offset += vdev_draid_asize(vd, size);
+		io_offset += vdev_draid_asize(vd, size, 0);
 		abd_offset += size;
 		abd_size -= size;
 		nrows++;
@@ -1151,7 +1138,6 @@ vdev_draid_map_alloc(zio_t *zio)
 	rm->rm_row[0] = rr[0];
 	if (nrows == 2)
 		rm->rm_row[1] = rr[1];
-
 	return (rm);
 }
 
@@ -1783,7 +1769,7 @@ vdev_draid_need_resilver(vdev_t *vd, const dva_t *dva, size_t psize,
     uint64_t phys_birth)
 {
 	uint64_t offset = DVA_GET_OFFSET(dva);
-	uint64_t asize = vdev_draid_asize(vd, psize);
+	uint64_t asize = vdev_draid_asize(vd, psize, 0);
 
 	if (phys_birth == TXG_UNKNOWN) {
 		/*
@@ -1840,7 +1826,7 @@ vdev_draid_io_verify(vdev_t *vd, raidz_row_t *rr, int col)
 	range_seg64_t logical_rs, physical_rs, remain_rs;
 	logical_rs.rs_start = rr->rr_offset;
 	logical_rs.rs_end = logical_rs.rs_start +
-	    vdev_draid_asize(vd, rr->rr_size);
+	    vdev_draid_asize(vd, rr->rr_size, 0);
 
 	raidz_col_t *rc = &rr->rr_col[col];
 	vdev_t *cvd = vd->vdev_child[rc->rc_devidx];
diff --git a/module/zfs/vdev_initialize.c b/module/zfs/vdev_initialize.c
index ffdcef197..5aaef1a69 100644
--- a/module/zfs/vdev_initialize.c
+++ b/module/zfs/vdev_initialize.c
@@ -48,7 +48,8 @@ static boolean_t
 vdev_initialize_should_stop(vdev_t *vd)
 {
 	return (vd->vdev_initialize_exit_wanted || !vdev_writeable(vd) ||
-	    vd->vdev_detached || vd->vdev_top->vdev_removing);
+	    vd->vdev_detached || vd->vdev_top->vdev_removing ||
+	    vd->vdev_top->vdev_rz_expanding);
 }
 
 static void
@@ -67,7 +68,8 @@ vdev_initialize_zap_update_sync(void *arg, dmu_tx_t *tx)
 	kmem_free(arg, sizeof (uint64_t));
 
 	vdev_t *vd = spa_lookup_by_guid(tx->tx_pool->dp_spa, guid, B_FALSE);
-	if (vd == NULL || vd->vdev_top->vdev_removing || !vdev_is_concrete(vd))
+	if (vd == NULL || vd->vdev_top->vdev_removing ||
+	    !vdev_is_concrete(vd) || vd->vdev_top->vdev_rz_expanding)
 		return;
 
 	uint64_t last_offset = vd->vdev_initialize_offset[txg & TXG_MASK];
@@ -631,6 +633,7 @@ vdev_initialize(vdev_t *vd)
 	ASSERT(!vd->vdev_detached);
 	ASSERT(!vd->vdev_initialize_exit_wanted);
 	ASSERT(!vd->vdev_top->vdev_removing);
+	ASSERT(!vd->vdev_top->vdev_rz_expanding);
 
 	vdev_initialize_change_state(vd, VDEV_INITIALIZE_ACTIVE);
 	vd->vdev_initialize_thread = thread_create(NULL, 0,
@@ -791,13 +794,14 @@ vdev_initialize_restart(vdev_t *vd)
 		ASSERT(err == 0 || err == ENOENT);
 		vd->vdev_initialize_action_time = timestamp;
 
-		if (vd->vdev_initialize_state == VDEV_INITIALIZE_SUSPENDED ||
-		    vd->vdev_offline) {
+		if ((vd->vdev_initialize_state == VDEV_INITIALIZE_SUSPENDED ||
+		    vd->vdev_offline) && !vd->vdev_top->vdev_rz_expanding) {
 			/* load progress for reporting, but don't resume */
 			VERIFY0(vdev_initialize_load(vd));
 		} else if (vd->vdev_initialize_state ==
 		    VDEV_INITIALIZE_ACTIVE && vdev_writeable(vd) &&
 		    !vd->vdev_top->vdev_removing &&
+		    !vd->vdev_top->vdev_rz_expanding &&
 		    vd->vdev_initialize_thread == NULL) {
 			vdev_initialize(vd);
 		}
diff --git a/module/zfs/vdev_label.c b/module/zfs/vdev_label.c
index a2e5524a8..e8f562a1a 100644
--- a/module/zfs/vdev_label.c
+++ b/module/zfs/vdev_label.c
@@ -142,6 +142,7 @@
 #include <sys/zap.h>
 #include <sys/vdev.h>
 #include <sys/vdev_impl.h>
+#include <sys/vdev_raidz.h>
 #include <sys/vdev_draid.h>
 #include <sys/uberblock_impl.h>
 #include <sys/metaslab.h>
@@ -423,6 +424,13 @@ root_vdev_actions_getprogress(vdev_t *vd, nvlist_t *nvl)
 		    ZPOOL_CONFIG_CHECKPOINT_STATS, (uint64_t *)&pcs,
 		    sizeof (pcs) / sizeof (uint64_t));
 	}
+
+	pool_raidz_expand_stat_t pres;
+	if (spa_raidz_expand_get_stats(spa, &pres) == 0) {
+		fnvlist_add_uint64_array(nvl,
+		    ZPOOL_CONFIG_RAIDZ_EXPAND_STATS, (uint64_t *)&pres,
+		    sizeof (pres) / sizeof (uint64_t));
+	}
 }
 
 static void
@@ -1504,7 +1512,8 @@ vdev_uberblock_compare(const uberblock_t *ub1, const uberblock_t *ub2)
 }
 
 struct ubl_cbdata {
-	uberblock_t	*ubl_ubbest;	/* Best uberblock */
+	uberblock_t	ubl_latest;	/* Most recent uberblock */
+	uberblock_t	*ubl_ubbest;	/* Best uberblock (w/r/t max_txg) */
 	vdev_t		*ubl_vd;	/* vdev associated with the above */
 };
 
@@ -1521,6 +1530,9 @@ vdev_uberblock_load_done(zio_t *zio)
 
 	if (zio->io_error == 0 && uberblock_verify(ub) == 0) {
 		mutex_enter(&rio->io_lock);
+		if (vdev_uberblock_compare(ub, &cbp->ubl_latest) > 0) {
+			cbp->ubl_latest = *ub;
+		}
 		if (ub->ub_txg <= spa->spa_load_max_txg &&
 		    vdev_uberblock_compare(ub, cbp->ubl_ubbest) > 0) {
 			/*
@@ -1578,10 +1590,10 @@ vdev_uberblock_load(vdev_t *rvd, uberblock_t *ub, nvlist_t **config)
 	ASSERT(config);
 
 	memset(ub, 0, sizeof (uberblock_t));
+	memset(&cb, 0, sizeof (cb));
 	*config = NULL;
 
 	cb.ubl_ubbest = ub;
-	cb.ubl_vd = NULL;
 
 	spa_config_enter(spa, SCL_ALL, FTAG, RW_WRITER);
 	zio = zio_root(spa, NULL, &cb, flags);
@@ -1598,6 +1610,22 @@ vdev_uberblock_load(vdev_t *rvd, uberblock_t *ub, nvlist_t **config)
 		vdev_dbgmsg(cb.ubl_vd, "best uberblock found for spa %s. "
 		    "txg %llu", spa->spa_name, (u_longlong_t)ub->ub_txg);
 
+		if (ub->ub_raidz_reflow_info !=
+		    cb.ubl_latest.ub_raidz_reflow_info) {
+			vdev_dbgmsg(cb.ubl_vd,
+			    "spa=%s best uberblock (txg=%llu info=0x%llx) "
+			    "has different raidz_reflow_info than latest "
+			    "uberblock (txg=%llu info=0x%llx)",
+			    spa->spa_name,
+			    (u_longlong_t)ub->ub_txg,
+			    (u_longlong_t)ub->ub_raidz_reflow_info,
+			    (u_longlong_t)cb.ubl_latest.ub_txg,
+			    (u_longlong_t)cb.ubl_latest.ub_raidz_reflow_info);
+			memset(ub, 0, sizeof (uberblock_t));
+			spa_config_exit(spa, SCL_ALL, FTAG);
+			return;
+		}
+
 		*config = vdev_label_read_config(cb.ubl_vd, ub->ub_txg);
 		if (*config == NULL && spa->spa_extreme_rewind) {
 			vdev_dbgmsg(cb.ubl_vd, "failed to read label config. "
@@ -1719,8 +1747,23 @@ vdev_uberblock_sync(zio_t *zio, uint64_t *good_writes,
 		vd->vdev_copy_uberblocks = B_FALSE;
 	}
 
+	/*
+	 * We chose a slot based on the txg.  If this uberblock has a special
+	 * RAIDZ expansion state, then it is essentially an update of the
+	 * current uberblock (it has the same txg).  However, the current
+	 * state is committed, so we want to write it to a different slot. If
+	 * we overwrote the same slot, and we lose power during the uberblock
+	 * write, and the disk does not do single-sector overwrites
+	 * atomically (even though it is required to - i.e. we should see
+	 * either the old or the new uberblock), then we could lose this
+	 * txg's uberblock. Rewinding to the previous txg's uberblock may not
+	 * be possible because RAIDZ expansion may have already overwritten
+	 * some of the data, so we need the progress indicator in the
+	 * uberblock.
+	 */
 	int m = spa_multihost(vd->vdev_spa) ? MMP_BLOCKS_PER_LABEL : 0;
-	int n = ub->ub_txg % (VDEV_UBERBLOCK_COUNT(vd) - m);
+	int n = (ub->ub_txg - (RRSS_GET_STATE(ub) == RRSS_SCRATCH_VALID)) %
+	    (VDEV_UBERBLOCK_COUNT(vd) - m);
 
 	/* Copy the uberblock_t into the ABD */
 	abd_t *ub_abd = abd_alloc_for_io(VDEV_UBERBLOCK_SIZE(vd), B_TRUE);
@@ -1737,7 +1780,7 @@ vdev_uberblock_sync(zio_t *zio, uint64_t *good_writes,
 }
 
 /* Sync the uberblocks to all vdevs in svd[] */
-static int
+int
 vdev_uberblock_sync_list(vdev_t **svd, int svdcount, uberblock_t *ub, int flags)
 {
 	spa_t *spa = svd[0]->vdev_spa;
diff --git a/module/zfs/vdev_raidz.c b/module/zfs/vdev_raidz.c
index 3445fa9d3..9d0b8763f 100644
--- a/module/zfs/vdev_raidz.c
+++ b/module/zfs/vdev_raidz.c
@@ -27,15 +27,22 @@
 
 #include <sys/zfs_context.h>
 #include <sys/spa.h>
+#include <sys/spa_impl.h>
+#include <sys/zap.h>
 #include <sys/vdev_impl.h>
+#include <sys/metaslab_impl.h>
 #include <sys/zio.h>
 #include <sys/zio_checksum.h>
+#include <sys/dmu_tx.h>
 #include <sys/abd.h>
+#include <sys/zfs_rlock.h>
 #include <sys/fs/zfs.h>
 #include <sys/fm/fs/zfs.h>
 #include <sys/vdev_raidz.h>
 #include <sys/vdev_raidz_impl.h>
 #include <sys/vdev_draid.h>
+#include <sys/uberblock_impl.h>
+#include <sys/dsl_scan.h>
 
 #ifdef ZFS_DEBUG
 #include <sys/vdev.h>	/* For vdev_xlate() in vdev_raidz_io_verify() */
@@ -135,6 +142,237 @@
 	VDEV_RAIDZ_64MUL_2((x), mask); \
 }
 
+
+/*
+ * Big Theory Statement for how a RAIDZ VDEV is expanded
+ *
+ * An existing RAIDZ VDEV can be expanded by attaching a new disk. Expansion
+ * works with all three RAIDZ parity choices, including RAIDZ1, 2, or 3. VDEVs
+ * that have been previously expanded can be expanded again.
+ *
+ * The RAIDZ VDEV must be healthy (must be able to write to all the drives in
+ * the VDEV) when an expansion starts.  And the expansion will pause if any
+ * disk in the VDEV fails, and resume once the VDEV is healthy again. All other
+ * operations on the pool can continue while an expansion is in progress (e.g.
+ * read/write, snapshot, zpool add, etc). Except zpool checkpoint, zpool trim,
+ * and zpool initialize which can't be run during an expansion.  Following a
+ * reboot or export/import, the expansion resumes where it left off.
+ *
+ * == Reflowing the Data ==
+ *
+ * The expansion involves reflowing (copying) the data from the current set
+ * of disks to spread it across the new set which now has one more disk. This
+ * reflow operation is similar to reflowing text when the column width of a
+ * text editor window is expanded. The text doesn’t change but the location of
+ * the text changes to accommodate the new width. An example reflow result for
+ * a 4-wide RAIDZ1 to a 5-wide is shown below.
+ *
+ *                            Reflow End State
+ *            Each letter indicates a parity group (logical stripe)
+ *
+ *         Before expansion                         After Expansion
+ *     D1     D2     D3     D4               D1     D2     D3     D4     D5
+ *  +------+------+------+------+         +------+------+------+------+------+
+ *  |      |      |      |      |         |      |      |      |      |      |
+ *  |  A   |  A   |  A   |  A   |         |  A   |  A   |  A   |  A   |  B   |
+ *  |     1|     2|     3|     4|         |     1|     2|     3|     4|     5|
+ *  +------+------+------+------+         +------+------+------+------+------+
+ *  |      |      |      |      |         |      |      |      |      |      |
+ *  |  B   |  B   |  C   |  C   |         |  B   |  C   |  C   |  C   |  C   |
+ *  |     5|     6|     7|     8|         |     6|     7|     8|     9|    10|
+ *  +------+------+------+------+         +------+------+------+------+------+
+ *  |      |      |      |      |         |      |      |      |      |      |
+ *  |  C   |  C   |  D   |  D   |         |  D   |  D   |  E   |  E   |  E   |
+ *  |     9|    10|    11|    12|         |    11|    12|    13|    14|    15|
+ *  +------+------+------+------+         +------+------+------+------+------+
+ *  |      |      |      |      |         |      |      |      |      |      |
+ *  |  E   |  E   |  E   |  E   |   -->   |  E   |  F   |  F   |  G   |  G   |
+ *  |    13|    14|    15|    16|         |    16|    17|    18|p   19|    20|
+ *  +------+------+------+------+         +------+------+------+------+------+
+ *  |      |      |      |      |         |      |      |      |      |      |
+ *  |  F   |  F   |  G   |  G   |         |  G   |  G   |  H   |  H   |  H   |
+ *  |    17|    18|    19|    20|         |    21|    22|    23|    24|    25|
+ *  +------+------+------+------+         +------+------+------+------+------+
+ *  |      |      |      |      |         |      |      |      |      |      |
+ *  |  G   |  G   |  H   |  H   |         |  H   |  I   |  I   |  J   |  J   |
+ *  |    21|    22|    23|    24|         |    26|    27|    28|    29|    30|
+ *  +------+------+------+------+         +------+------+------+------+------+
+ *  |      |      |      |      |         |      |      |      |      |      |
+ *  |  H   |  H   |  I   |  I   |         |  J   |  J   |      |      |  K   |
+ *  |    25|    26|    27|    28|         |    31|    32|    33|    34|    35|
+ *  +------+------+------+------+         +------+------+------+------+------+
+ *
+ * This reflow approach has several advantages. There is no need to read or
+ * modify the block pointers or recompute any block checksums.  The reflow
+ * doesn’t need to know where the parity sectors reside. We can read and write
+ * data sequentially and the copy can occur in a background thread in open
+ * context. The design also allows for fast discovery of what data to copy.
+ *
+ * The VDEV metaslabs are processed, one at a time, to copy the block data to
+ * have it flow across all the disks. The metaslab is disabled for allocations
+ * during the copy. As an optimization, we only copy the allocated data which
+ * can be determined by looking at the metaslab range tree. During the copy we
+ * must maintain the redundancy guarantees of the RAIDZ VDEV (i.e., we still
+ * need to be able to survive losing parity count disks).  This means we
+ * cannot overwrite data during the reflow that would be needed if a disk is
+ * lost.
+ *
+ * After the reflow completes, all newly-written blocks will have the new
+ * layout, i.e., they will have the parity to data ratio implied by the new
+ * number of disks in the RAIDZ group.  Even though the reflow copies all of
+ * the allocated space (data and parity), it is only rearranged, not changed.
+ *
+ * This act of reflowing the data has a few implications about blocks
+ * that were written before the reflow completes:
+ *
+ *  - Old blocks will still use the same amount of space (i.e., they will have
+ *    the parity to data ratio implied by the old number of disks in the RAIDZ
+ *    group).
+ *  - Reading old blocks will be slightly slower than before the reflow, for
+ *    two reasons. First, we will have to read from all disks in the RAIDZ
+ *    VDEV, rather than being able to skip the children that contain only
+ *    parity of this block (because the data of a single block is now spread
+ *    out across all the disks).  Second, in most cases there will be an extra
+ *    bcopy, needed to rearrange the data back to its original layout in memory.
+ *
+ * == Scratch Area ==
+ *
+ * As we copy the block data, we can only progress to the point that writes
+ * will not overlap with blocks whose progress has not yet been recorded on
+ * disk.  Since partially-copied rows are always read from the old location,
+ * we need to stop one row before the sector-wise overlap, to prevent any
+ * row-wise overlap. For example, in the diagram above, when we reflow sector
+ * B6 it will overwite the original location for B5.
+ *
+ * To get around this, a scratch space is used so that we can start copying
+ * without risking data loss by overlapping the row. As an added benefit, it
+ * improves performance at the beginning of the reflow, but that small perf
+ * boost wouldn't be worth the complexity on its own.
+ *
+ * Ideally we want to copy at least 2 * (new_width)^2 so that we have a
+ * separation of 2*(new_width+1) and a chunk size of new_width+2. With the max
+ * RAIDZ width of 255 and 4K sectors this would be 2MB per disk. In practice
+ * the widths will likely be single digits so we can get a substantial chuck
+ * size using only a few MB of scratch per disk.
+ *
+ * The scratch area is persisted to disk which holds a large amount of reflowed
+ * state. We can always read the partially written stripes when a disk fails or
+ * the copy is interrupted (crash) during the initial copying phase and also
+ * get past a small chunk size restriction.  At a minimum, the scratch space
+ * must be large enough to get us to the point that one row does not overlap
+ * itself when moved (i.e new_width^2).  But going larger is even better. We
+ * use the 3.5 MiB reserved "boot" space that resides after the ZFS disk labels
+ * as our scratch space to handle overwriting the initial part of the VDEV.
+ *
+ *	0     256K   512K                    4M
+ *	+------+------+-----------------------+-----------------------------
+ *	| VDEV | VDEV |   Boot Block (3.5M)   |  Allocatable space ...
+ *	|  L0  |  L1  |       Reserved        |     (Metaslabs)
+ *	+------+------+-----------------------+-------------------------------
+ *                        Scratch Area
+ *
+ * == Reflow Progress Updates ==
+ * After the initial scratch-based reflow, the expansion process works
+ * similarly to device removal. We create a new open context thread which
+ * reflows the data, and periodically kicks off sync tasks to update logical
+ * state. In this case, state is the committed progress (offset of next data
+ * to copy). We need to persist the completed offset on disk, so that if we
+ * crash we know which format each VDEV offset is in.
+ *
+ * == Time Dependent Geometry ==
+ *
+ * In non-expanded RAIDZ, blocks are read from disk in a column by column
+ * fashion. For a multi-row block, the second sector is in the first column
+ * not in the second column. This allows us to issue full reads for each
+ * column directly into the request buffer. The block data is thus laid out
+ * sequentially in a column-by-column fashion.
+ *
+ * For example, in the before expansion diagram above, one logical block might
+ * be sectors G19-H26. The parity is in G19,H23; and the data is in
+ * G20,H24,G21,H25,G22,H26.
+ *
+ * After a block is reflowed, the sectors that were all in the original column
+ * data can now reside in different columns. When reading from an expanded
+ * VDEV, we need to know the logical stripe width for each block so we can
+ * reconstitute the block’s data after the reads are completed. Likewise,
+ * when we perform the combinatorial reconstruction we need to know the
+ * original width so we can retry combinations from the past layouts.
+ *
+ * Time dependent geometry is what we call having blocks with different layouts
+ * (stripe widths) in the same VDEV. This time-dependent geometry uses the
+ * block’s birth time (+ the time expansion ended) to establish the correct
+ * width for a given block. After an expansion completes, we record the time
+ * for blocks written with a particular width (geometry).
+ *
+ * == On Disk Format Changes ==
+ *
+ * New pool feature flag, 'raidz_expansion' whose reference count is the number
+ * of RAIDZ VDEVs that have been expanded.
+ *
+ * The blocks on expanded RAIDZ VDEV can have different logical stripe widths.
+ *
+ * Since the uberblock can point to arbitrary blocks, which might be on the
+ * expanding RAIDZ, and might or might not have been expanded. We need to know
+ * which way a block is laid out before reading it. This info is the next
+ * offset that needs to be reflowed and we persist that in the uberblock, in
+ * the new ub_raidz_reflow_info field, as opposed to the MOS or the vdev label.
+ * After the expansion is complete, we then use the raidz_expand_txgs array
+ * (see below) to determine how to read a block and the ub_raidz_reflow_info
+ * field no longer required.
+ *
+ * The uberblock's ub_raidz_reflow_info field also holds the scratch space
+ * state (i.e., active or not) which is also required before reading a block
+ * during the initial phase of reflowing the data.
+ *
+ * The top-level RAIDZ VDEV has two new entries in the nvlist:
+ *
+ * 'raidz_expand_txgs' array: logical stripe widths by txg are recorded here
+ *                            and used after the expansion is complete to
+ *                            determine how to read a raidz block
+ * 'raidz_expanding' boolean: present during reflow and removed after completion
+ *                            used during a spa import to resume an unfinished
+ *                            expansion
+ *
+ * And finally the VDEVs top zap adds the following informational entries:
+ *   VDEV_TOP_ZAP_RAIDZ_EXPAND_STATE
+ *   VDEV_TOP_ZAP_RAIDZ_EXPAND_START_TIME
+ *   VDEV_TOP_ZAP_RAIDZ_EXPAND_END_TIME
+ *   VDEV_TOP_ZAP_RAIDZ_EXPAND_BYTES_COPIED
+ */
+
+/*
+ * For testing only: pause the raidz expansion after reflowing this amount.
+ * (accessed by ZTS and ztest)
+ */
+#ifdef	_KERNEL
+static
+#endif	/* _KERNEL */
+unsigned long raidz_expand_max_reflow_bytes = 0;
+
+/*
+ * For testing only: pause the raidz expansion at a certain point.
+ */
+uint_t raidz_expand_pause_point = 0;
+
+/*
+ * Maximum amount of copy io's outstanding at once.
+ */
+static unsigned long raidz_expand_max_copy_bytes = 10 * SPA_MAXBLOCKSIZE;
+
+/*
+ * Apply raidz map abds aggregation if the number of rows in the map is equal
+ * or greater than the value below.
+ */
+static unsigned long raidz_io_aggregate_rows = 4;
+
+/*
+ * Automatically start a pool scrub when a RAIDZ expansion completes in
+ * order to verify the checksums of all blocks which have been copied
+ * during the expansion.  Automatic scrubbing is enabled by default and
+ * is strongly recommended.
+ */
+static int zfs_scrub_after_expand = 1;
+
 static void
 vdev_raidz_row_free(raidz_row_t *rr)
 {
@@ -159,6 +397,17 @@ vdev_raidz_map_free(raidz_map_t *rm)
 	for (int i = 0; i < rm->rm_nrows; i++)
 		vdev_raidz_row_free(rm->rm_row[i]);
 
+	if (rm->rm_nphys_cols) {
+		for (int i = 0; i < rm->rm_nphys_cols; i++) {
+			if (rm->rm_phys_col[i].rc_abd != NULL)
+				abd_free(rm->rm_phys_col[i].rc_abd);
+		}
+
+		kmem_free(rm->rm_phys_col, sizeof (raidz_col_t) *
+		    rm->rm_nphys_cols);
+	}
+
+	ASSERT3P(rm->rm_lr, ==, NULL);
 	kmem_free(rm, offsetof(raidz_map_t, rm_row[rm->rm_nrows]));
 }
 
@@ -170,10 +419,37 @@ vdev_raidz_map_free_vsd(zio_t *zio)
 	vdev_raidz_map_free(rm);
 }
 
+static int
+vdev_raidz_reflow_compare(const void *x1, const void *x2)
+{
+	const reflow_node_t *l = x1;
+	const reflow_node_t *r = x2;
+
+	return (TREE_CMP(l->re_txg, r->re_txg));
+}
+
 const zio_vsd_ops_t vdev_raidz_vsd_ops = {
 	.vsd_free = vdev_raidz_map_free_vsd,
 };
 
+raidz_row_t *
+vdev_raidz_row_alloc(int cols)
+{
+	raidz_row_t *rr =
+	    kmem_zalloc(offsetof(raidz_row_t, rr_col[cols]), KM_SLEEP);
+
+	rr->rr_cols = cols;
+	rr->rr_scols = cols;
+
+	for (int c = 0; c < cols; c++) {
+		raidz_col_t *rc = &rr->rr_col[c];
+		rc->rc_shadow_devidx = INT_MAX;
+		rc->rc_shadow_offset = UINT64_MAX;
+		rc->rc_allow_repair = 1;
+	}
+	return (rr);
+}
+
 static void
 vdev_raidz_map_alloc_write(zio_t *zio, raidz_map_t *rm, uint64_t ashift)
 {
@@ -302,7 +578,7 @@ vdev_raidz_map_alloc(zio_t *zio, uint64_t ashift, uint64_t dcols,
 	uint64_t f = b % dcols;
 	/* The starting byte offset on each child vdev. */
 	uint64_t o = (b / dcols) << ashift;
-	uint64_t q, r, c, bc, col, acols, scols, coff, devidx, asize, tot;
+	uint64_t acols, scols;
 
 	raidz_map_t *rm =
 	    kmem_zalloc(offsetof(raidz_map_t, rm_row[1]), KM_SLEEP);
@@ -312,22 +588,22 @@ vdev_raidz_map_alloc(zio_t *zio, uint64_t ashift, uint64_t dcols,
 	 * "Quotient": The number of data sectors for this stripe on all but
 	 * the "big column" child vdevs that also contain "remainder" data.
 	 */
-	q = s / (dcols - nparity);
+	uint64_t q = s / (dcols - nparity);
 
 	/*
 	 * "Remainder": The number of partial stripe data sectors in this I/O.
 	 * This will add a sector to some, but not all, child vdevs.
 	 */
-	r = s - q * (dcols - nparity);
+	uint64_t r = s - q * (dcols - nparity);
 
 	/* The number of "big columns" - those which contain remainder data. */
-	bc = (r == 0 ? 0 : r + nparity);
+	uint64_t bc = (r == 0 ? 0 : r + nparity);
 
 	/*
 	 * The total number of data and parity sectors associated with
 	 * this I/O.
 	 */
-	tot = s + nparity * (q + (r == 0 ? 0 : 1));
+	uint64_t tot = s + nparity * (q + (r == 0 ? 0 : 1));
 
 	/*
 	 * acols: The columns that will be accessed.
@@ -343,43 +619,28 @@ vdev_raidz_map_alloc(zio_t *zio, uint64_t ashift, uint64_t dcols,
 	}
 
 	ASSERT3U(acols, <=, scols);
-
-	rr = kmem_alloc(offsetof(raidz_row_t, rr_col[scols]), KM_SLEEP);
+	rr = vdev_raidz_row_alloc(scols);
 	rm->rm_row[0] = rr;
-
 	rr->rr_cols = acols;
-	rr->rr_scols = scols;
 	rr->rr_bigcols = bc;
-	rr->rr_missingdata = 0;
-	rr->rr_missingparity = 0;
 	rr->rr_firstdatacol = nparity;
-	rr->rr_abd_empty = NULL;
-	rr->rr_nempty = 0;
 #ifdef ZFS_DEBUG
 	rr->rr_offset = zio->io_offset;
 	rr->rr_size = zio->io_size;
 #endif
 
-	asize = 0;
+	uint64_t asize = 0;
 
-	for (c = 0; c < scols; c++) {
+	for (uint64_t c = 0; c < scols; c++) {
 		raidz_col_t *rc = &rr->rr_col[c];
-		col = f + c;
-		coff = o;
+		uint64_t col = f + c;
+		uint64_t coff = o;
 		if (col >= dcols) {
 			col -= dcols;
 			coff += 1ULL << ashift;
 		}
 		rc->rc_devidx = col;
 		rc->rc_offset = coff;
-		rc->rc_abd = NULL;
-		rc->rc_orig_data = NULL;
-		rc->rc_error = 0;
-		rc->rc_tried = 0;
-		rc->rc_skipped = 0;
-		rc->rc_force_repair = 0;
-		rc->rc_allow_repair = 1;
-		rc->rc_need_orig_restore = B_FALSE;
 
 		if (c >= acols)
 			rc->rc_size = 0;
@@ -419,13 +680,12 @@ vdev_raidz_map_alloc(zio_t *zio, uint64_t ashift, uint64_t dcols,
 	ASSERT(rr->rr_col[0].rc_size == rr->rr_col[1].rc_size);
 
 	if (rr->rr_firstdatacol == 1 && (zio->io_offset & (1ULL << 20))) {
-		devidx = rr->rr_col[0].rc_devidx;
+		uint64_t devidx = rr->rr_col[0].rc_devidx;
 		o = rr->rr_col[0].rc_offset;
 		rr->rr_col[0].rc_devidx = rr->rr_col[1].rc_devidx;
 		rr->rr_col[0].rc_offset = rr->rr_col[1].rc_offset;
 		rr->rr_col[1].rc_devidx = devidx;
 		rr->rr_col[1].rc_offset = o;
-
 		if (rm->rm_skipstart == 0)
 			rm->rm_skipstart = 1;
 	}
@@ -435,7 +695,338 @@ vdev_raidz_map_alloc(zio_t *zio, uint64_t ashift, uint64_t dcols,
 	} else {
 		vdev_raidz_map_alloc_read(zio, rm);
 	}
+	/* init RAIDZ parity ops */
+	rm->rm_ops = vdev_raidz_math_get_ops();
+
+	return (rm);
+}
+
+/*
+ * Everything before reflow_offset_synced should have been moved to the new
+ * location (read and write completed).  However, this may not yet be reflected
+ * in the on-disk format (e.g. raidz_reflow_sync() has been called but the
+ * uberblock has not yet been written). If reflow is not in progress,
+ * reflow_offset_synced should be UINT64_MAX. For each row, if the row is
+ * entirely before reflow_offset_synced, it will come from the new location.
+ * Otherwise this row will come from the old location.  Therefore, rows that
+ * straddle the reflow_offset_synced will come from the old location.
+ *
+ * For writes, reflow_offset_next is the next offset to copy.  If a sector has
+ * been copied, but not yet reflected in the on-disk progress
+ * (reflow_offset_synced), it will also be written to the new (already copied)
+ * offset.
+ */
+noinline raidz_map_t *
+vdev_raidz_map_alloc_expanded(zio_t *zio,
+    uint64_t ashift, uint64_t physical_cols, uint64_t logical_cols,
+    uint64_t nparity, uint64_t reflow_offset_synced,
+    uint64_t reflow_offset_next, boolean_t use_scratch)
+{
+	abd_t *abd = zio->io_abd;
+	uint64_t offset = zio->io_offset;
+	uint64_t size = zio->io_size;
+
+	/* The zio's size in units of the vdev's minimum sector size. */
+	uint64_t s = size >> ashift;
+
+	/*
+	 * "Quotient": The number of data sectors for this stripe on all but
+	 * the "big column" child vdevs that also contain "remainder" data.
+	 * AKA "full rows"
+	 */
+	uint64_t q = s / (logical_cols - nparity);
 
+	/*
+	 * "Remainder": The number of partial stripe data sectors in this I/O.
+	 * This will add a sector to some, but not all, child vdevs.
+	 */
+	uint64_t r = s - q * (logical_cols - nparity);
+
+	/* The number of "big columns" - those which contain remainder data. */
+	uint64_t bc = (r == 0 ? 0 : r + nparity);
+
+	/*
+	 * The total number of data and parity sectors associated with
+	 * this I/O.
+	 */
+	uint64_t tot = s + nparity * (q + (r == 0 ? 0 : 1));
+
+	/* How many rows contain data (not skip) */
+	uint64_t rows = howmany(tot, logical_cols);
+	int cols = MIN(tot, logical_cols);
+
+	raidz_map_t *rm =
+	    kmem_zalloc(offsetof(raidz_map_t, rm_row[rows]),
+	    KM_SLEEP);
+	rm->rm_nrows = rows;
+	rm->rm_nskip = roundup(tot, nparity + 1) - tot;
+	rm->rm_skipstart = bc;
+	uint64_t asize = 0;
+
+	for (uint64_t row = 0; row < rows; row++) {
+		boolean_t row_use_scratch = B_FALSE;
+		raidz_row_t *rr = vdev_raidz_row_alloc(cols);
+		rm->rm_row[row] = rr;
+
+		/* The starting RAIDZ (parent) vdev sector of the row. */
+		uint64_t b = (offset >> ashift) + row * logical_cols;
+
+		/*
+		 * If we are in the middle of a reflow, and the copying has
+		 * not yet completed for any part of this row, then use the
+		 * old location of this row.  Note that reflow_offset_synced
+		 * reflects the i/o that's been completed, because it's
+		 * updated by a synctask, after zio_wait(spa_txg_zio[]).
+		 * This is sufficient for our check, even if that progress
+		 * has not yet been recorded to disk (reflected in
+		 * spa_ubsync).  Also note that we consider the last row to
+		 * be "full width" (`cols`-wide rather than `bc`-wide) for
+		 * this calculation. This causes a tiny bit of unnecessary
+		 * double-writes but is safe and simpler to calculate.
+		 */
+		int row_phys_cols = physical_cols;
+		if (b + cols > reflow_offset_synced >> ashift)
+			row_phys_cols--;
+		else if (use_scratch)
+			row_use_scratch = B_TRUE;
+
+		/* starting child of this row */
+		uint64_t child_id = b % row_phys_cols;
+		/* The starting byte offset on each child vdev. */
+		uint64_t child_offset = (b / row_phys_cols) << ashift;
+
+		/*
+		 * Note, rr_cols is the entire width of the block, even
+		 * if this row is shorter.  This is needed because parity
+		 * generation (for Q and R) needs to know the entire width,
+		 * because it treats the short row as though it was
+		 * full-width (and the "phantom" sectors were zero-filled).
+		 *
+		 * Another approach to this would be to set cols shorter
+		 * (to just the number of columns that we might do i/o to)
+		 * and have another mechanism to tell the parity generation
+		 * about the "entire width".  Reconstruction (at least
+		 * vdev_raidz_reconstruct_general()) would also need to
+		 * know about the "entire width".
+		 */
+		rr->rr_firstdatacol = nparity;
+#ifdef ZFS_DEBUG
+		/*
+		 * note: rr_size is PSIZE, not ASIZE
+		 */
+		rr->rr_offset = b << ashift;
+		rr->rr_size = (rr->rr_cols - rr->rr_firstdatacol) << ashift;
+#endif
+
+		for (int c = 0; c < rr->rr_cols; c++, child_id++) {
+			if (child_id >= row_phys_cols) {
+				child_id -= row_phys_cols;
+				child_offset += 1ULL << ashift;
+			}
+			raidz_col_t *rc = &rr->rr_col[c];
+			rc->rc_devidx = child_id;
+			rc->rc_offset = child_offset;
+
+			/*
+			 * Get this from the scratch space if appropriate.
+			 * This only happens if we crashed in the middle of
+			 * raidz_reflow_scratch_sync() (while it's running,
+			 * the rangelock prevents us from doing concurrent
+			 * io), and even then only during zpool import or
+			 * when the pool is imported readonly.
+			 */
+			if (row_use_scratch)
+				rc->rc_offset -= VDEV_BOOT_SIZE;
+
+			uint64_t dc = c - rr->rr_firstdatacol;
+			if (c < rr->rr_firstdatacol) {
+				rc->rc_size = 1ULL << ashift;
+
+				/*
+				 * Parity sectors' rc_abd's are set below
+				 * after determining if this is an aggregation.
+				 */
+			} else if (row == rows - 1 && bc != 0 && c >= bc) {
+				/*
+				 * Past the end of the block (even including
+				 * skip sectors).  This sector is part of the
+				 * map so that we have full rows for p/q parity
+				 * generation.
+				 */
+				rc->rc_size = 0;
+				rc->rc_abd = NULL;
+			} else {
+				/* "data column" (col excluding parity) */
+				uint64_t off;
+
+				if (c < bc || r == 0) {
+					off = dc * rows + row;
+				} else {
+					off = r * rows +
+					    (dc - r) * (rows - 1) + row;
+				}
+				rc->rc_size = 1ULL << ashift;
+				rc->rc_abd = abd_get_offset_struct(
+				    &rc->rc_abdstruct, abd, off << ashift,
+				    rc->rc_size);
+			}
+
+			if (rc->rc_size == 0)
+				continue;
+
+			/*
+			 * If any part of this row is in both old and new
+			 * locations, the primary location is the old
+			 * location. If this sector was already copied to the
+			 * new location, we need to also write to the new,
+			 * "shadow" location.
+			 *
+			 * Note, `row_phys_cols != physical_cols` indicates
+			 * that the primary location is the old location.
+			 * `b+c < reflow_offset_next` indicates that the copy
+			 * to the new location has been initiated. We know
+			 * that the copy has completed because we have the
+			 * rangelock, which is held exclusively while the
+			 * copy is in progress.
+			 */
+			if (row_use_scratch ||
+			    (row_phys_cols != physical_cols &&
+			    b + c < reflow_offset_next >> ashift)) {
+				rc->rc_shadow_devidx = (b + c) % physical_cols;
+				rc->rc_shadow_offset =
+				    ((b + c) / physical_cols) << ashift;
+				if (row_use_scratch)
+					rc->rc_shadow_offset -= VDEV_BOOT_SIZE;
+			}
+
+			asize += rc->rc_size;
+		}
+
+		/*
+		 * See comment in vdev_raidz_map_alloc()
+		 */
+		if (rr->rr_firstdatacol == 1 && rr->rr_cols > 1 &&
+		    (offset & (1ULL << 20))) {
+			ASSERT(rr->rr_cols >= 2);
+			ASSERT(rr->rr_col[0].rc_size == rr->rr_col[1].rc_size);
+
+			int devidx0 = rr->rr_col[0].rc_devidx;
+			uint64_t offset0 = rr->rr_col[0].rc_offset;
+			int shadow_devidx0 = rr->rr_col[0].rc_shadow_devidx;
+			uint64_t shadow_offset0 =
+			    rr->rr_col[0].rc_shadow_offset;
+
+			rr->rr_col[0].rc_devidx = rr->rr_col[1].rc_devidx;
+			rr->rr_col[0].rc_offset = rr->rr_col[1].rc_offset;
+			rr->rr_col[0].rc_shadow_devidx =
+			    rr->rr_col[1].rc_shadow_devidx;
+			rr->rr_col[0].rc_shadow_offset =
+			    rr->rr_col[1].rc_shadow_offset;
+
+			rr->rr_col[1].rc_devidx = devidx0;
+			rr->rr_col[1].rc_offset = offset0;
+			rr->rr_col[1].rc_shadow_devidx = shadow_devidx0;
+			rr->rr_col[1].rc_shadow_offset = shadow_offset0;
+		}
+	}
+	ASSERT3U(asize, ==, tot << ashift);
+
+	/*
+	 * Determine if the block is contiguous, in which case we can use
+	 * an aggregation.
+	 */
+	if (rows >= raidz_io_aggregate_rows) {
+		rm->rm_nphys_cols = physical_cols;
+		rm->rm_phys_col =
+		    kmem_zalloc(sizeof (raidz_col_t) * rm->rm_nphys_cols,
+		    KM_SLEEP);
+
+		/*
+		 * Determine the aggregate io's offset and size, and check
+		 * that the io is contiguous.
+		 */
+		for (int i = 0;
+		    i < rm->rm_nrows && rm->rm_phys_col != NULL; i++) {
+			raidz_row_t *rr = rm->rm_row[i];
+			for (int c = 0; c < rr->rr_cols; c++) {
+				raidz_col_t *rc = &rr->rr_col[c];
+				raidz_col_t *prc =
+				    &rm->rm_phys_col[rc->rc_devidx];
+
+				if (rc->rc_size == 0)
+					continue;
+
+				if (prc->rc_size == 0) {
+					ASSERT0(prc->rc_offset);
+					prc->rc_offset = rc->rc_offset;
+				} else if (prc->rc_offset + prc->rc_size !=
+				    rc->rc_offset) {
+					/*
+					 * This block is not contiguous and
+					 * therefore can't be aggregated.
+					 * This is expected to be rare, so
+					 * the cost of allocating and then
+					 * freeing rm_phys_col is not
+					 * significant.
+					 */
+					kmem_free(rm->rm_phys_col,
+					    sizeof (raidz_col_t) *
+					    rm->rm_nphys_cols);
+					rm->rm_phys_col = NULL;
+					rm->rm_nphys_cols = 0;
+					break;
+				}
+				prc->rc_size += rc->rc_size;
+			}
+		}
+	}
+	if (rm->rm_phys_col != NULL) {
+		/*
+		 * Allocate aggregate ABD's.
+		 */
+		for (int i = 0; i < rm->rm_nphys_cols; i++) {
+			raidz_col_t *prc = &rm->rm_phys_col[i];
+
+			prc->rc_devidx = i;
+
+			if (prc->rc_size == 0)
+				continue;
+
+			prc->rc_abd =
+			    abd_alloc_linear(rm->rm_phys_col[i].rc_size,
+			    B_FALSE);
+		}
+
+		/*
+		 * Point the parity abd's into the aggregate abd's.
+		 */
+		for (int i = 0; i < rm->rm_nrows; i++) {
+			raidz_row_t *rr = rm->rm_row[i];
+			for (int c = 0; c < rr->rr_firstdatacol; c++) {
+				raidz_col_t *rc = &rr->rr_col[c];
+				raidz_col_t *prc =
+				    &rm->rm_phys_col[rc->rc_devidx];
+				rc->rc_abd =
+				    abd_get_offset_struct(&rc->rc_abdstruct,
+				    prc->rc_abd,
+				    rc->rc_offset - prc->rc_offset,
+				    rc->rc_size);
+			}
+		}
+	} else {
+		/*
+		 * Allocate new abd's for the parity sectors.
+		 */
+		for (int i = 0; i < rm->rm_nrows; i++) {
+			raidz_row_t *rr = rm->rm_row[i];
+			for (int c = 0; c < rr->rr_firstdatacol; c++) {
+				raidz_col_t *rc = &rr->rr_col[c];
+				rc->rc_abd =
+				    abd_alloc_linear(rc->rc_size,
+				    B_TRUE);
+			}
+		}
+	}
 	/* init RAIDZ parity ops */
 	rm->rm_ops = vdev_raidz_math_get_ops();
 
@@ -453,11 +1044,11 @@ vdev_raidz_p_func(void *buf, size_t size, void *private)
 {
 	struct pqr_struct *pqr = private;
 	const uint64_t *src = buf;
-	int i, cnt = size / sizeof (src[0]);
+	int cnt = size / sizeof (src[0]);
 
 	ASSERT(pqr->p && !pqr->q && !pqr->r);
 
-	for (i = 0; i < cnt; i++, src++, pqr->p++)
+	for (int i = 0; i < cnt; i++, src++, pqr->p++)
 		*pqr->p ^= *src;
 
 	return (0);
@@ -469,11 +1060,11 @@ vdev_raidz_pq_func(void *buf, size_t size, void *private)
 	struct pqr_struct *pqr = private;
 	const uint64_t *src = buf;
 	uint64_t mask;
-	int i, cnt = size / sizeof (src[0]);
+	int cnt = size / sizeof (src[0]);
 
 	ASSERT(pqr->p && pqr->q && !pqr->r);
 
-	for (i = 0; i < cnt; i++, src++, pqr->p++, pqr->q++) {
+	for (int i = 0; i < cnt; i++, src++, pqr->p++, pqr->q++) {
 		*pqr->p ^= *src;
 		VDEV_RAIDZ_64MUL_2(*pqr->q, mask);
 		*pqr->q ^= *src;
@@ -488,11 +1079,11 @@ vdev_raidz_pqr_func(void *buf, size_t size, void *private)
 	struct pqr_struct *pqr = private;
 	const uint64_t *src = buf;
 	uint64_t mask;
-	int i, cnt = size / sizeof (src[0]);
+	int cnt = size / sizeof (src[0]);
 
 	ASSERT(pqr->p && pqr->q && pqr->r);
 
-	for (i = 0; i < cnt; i++, src++, pqr->p++, pqr->q++, pqr->r++) {
+	for (int i = 0; i < cnt; i++, src++, pqr->p++, pqr->q++, pqr->r++) {
 		*pqr->p ^= *src;
 		VDEV_RAIDZ_64MUL_2(*pqr->q, mask);
 		*pqr->q ^= *src;
@@ -618,7 +1209,15 @@ vdev_raidz_generate_parity_pqr(raidz_row_t *rr)
 void
 vdev_raidz_generate_parity_row(raidz_map_t *rm, raidz_row_t *rr)
 {
-	ASSERT3U(rr->rr_cols, !=, 0);
+	if (rr->rr_cols == 0) {
+		/*
+		 * We are handling this block one row at a time (because
+		 * this block has a different logical vs physical width,
+		 * due to RAIDZ expansion), and this is a pad-only row,
+		 * which has no parity.
+		 */
+		return;
+	}
 
 	/* Generate using the new math implementation */
 	if (vdev_raidz_math_generate(rm, rr) != RAIDZ_ORIGINAL_IMPL)
@@ -770,6 +1369,9 @@ vdev_raidz_reconstruct_p(raidz_row_t *rr, int *tgts, int ntgts)
 	int x = tgts[0];
 	abd_t *dst, *src;
 
+	if (zfs_flags & ZFS_DEBUG_RAIDZ_RECONSTRUCT)
+		zfs_dbgmsg("reconstruct_p(rm=%px x=%u)", rr, x);
+
 	ASSERT3U(ntgts, ==, 1);
 	ASSERT3U(x, >=, rr->rr_firstdatacol);
 	ASSERT3U(x, <, rr->rr_cols);
@@ -802,6 +1404,9 @@ vdev_raidz_reconstruct_q(raidz_row_t *rr, int *tgts, int ntgts)
 	int c, exp;
 	abd_t *dst, *src;
 
+	if (zfs_flags & ZFS_DEBUG_RAIDZ_RECONSTRUCT)
+		zfs_dbgmsg("reconstruct_q(rm=%px x=%u)", rr, x);
+
 	ASSERT(ntgts == 1);
 
 	ASSERT(rr->rr_col[x].rc_size <= rr->rr_col[VDEV_RAIDZ_Q].rc_size);
@@ -848,6 +1453,9 @@ vdev_raidz_reconstruct_pq(raidz_row_t *rr, int *tgts, int ntgts)
 	int y = tgts[1];
 	abd_t *xd, *yd;
 
+	if (zfs_flags & ZFS_DEBUG_RAIDZ_RECONSTRUCT)
+		zfs_dbgmsg("reconstruct_pq(rm=%px x=%u y=%u)", rr, x, y);
+
 	ASSERT(ntgts == 2);
 	ASSERT(x < y);
 	ASSERT(x >= rr->rr_firstdatacol);
@@ -1295,11 +1903,14 @@ vdev_raidz_reconstruct_general(raidz_row_t *rr, int *tgts, int ntgts)
 
 	abd_t **bufs = NULL;
 
+	if (zfs_flags & ZFS_DEBUG_RAIDZ_RECONSTRUCT)
+		zfs_dbgmsg("reconstruct_general(rm=%px ntgts=%u)", rr, ntgts);
 	/*
 	 * Matrix reconstruction can't use scatter ABDs yet, so we allocate
 	 * temporary linear ABDs if any non-linear ABDs are found.
 	 */
 	for (i = rr->rr_firstdatacol; i < rr->rr_cols; i++) {
+		ASSERT(rr->rr_col[i].rc_abd != NULL);
 		if (!abd_is_linear(rr->rr_col[i].rc_abd)) {
 			bufs = kmem_alloc(rr->rr_cols * sizeof (abd_t *),
 			    KM_PUSHPAGE);
@@ -1427,10 +2038,23 @@ vdev_raidz_reconstruct_row(raidz_map_t *rm, raidz_row_t *rr,
 	int nbadparity, nbaddata;
 	int parity_valid[VDEV_RAIDZ_MAXPARITY];
 
+	if (zfs_flags & ZFS_DEBUG_RAIDZ_RECONSTRUCT) {
+		zfs_dbgmsg("reconstruct(rm=%px nt=%u cols=%u md=%u mp=%u)",
+		    rr, nt, (int)rr->rr_cols, (int)rr->rr_missingdata,
+		    (int)rr->rr_missingparity);
+	}
+
 	nbadparity = rr->rr_firstdatacol;
 	nbaddata = rr->rr_cols - nbadparity;
 	ntgts = 0;
 	for (i = 0, c = 0; c < rr->rr_cols; c++) {
+		if (zfs_flags & ZFS_DEBUG_RAIDZ_RECONSTRUCT) {
+			zfs_dbgmsg("reconstruct(rm=%px col=%u devid=%u "
+			    "offset=%llx error=%u)",
+			    rr, c, (int)rr->rr_col[c].rc_devidx,
+			    (long long)rr->rr_col[c].rc_offset,
+			    (int)rr->rr_col[c].rc_error);
+		}
 		if (c < rr->rr_firstdatacol)
 			parity_valid[c] = B_FALSE;
 
@@ -1537,8 +2161,15 @@ vdev_raidz_open(vdev_t *vd, uint64_t *asize, uint64_t *max_asize,
 		    *physical_ashift, cvd->vdev_physical_ashift);
 	}
 
-	*asize *= vd->vdev_children;
-	*max_asize *= vd->vdev_children;
+	if (vd->vdev_rz_expanding) {
+		*asize *= vd->vdev_children - 1;
+		*max_asize *= vd->vdev_children - 1;
+
+		vd->vdev_min_asize = *asize;
+	} else {
+		*asize *= vd->vdev_children;
+		*max_asize *= vd->vdev_children;
+	}
 
 	if (numerrors > nparity) {
 		vd->vdev_stat.vs_aux = VDEV_AUX_NO_REPLICAS;
@@ -1557,19 +2188,71 @@ vdev_raidz_close(vdev_t *vd)
 	}
 }
 
+/*
+ * Return the logical width to use, given the txg in which the allocation
+ * happened.  Note that BP_PHYSICAL_BIRTH() is usually the txg in which the
+ * BP was allocated.  Remapped BP's (that were relocated due to device
+ * removal, see remap_blkptr_cb()), will have a more recent
+ * BP_PHYSICAL_BIRTH() which reflects when the BP was relocated, but we can
+ * ignore these because they can't be on RAIDZ (device removal doesn't
+ * support RAIDZ).
+ */
+static uint64_t
+vdev_raidz_get_logical_width(vdev_raidz_t *vdrz, uint64_t txg)
+{
+	reflow_node_t lookup = {
+		.re_txg = txg,
+	};
+	avl_index_t where;
+
+	uint64_t width;
+	mutex_enter(&vdrz->vd_expand_lock);
+	reflow_node_t *re = avl_find(&vdrz->vd_expand_txgs, &lookup, &where);
+	if (re != NULL) {
+		width = re->re_logical_width;
+	} else {
+		re = avl_nearest(&vdrz->vd_expand_txgs, where, AVL_BEFORE);
+		if (re != NULL)
+			width = re->re_logical_width;
+		else
+			width = vdrz->vd_original_width;
+	}
+	mutex_exit(&vdrz->vd_expand_lock);
+	return (width);
+}
+
+/*
+ * Note: If the RAIDZ vdev has been expanded, older BP's may have allocated
+ * more space due to the lower data-to-parity ratio.  In this case it's
+ * important to pass in the correct txg.  Note that vdev_gang_header_asize()
+ * relies on a constant asize for psize=SPA_GANGBLOCKSIZE=SPA_MINBLOCKSIZE,
+ * regardless of txg.  This is assured because for a single data sector, we
+ * allocate P+1 sectors regardless of width ("cols", which is at least P+1).
+ */
 static uint64_t
-vdev_raidz_asize(vdev_t *vd, uint64_t psize)
+vdev_raidz_asize(vdev_t *vd, uint64_t psize, uint64_t txg)
 {
 	vdev_raidz_t *vdrz = vd->vdev_tsd;
 	uint64_t asize;
 	uint64_t ashift = vd->vdev_top->vdev_ashift;
-	uint64_t cols = vdrz->vd_logical_width;
+	uint64_t cols = vdrz->vd_original_width;
 	uint64_t nparity = vdrz->vd_nparity;
 
+	cols = vdev_raidz_get_logical_width(vdrz, txg);
+
 	asize = ((psize - 1) >> ashift) + 1;
 	asize += nparity * ((asize + cols - nparity - 1) / (cols - nparity));
 	asize = roundup(asize, nparity + 1) << ashift;
 
+#ifdef ZFS_DEBUG
+	uint64_t asize_new = ((psize - 1) >> ashift) + 1;
+	uint64_t ncols_new = vdrz->vd_physical_width;
+	asize_new += nparity * ((asize_new + ncols_new - nparity - 1) /
+	    (ncols_new - nparity));
+	asize_new = roundup(asize_new, nparity + 1) << ashift;
+	VERIFY3U(asize_new, <=, asize);
+#endif
+
 	return (asize);
 }
 
@@ -1596,21 +2279,37 @@ vdev_raidz_child_done(zio_t *zio)
 }
 
 static void
-vdev_raidz_io_verify(vdev_t *vd, raidz_row_t *rr, int col)
+vdev_raidz_shadow_child_done(zio_t *zio)
 {
-#ifdef ZFS_DEBUG
-	vdev_t *tvd = vd->vdev_top;
+	raidz_col_t *rc = zio->io_private;
+
+	rc->rc_shadow_error = zio->io_error;
+}
 
+static void
+vdev_raidz_io_verify(zio_t *zio, raidz_map_t *rm, raidz_row_t *rr, int col)
+{
+	(void) rm;
+#ifdef ZFS_DEBUG
 	range_seg64_t logical_rs, physical_rs, remain_rs;
 	logical_rs.rs_start = rr->rr_offset;
 	logical_rs.rs_end = logical_rs.rs_start +
-	    vdev_raidz_asize(vd, rr->rr_size);
+	    vdev_raidz_asize(zio->io_vd, rr->rr_size,
+	    BP_PHYSICAL_BIRTH(zio->io_bp));
 
 	raidz_col_t *rc = &rr->rr_col[col];
-	vdev_t *cvd = vd->vdev_child[rc->rc_devidx];
+	vdev_t *cvd = zio->io_vd->vdev_child[rc->rc_devidx];
 
 	vdev_xlate(cvd, &logical_rs, &physical_rs, &remain_rs);
 	ASSERT(vdev_xlate_is_empty(&remain_rs));
+	if (vdev_xlate_is_empty(&physical_rs)) {
+		/*
+		 * If we are in the middle of expansion, the
+		 * physical->logical mapping is changing so vdev_xlate()
+		 * can't give us a reliable answer.
+		 */
+		return;
+	}
 	ASSERT3U(rc->rc_offset, ==, physical_rs.rs_start);
 	ASSERT3U(rc->rc_offset, <, physical_rs.rs_end);
 	/*
@@ -1621,7 +2320,7 @@ vdev_raidz_io_verify(vdev_t *vd, raidz_row_t *rr, int col)
 	 */
 	if (physical_rs.rs_end > rc->rc_offset + rc->rc_size) {
 		ASSERT3U(physical_rs.rs_end, ==, rc->rc_offset +
-		    rc->rc_size + (1 << tvd->vdev_ashift));
+		    rc->rc_size + (1 << zio->io_vd->vdev_top->vdev_ashift));
 	} else {
 		ASSERT3U(physical_rs.rs_end, ==, rc->rc_offset + rc->rc_size);
 	}
@@ -1629,7 +2328,7 @@ vdev_raidz_io_verify(vdev_t *vd, raidz_row_t *rr, int col)
 }
 
 static void
-vdev_raidz_io_start_write(zio_t *zio, raidz_row_t *rr, uint64_t ashift)
+vdev_raidz_io_start_write(zio_t *zio, raidz_row_t *rr)
 {
 	vdev_t *vd = zio->io_vd;
 	raidz_map_t *rm = zio->io_vsd;
@@ -1641,31 +2340,66 @@ vdev_raidz_io_start_write(zio_t *zio, raidz_row_t *rr, uint64_t ashift)
 		vdev_t *cvd = vd->vdev_child[rc->rc_devidx];
 
 		/* Verify physical to logical translation */
-		vdev_raidz_io_verify(vd, rr, c);
+		vdev_raidz_io_verify(zio, rm, rr, c);
 
-		if (rc->rc_size > 0) {
-			ASSERT3P(rc->rc_abd, !=, NULL);
-			zio_nowait(zio_vdev_child_io(zio, NULL, cvd,
-			    rc->rc_offset, rc->rc_abd,
-			    abd_get_size(rc->rc_abd), zio->io_type,
-			    zio->io_priority, 0, vdev_raidz_child_done, rc));
-		} else {
-			/*
-			 * Generate optional write for skip sector to improve
-			 * aggregation contiguity.
-			 */
-			ASSERT3P(rc->rc_abd, ==, NULL);
-			zio_nowait(zio_vdev_child_io(zio, NULL, cvd,
-			    rc->rc_offset, NULL, 1ULL << ashift,
-			    zio->io_type, zio->io_priority,
-			    ZIO_FLAG_NODATA | ZIO_FLAG_OPTIONAL, NULL,
-			    NULL));
+		if (rc->rc_size == 0)
+			continue;
+
+		ASSERT3U(rc->rc_offset + rc->rc_size, <,
+		    cvd->vdev_psize - VDEV_LABEL_END_SIZE);
+
+		ASSERT3P(rc->rc_abd, !=, NULL);
+		zio_nowait(zio_vdev_child_io(zio, NULL, cvd,
+		    rc->rc_offset, rc->rc_abd,
+		    abd_get_size(rc->rc_abd), zio->io_type,
+		    zio->io_priority, 0, vdev_raidz_child_done, rc));
+
+		if (rc->rc_shadow_devidx != INT_MAX) {
+			vdev_t *cvd2 = vd->vdev_child[rc->rc_shadow_devidx];
+
+			ASSERT3U(
+			    rc->rc_shadow_offset + abd_get_size(rc->rc_abd), <,
+			    cvd2->vdev_psize - VDEV_LABEL_END_SIZE);
+
+			zio_nowait(zio_vdev_child_io(zio, NULL, cvd2,
+			    rc->rc_shadow_offset, rc->rc_abd,
+			    abd_get_size(rc->rc_abd),
+			    zio->io_type, zio->io_priority, 0,
+			    vdev_raidz_shadow_child_done, rc));
 		}
 	}
 }
 
+/*
+ * Generate optional I/Os for skip sectors to improve aggregation contiguity.
+ * This only works for vdev_raidz_map_alloc() (not _expanded()).
+ */
+static void
+raidz_start_skip_writes(zio_t *zio)
+{
+	vdev_t *vd = zio->io_vd;
+	uint64_t ashift = vd->vdev_top->vdev_ashift;
+	raidz_map_t *rm = zio->io_vsd;
+	ASSERT3U(rm->rm_nrows, ==, 1);
+	raidz_row_t *rr = rm->rm_row[0];
+	for (int c = 0; c < rr->rr_scols; c++) {
+		raidz_col_t *rc = &rr->rr_col[c];
+		vdev_t *cvd = vd->vdev_child[rc->rc_devidx];
+		if (rc->rc_size != 0)
+			continue;
+		ASSERT3P(rc->rc_abd, ==, NULL);
+
+		ASSERT3U(rc->rc_offset, <,
+		    cvd->vdev_psize - VDEV_LABEL_END_SIZE);
+
+		zio_nowait(zio_vdev_child_io(zio, NULL, cvd, rc->rc_offset,
+		    NULL, 1ULL << ashift, zio->io_type, zio->io_priority,
+		    ZIO_FLAG_NODATA | ZIO_FLAG_OPTIONAL, NULL, NULL));
+	}
+}
+
 static void
-vdev_raidz_io_start_read(zio_t *zio, raidz_row_t *rr)
+vdev_raidz_io_start_read_row(zio_t *zio, raidz_row_t *rr, boolean_t forceparity)
 {
 	vdev_t *vd = zio->io_vd;
 
@@ -1697,7 +2431,8 @@ vdev_raidz_io_start_read(zio_t *zio, raidz_row_t *rr)
 			rc->rc_skipped = 1;
 			continue;
 		}
-		if (c >= rr->rr_firstdatacol || rr->rr_missingdata > 0 ||
+		if (forceparity ||
+		    c >= rr->rr_firstdatacol || rr->rr_missingdata > 0 ||
 		    (zio->io_flags & (ZIO_FLAG_SCRUB | ZIO_FLAG_RESILVER))) {
 			zio_nowait(zio_vdev_child_io(zio, NULL, cvd,
 			    rc->rc_offset, rc->rc_abd, rc->rc_size,
@@ -1707,6 +2442,56 @@ vdev_raidz_io_start_read(zio_t *zio, raidz_row_t *rr)
 	}
 }
 
+static void
+vdev_raidz_io_start_read_phys_cols(zio_t *zio, raidz_map_t *rm)
+{
+	vdev_t *vd = zio->io_vd;
+
+	for (int i = 0; i < rm->rm_nphys_cols; i++) {
+		raidz_col_t *prc = &rm->rm_phys_col[i];
+		if (prc->rc_size == 0)
+			continue;
+
+		ASSERT3U(prc->rc_devidx, ==, i);
+		vdev_t *cvd = vd->vdev_child[i];
+		if (!vdev_readable(cvd)) {
+			prc->rc_error = SET_ERROR(ENXIO);
+			prc->rc_tried = 1;	/* don't even try */
+			prc->rc_skipped = 1;
+			continue;
+		}
+		if (vdev_dtl_contains(cvd, DTL_MISSING, zio->io_txg, 1)) {
+			prc->rc_error = SET_ERROR(ESTALE);
+			prc->rc_skipped = 1;
+			continue;
+		}
+		zio_nowait(zio_vdev_child_io(zio, NULL, cvd,
+		    prc->rc_offset, prc->rc_abd, prc->rc_size,
+		    zio->io_type, zio->io_priority, 0,
+		    vdev_raidz_child_done, prc));
+	}
+}
+
+static void
+vdev_raidz_io_start_read(zio_t *zio, raidz_map_t *rm)
+{
+	/*
+	 * If there are multiple rows, we will be hitting
+	 * all disks, so go ahead and read the parity so
+	 * that we are reading in decent size chunks.
+	 */
+	boolean_t forceparity = rm->rm_nrows > 1;
+
+	if (rm->rm_phys_col) {
+		vdev_raidz_io_start_read_phys_cols(zio, rm);
+	} else {
+		for (int i = 0; i < rm->rm_nrows; i++) {
+			raidz_row_t *rr = rm->rm_row[i];
+			vdev_raidz_io_start_read_row(zio, rr, forceparity);
+		}
+	}
+}
+
 /*
  * Start an IO operation on a RAIDZ VDev
  *
@@ -1730,24 +2515,83 @@ vdev_raidz_io_start(zio_t *zio)
 	vdev_t *vd = zio->io_vd;
 	vdev_t *tvd = vd->vdev_top;
 	vdev_raidz_t *vdrz = vd->vdev_tsd;
+	raidz_map_t *rm;
+
+	uint64_t logical_width = vdev_raidz_get_logical_width(vdrz,
+	    BP_PHYSICAL_BIRTH(zio->io_bp));
+	if (logical_width != vdrz->vd_physical_width) {
+		zfs_locked_range_t *lr = NULL;
+		uint64_t synced_offset = UINT64_MAX;
+		uint64_t next_offset = UINT64_MAX;
+		boolean_t use_scratch = B_FALSE;
+		/*
+		 * Note: when the expansion is completing, we set
+		 * vre_state=DSS_FINISHED (in raidz_reflow_complete_sync())
+		 * in a later txg than when we last update spa_ubsync's state
+		 * (see the end of spa_raidz_expand_thread()).  Therefore we
+		 * may see vre_state!=SCANNING before
+		 * VDEV_TOP_ZAP_RAIDZ_EXPAND_STATE=DSS_FINISHED is reflected
+		 * on disk, but the copying progress has been synced to disk
+		 * (and reflected in spa_ubsync).  In this case it's fine to
+		 * treat the expansion as completed, since if we crash there's
+		 * no additional copying to do.
+		 */
+		if (vdrz->vn_vre.vre_state == DSS_SCANNING) {
+			ASSERT3P(vd->vdev_spa->spa_raidz_expand, ==,
+			    &vdrz->vn_vre);
+			lr = zfs_rangelock_enter(&vdrz->vn_vre.vre_rangelock,
+			    zio->io_offset, zio->io_size, RL_READER);
+			use_scratch =
+			    (RRSS_GET_STATE(&vd->vdev_spa->spa_ubsync) ==
+			    RRSS_SCRATCH_VALID);
+			synced_offset =
+			    RRSS_GET_OFFSET(&vd->vdev_spa->spa_ubsync);
+			next_offset = vdrz->vn_vre.vre_offset;
+			/*
+			 * If we haven't resumed expanding since importing the
+			 * pool, vre_offset won't have been set yet.  In
+			 * this case the next offset to be copied is the same
+			 * as what was synced.
+			 */
+			if (next_offset == UINT64_MAX) {
+				next_offset = synced_offset;
+			}
+		}
+		if (use_scratch) {
+			zfs_dbgmsg("zio=%px %s io_offset=%llu offset_synced="
+			    "%lld next_offset=%lld use_scratch=%u",
+			    zio,
+			    zio->io_type == ZIO_TYPE_WRITE ? "WRITE" : "READ",
+			    (long long)zio->io_offset,
+			    (long long)synced_offset,
+			    (long long)next_offset,
+			    use_scratch);
+		}
+
+		rm = vdev_raidz_map_alloc_expanded(zio,
+		    tvd->vdev_ashift, vdrz->vd_physical_width,
+		    logical_width, vdrz->vd_nparity,
+		    synced_offset, next_offset, use_scratch);
+		rm->rm_lr = lr;
+	} else {
+		rm = vdev_raidz_map_alloc(zio,
+		    tvd->vdev_ashift, logical_width, vdrz->vd_nparity);
+	}
+	rm->rm_original_width = vdrz->vd_original_width;
 
-	raidz_map_t *rm = vdev_raidz_map_alloc(zio, tvd->vdev_ashift,
-	    vdrz->vd_logical_width, vdrz->vd_nparity);
 	zio->io_vsd = rm;
 	zio->io_vsd_ops = &vdev_raidz_vsd_ops;
-
-	/*
-	 * Until raidz expansion is implemented all maps for a raidz vdev
-	 * contain a single row.
-	 */
-	ASSERT3U(rm->rm_nrows, ==, 1);
-	raidz_row_t *rr = rm->rm_row[0];
-
 	if (zio->io_type == ZIO_TYPE_WRITE) {
-		vdev_raidz_io_start_write(zio, rr, tvd->vdev_ashift);
+		for (int i = 0; i < rm->rm_nrows; i++) {
+			vdev_raidz_io_start_write(zio, rm->rm_row[i]);
+		}
+
+		if (logical_width == vdrz->vd_physical_width) {
+			raidz_start_skip_writes(zio);
+		}
 	} else {
 		ASSERT(zio->io_type == ZIO_TYPE_READ);
-		vdev_raidz_io_start_read(zio, rr);
+		vdev_raidz_io_start_read(zio, rm);
 	}
 
 	zio_execute(zio);
@@ -1847,6 +2691,8 @@ raidz_parity_verify(zio_t *zio, raidz_row_t *rr)
 			continue;
 
 		if (abd_cmp(orig[c], rc->rc_abd) != 0) {
+			zfs_dbgmsg("found error on col=%u devidx=%u off %llx",
+			    c, (int)rc->rc_devidx, (u_longlong_t)rc->rc_offset);
 			vdev_raidz_checksum_error(zio, rc, orig[c]);
 			rc->rc_error = SET_ERROR(ECKSUM);
 			ret++;
@@ -1862,8 +2708,10 @@ vdev_raidz_worst_error(raidz_row_t *rr)
 {
 	int error = 0;
 
-	for (int c = 0; c < rr->rr_cols; c++)
+	for (int c = 0; c < rr->rr_cols; c++) {
 		error = zio_worst_error(error, rr->rr_col[c].rc_error);
+		error = zio_worst_error(error, rr->rr_col[c].rc_shadow_error);
+	}
 
 	return (error);
 }
@@ -1929,6 +2777,10 @@ vdev_raidz_io_done_verified(zio_t *zio, raidz_row_t *rr)
 				continue;
 			}
 
+			zfs_dbgmsg("zio=%px repairing c=%u devidx=%u "
+			    "offset=%llx",
+			    zio, c, rc->rc_devidx, (long long)rc->rc_offset);
+
 			zio_nowait(zio_vdev_child_io(zio, NULL, cvd,
 			    rc->rc_offset, rc->rc_abd, rc->rc_size,
 			    ZIO_TYPE_WRITE,
@@ -1938,6 +2790,42 @@ vdev_raidz_io_done_verified(zio_t *zio, raidz_row_t *rr)
 			    ZIO_FLAG_SELF_HEAL : 0), NULL, NULL));
 		}
 	}
+
+	/*
+	 * Scrub or resilver i/o's: overwrite any shadow locations with the
+	 * good data.  This ensures that if we've already copied this sector,
+	 * it will be corrected if it was damaged.  This writes more than is
+	 * necessary, but since expansion is paused during scrub/resilver, at
+	 * most a single row will have a shadow location.
+	 */
+	if (zio->io_error == 0 && spa_writeable(zio->io_spa) &&
+	    (zio->io_flags & (ZIO_FLAG_RESILVER | ZIO_FLAG_SCRUB))) {
+		for (int c = 0; c < rr->rr_cols; c++) {
+			raidz_col_t *rc = &rr->rr_col[c];
+			vdev_t *vd = zio->io_vd;
+
+			if (rc->rc_shadow_devidx == INT_MAX || rc->rc_size == 0)
+				continue;
+			vdev_t *cvd = vd->vdev_child[rc->rc_shadow_devidx];
+
+			/*
+			 * Note: We don't want to update the repair stats
+			 * because that would incorrectly indicate that there
+			 * was bad data to repair, which we aren't sure about.
+			 * By clearing the SCAN_THREAD flag, we prevent this
+			 * from happening, despite having the REPAIR flag set.
+			 * We need to set SELF_HEAL so that this i/o can't be
+			 * bypassed by zio_vdev_io_start().
+			 */
+			zio_t *cio = zio_vdev_child_io(zio, NULL, cvd,
+			    rc->rc_shadow_offset, rc->rc_abd, rc->rc_size,
+			    ZIO_TYPE_WRITE, ZIO_PRIORITY_ASYNC_WRITE,
+			    ZIO_FLAG_IO_REPAIR | ZIO_FLAG_SELF_HEAL,
+			    NULL, NULL);
+			cio->io_flags &= ~ZIO_FLAG_SCAN_THREAD;
+			zio_nowait(cio);
+		}
+	}
 }
 
 static void
@@ -1957,6 +2845,43 @@ raidz_restore_orig_data(raidz_map_t *rm)
 }
 
 /*
+ * During raidz_reconstruct() for expanded VDEV, we need special consideration
+ * failure simulations.  See note in raidz_reconstruct() on simulating failure
+ * of a pre-expansion device.
+ *
+ * Treating logical child i as failed, return TRUE if the given column should
+ * be treated as failed.  The idea of logical children allows us to imagine
+ * that a disk silently failed before a RAIDZ expansion (reads from this disk
+ * succeed but return the wrong data).  Since the expansion doesn't verify
+ * checksums, the incorrect data will be moved to new locations spread among
+ * the children (going diagonally across them).
+ *
+ * Higher "logical child failures" (values of `i`) indicate these
+ * "pre-expansion failures".  The first physical_width values imagine that a
+ * current child failed; the next physical_width-1 values imagine that a
+ * child failed before the most recent expansion; the next physical_width-2
+ * values imagine a child failed in the expansion before that, etc.
+ */
+static boolean_t
+raidz_simulate_failure(int physical_width, int original_width, int ashift,
+    int i, raidz_col_t *rc)
+{
+	uint64_t sector_id =
+	    physical_width * (rc->rc_offset >> ashift) +
+	    rc->rc_devidx;
+
+	for (int w = physical_width; w >= original_width; w--) {
+		if (i < w) {
+			return (sector_id % w == i);
+		} else {
+			i -= w;
+		}
+	}
+	ASSERT(!"invalid logical child id");
+	return (B_FALSE);
+}
+
+/*
  * returns EINVAL if reconstruction of the block will not be possible
  * returns ECKSUM if this specific reconstruction failed
  * returns 0 on successful reconstruction
@@ -1965,6 +2890,15 @@ static int
 raidz_reconstruct(zio_t *zio, int *ltgts, int ntgts, int nparity)
 {
 	raidz_map_t *rm = zio->io_vsd;
+	int physical_width = zio->io_vd->vdev_children;
+	int original_width = (rm->rm_original_width != 0) ?
+	    rm->rm_original_width : physical_width;
+	int dbgmsg = zfs_flags & ZFS_DEBUG_RAIDZ_RECONSTRUCT;
+
+	if (dbgmsg) {
+		zfs_dbgmsg("raidz_reconstruct_expanded(zio=%px ltgts=%u,%u,%u "
+		    "ntgts=%u", zio, ltgts[0], ltgts[1], ltgts[2], ntgts);
+	}
 
 	/* Reconstruct each row */
 	for (int r = 0; r < rm->rm_nrows; r++) {
@@ -1974,6 +2908,9 @@ raidz_reconstruct(zio_t *zio, int *ltgts, int ntgts, int nparity)
 		int dead = 0;
 		int dead_data = 0;
 
+		if (dbgmsg)
+			zfs_dbgmsg("raidz_reconstruct_expanded(row=%u)", r);
+
 		for (int c = 0; c < rr->rr_cols; c++) {
 			raidz_col_t *rc = &rr->rr_col[c];
 			ASSERT0(rc->rc_need_orig_restore);
@@ -1986,7 +2923,10 @@ raidz_reconstruct(zio_t *zio, int *ltgts, int ntgts, int nparity)
 			if (rc->rc_size == 0)
 				continue;
 			for (int lt = 0; lt < ntgts; lt++) {
-				if (rc->rc_devidx == ltgts[lt]) {
+				if (raidz_simulate_failure(physical_width,
+				    original_width,
+				    zio->io_vd->vdev_top->vdev_ashift,
+				    ltgts[lt], rc)) {
 					if (rc->rc_orig_data == NULL) {
 						rc->rc_orig_data =
 						    abd_alloc_linear(
@@ -1999,13 +2939,37 @@ raidz_reconstruct(zio_t *zio, int *ltgts, int ntgts, int nparity)
 					dead++;
 					if (c >= nparity)
 						dead_data++;
-					my_tgts[t++] = c;
+					/*
+					 * Note: simulating failure of a
+					 * pre-expansion device can hit more
+					 * than one column, in which case we
+					 * might try to simulate more failures
+					 * than can be reconstructed, which is
+					 * also more than the size of my_tgts.
+					 * This check prevents accessing past
+					 * the end of my_tgts.  The "dead >
+					 * nparity" check below will fail this
+					 * reconstruction attempt.
+					 */
+					if (t < VDEV_RAIDZ_MAXPARITY) {
+						my_tgts[t++] = c;
+						if (dbgmsg) {
+							zfs_dbgmsg("simulating "
+							    "failure of col %u "
+							    "devidx %u", c,
+							    (int)rc->rc_devidx);
+						}
+					}
 					break;
 				}
 			}
 		}
 		if (dead > nparity) {
 			/* reconstruction not possible */
+			if (dbgmsg) {
+				zfs_dbgmsg("reconstruction not possible; "
+				    "too many failures");
+			}
 			raidz_restore_orig_data(rm);
 			return (EINVAL);
 		}
@@ -2049,11 +3013,19 @@ raidz_reconstruct(zio_t *zio, int *ltgts, int ntgts, int nparity)
 
 		zio_checksum_verified(zio);
 
+		if (dbgmsg) {
+			zfs_dbgmsg("reconstruction successful "
+			    "(checksum verified)");
+		}
 		return (0);
 	}
 
 	/* Reconstruction failed - restore original data */
 	raidz_restore_orig_data(rm);
+	if (dbgmsg) {
+		zfs_dbgmsg("raidz_reconstruct_expanded(zio=%px) checksum "
+		    "failed", zio);
+	}
 	return (ECKSUM);
 }
 
@@ -2068,7 +3040,7 @@ raidz_reconstruct(zio_t *zio, int *ltgts, int ntgts, int nparity)
  * The order that we find the various possible combinations of failed
  * disks is dictated by these rules:
  * - Examine each "slot" (the "i" in tgts[i])
- *   - Try to increment this slot (tgts[i] = tgts[i] + 1)
+ *   - Try to increment this slot (tgts[i] += 1)
  *   - if we can't increment because it runs into the next slot,
  *     reset our slot to the minimum, and examine the next slot
  *
@@ -2099,18 +3071,22 @@ raidz_reconstruct(zio_t *zio, int *ltgts, int ntgts, int nparity)
  *
  * This strategy works for dRAID but is less efficient when there are a large
  * number of child vdevs and therefore permutations to check. Furthermore,
- * since the raidz_map_t rows likely do not overlap reconstruction would be
+ * since the raidz_map_t rows likely do not overlap, reconstruction would be
  * possible as long as there are no more than nparity data errors per row.
  * These additional permutations are not currently checked but could be as
  * a future improvement.
+ *
+ * Returns 0 on success, ECKSUM on failure.
  */
 static int
 vdev_raidz_combrec(zio_t *zio)
 {
 	int nparity = vdev_get_nparity(zio->io_vd);
 	raidz_map_t *rm = zio->io_vsd;
+	int physical_width = zio->io_vd->vdev_children;
+	int original_width = (rm->rm_original_width != 0) ?
+	    rm->rm_original_width : physical_width;
 
-	/* Check if there's enough data to attempt reconstrution. */
 	for (int i = 0; i < rm->rm_nrows; i++) {
 		raidz_row_t *rr = rm->rm_row[i];
 		int total_errors = 0;
@@ -2128,8 +3104,16 @@ vdev_raidz_combrec(zio_t *zio)
 		int tstore[VDEV_RAIDZ_MAXPARITY + 2];
 		int *ltgts = &tstore[1]; /* value is logical child ID */
 
-		/* Determine number of logical children, n */
-		int n = zio->io_vd->vdev_children;
+
+		/*
+		 * Determine number of logical children, n.  See comment
+		 * above raidz_simulate_failure().
+		 */
+		int n = 0;
+		for (int w = physical_width;
+		    w >= original_width; w--) {
+			n += w;
+		}
 
 		ASSERT3U(num_failures, <=, nparity);
 		ASSERT3U(num_failures, <=, VDEV_RAIDZ_MAXPARITY);
@@ -2160,6 +3144,14 @@ vdev_raidz_combrec(zio_t *zio)
 				if (ltgts[t] == n) {
 					/* try more failures */
 					ASSERT3U(t, ==, num_failures - 1);
+					if (zfs_flags &
+					    ZFS_DEBUG_RAIDZ_RECONSTRUCT) {
+						zfs_dbgmsg("reconstruction "
+						    "failed for num_failures="
+						    "%u; tried all "
+						    "combinations",
+						    num_failures);
+					}
 					break;
 				}
 
@@ -2171,7 +3163,7 @@ vdev_raidz_combrec(zio_t *zio)
 				 * Try the next combination.
 				 */
 				if (ltgts[t] != ltgts[t + 1])
-					break;
+					break; // found next combination
 
 				/*
 				 * Otherwise, reset this tgt to the minimum,
@@ -2186,7 +3178,8 @@ vdev_raidz_combrec(zio_t *zio)
 				break;
 		}
 	}
-
+	if (zfs_flags & ZFS_DEBUG_RAIDZ_RECONSTRUCT)
+		zfs_dbgmsg("reconstruction failed for all num_failures");
 	return (ECKSUM);
 }
 
@@ -2211,7 +3204,8 @@ vdev_raidz_reconstruct(raidz_map_t *rm, const int *t, int nt)
 static void
 vdev_raidz_io_done_write_impl(zio_t *zio, raidz_row_t *rr)
 {
-	int total_errors = 0;
+	int normal_errors = 0;
+	int shadow_errors = 0;
 
 	ASSERT3U(rr->rr_missingparity, <=, rr->rr_firstdatacol);
 	ASSERT3U(rr->rr_missingdata, <=, rr->rr_cols - rr->rr_firstdatacol);
@@ -2220,24 +3214,31 @@ vdev_raidz_io_done_write_impl(zio_t *zio, raidz_row_t *rr)
 	for (int c = 0; c < rr->rr_cols; c++) {
 		raidz_col_t *rc = &rr->rr_col[c];
 
-		if (rc->rc_error) {
+		if (rc->rc_error != 0) {
 			ASSERT(rc->rc_error != ECKSUM);	/* child has no bp */
-
-			total_errors++;
+			normal_errors++;
+		}
+		if (rc->rc_shadow_error != 0) {
+			ASSERT(rc->rc_shadow_error != ECKSUM);
+			shadow_errors++;
 		}
 	}
 
 	/*
 	 * Treat partial writes as a success. If we couldn't write enough
-	 * columns to reconstruct the data, the I/O failed.  Otherwise,
-	 * good enough.
+	 * columns to reconstruct the data, the I/O failed.  Otherwise, good
+	 * enough.  Note that in the case of a shadow write (during raidz
+	 * expansion), depending on if we crash, either the normal (old) or
+	 * shadow (new) location may become the "real" version of the block,
+	 * so both locations must have sufficient redundancy.
 	 *
 	 * Now that we support write reallocation, it would be better
 	 * to treat partial failure as real failure unless there are
 	 * no non-degraded top-level vdevs left, and not update DTLs
 	 * if we intend to reallocate.
 	 */
-	if (total_errors > rr->rr_firstdatacol) {
+	if (normal_errors > rr->rr_firstdatacol ||
+	    shadow_errors > rr->rr_firstdatacol) {
 		zio->io_error = zio_worst_error(zio->io_error,
 		    vdev_raidz_worst_error(rr));
 	}
@@ -2254,7 +3255,6 @@ vdev_raidz_io_done_reconstruct_known_missing(zio_t *zio, raidz_map_t *rm,
 
 	ASSERT3U(rr->rr_missingparity, <=, rr->rr_firstdatacol);
 	ASSERT3U(rr->rr_missingdata, <=, rr->rr_cols - rr->rr_firstdatacol);
-	ASSERT3U(zio->io_type, ==, ZIO_TYPE_READ);
 
 	for (int c = 0; c < rr->rr_cols; c++) {
 		raidz_col_t *rc = &rr->rr_col[c];
@@ -2337,7 +3337,7 @@ vdev_raidz_read_all(zio_t *zio, raidz_row_t *rr)
 	 * for a normal read then allocate an ABD for them now so they
 	 * may be read, verified, and any needed repairs performed.
 	 */
-	if (rr->rr_nempty && rr->rr_abd_empty == NULL)
+	if (rr->rr_nempty != 0 && rr->rr_abd_empty == NULL)
 		vdev_draid_map_alloc_empty(zio, rr);
 
 	for (int c = 0; c < rr->rr_cols; c++) {
@@ -2395,11 +3395,48 @@ vdev_raidz_io_done(zio_t *zio)
 {
 	raidz_map_t *rm = zio->io_vsd;
 
+	ASSERT(zio->io_bp != NULL);
 	if (zio->io_type == ZIO_TYPE_WRITE) {
 		for (int i = 0; i < rm->rm_nrows; i++) {
 			vdev_raidz_io_done_write_impl(zio, rm->rm_row[i]);
 		}
 	} else {
+		if (rm->rm_phys_col) {
+			/*
+			 * This is an aggregated read.  Copy the data and status
+			 * from the aggregate abd's to the individual rows.
+			 */
+			for (int i = 0; i < rm->rm_nrows; i++) {
+				raidz_row_t *rr = rm->rm_row[i];
+
+				for (int c = 0; c < rr->rr_cols; c++) {
+					raidz_col_t *rc = &rr->rr_col[c];
+					if (rc->rc_tried || rc->rc_size == 0)
+						continue;
+
+					raidz_col_t *prc =
+					    &rm->rm_phys_col[rc->rc_devidx];
+					rc->rc_error = prc->rc_error;
+					rc->rc_tried = prc->rc_tried;
+					rc->rc_skipped = prc->rc_skipped;
+					if (c >= rr->rr_firstdatacol) {
+						/*
+						 * Note: this is slightly faster
+						 * than using abd_copy_off().
+						 */
+						char *physbuf = abd_to_buf(
+						    prc->rc_abd);
+						void *physloc = physbuf +
+						    rc->rc_offset -
+						    prc->rc_offset;
+
+						abd_copy_from_buf(rc->rc_abd,
+						    physloc, rc->rc_size);
+					}
+				}
+			}
+		}
+
 		for (int i = 0; i < rm->rm_nrows; i++) {
 			raidz_row_t *rr = rm->rm_row[i];
 			vdev_raidz_io_done_reconstruct_known_missing(zio,
@@ -2446,7 +3483,54 @@ vdev_raidz_io_done(zio_t *zio)
 					zio_vdev_io_redone(zio);
 				return;
 			}
-
+			/*
+			 * It would be too expensive to try every possible
+			 * combination of failed sectors in every row, so
+			 * instead we try every combination of failed current or
+			 * past physical disk. This means that if the incorrect
+			 * sectors were all on Nparity disks at any point in the
+			 * past, we will find the correct data.  The only known
+			 * case where this is less durable than a non-expanded
+			 * RAIDZ, is if we have a silent failure during
+			 * expansion.  In that case, one block could be
+			 * partially in the old format and partially in the
+			 * new format, so we'd lost some sectors from the old
+			 * format and some from the new format.
+			 *
+			 * e.g. logical_width=4 physical_width=6
+			 * the 15 (6+5+4) possible failed disks are:
+			 * width=6 child=0
+			 * width=6 child=1
+			 * width=6 child=2
+			 * width=6 child=3
+			 * width=6 child=4
+			 * width=6 child=5
+			 * width=5 child=0
+			 * width=5 child=1
+			 * width=5 child=2
+			 * width=5 child=3
+			 * width=5 child=4
+			 * width=4 child=0
+			 * width=4 child=1
+			 * width=4 child=2
+			 * width=4 child=3
+			 * And we will try every combination of Nparity of these
+			 * failing.
+			 *
+			 * As a first pass, we can generate every combo,
+			 * and try reconstructing, ignoring any known
+			 * failures.  If any row has too many known + simulated
+			 * failures, then we bail on reconstructing with this
+			 * number of simulated failures.  As an improvement,
+			 * we could detect the number of whole known failures
+			 * (i.e. we have known failures on these disks for
+			 * every row; the disks never succeeded), and
+			 * subtract that from the max # failures to simulate.
+			 * We could go even further like the current
+			 * combrec code, but that doesn't seem like it
+			 * gains us very much.  If we simulate a failure
+			 * that is also a known failure, that's fine.
+			 */
 			zio->io_error = vdev_raidz_combrec(zio);
 			if (zio->io_error == ECKSUM &&
 			    !(zio->io_flags & ZIO_FLAG_SPECULATIVE)) {
@@ -2454,6 +3538,10 @@ vdev_raidz_io_done(zio_t *zio)
 			}
 		}
 	}
+	if (rm->rm_lr != NULL) {
+		zfs_rangelock_exit(rm->rm_lr);
+		rm->rm_lr = NULL;
+	}
 }
 
 static void
@@ -2480,6 +3568,14 @@ vdev_raidz_need_resilver(vdev_t *vd, const dva_t *dva, size_t psize,
     uint64_t phys_birth)
 {
 	vdev_raidz_t *vdrz = vd->vdev_tsd;
+
+	/*
+	 * If we're in the middle of a RAIDZ expansion, this block may be in
+	 * the old and/or new location.  For simplicity, always resilver it.
+	 */
+	if (vdrz->vn_vre.vre_state == DSS_SCANNING)
+		return (B_TRUE);
+
 	uint64_t dcols = vd->vdev_children;
 	uint64_t nparity = vdrz->vd_nparity;
 	uint64_t ashift = vd->vdev_top->vdev_ashift;
@@ -2524,7 +3620,24 @@ vdev_raidz_xlate(vdev_t *cvd, const range_seg64_t *logical_rs,
 	vdev_t *raidvd = cvd->vdev_parent;
 	ASSERT(raidvd->vdev_ops == &vdev_raidz_ops);
 
-	uint64_t width = raidvd->vdev_children;
+	vdev_raidz_t *vdrz = raidvd->vdev_tsd;
+
+	if (vdrz->vn_vre.vre_state == DSS_SCANNING) {
+		/*
+		 * We're in the middle of expansion, in which case the
+		 * translation is in flux.  Any answer we give may be wrong
+		 * by the time we return, so it isn't safe for the caller to
+		 * act on it.  Therefore we say that this range isn't present
+		 * on any children.  The only consumers of this are "zpool
+		 * initialize" and trimming, both of which are "best effort"
+		 * anyway.
+		 */
+		physical_rs->rs_start = physical_rs->rs_end = 0;
+		remain_rs->rs_start = remain_rs->rs_end = 0;
+		return;
+	}
+
+	uint64_t width = vdrz->vd_physical_width;
 	uint64_t tgt_col = cvd->vdev_id;
 	uint64_t ashift = raidvd->vdev_top->vdev_ashift;
 
@@ -2550,15 +3663,1155 @@ vdev_raidz_xlate(vdev_t *cvd, const range_seg64_t *logical_rs,
 	    logical_rs->rs_end - logical_rs->rs_start);
 }
 
+static void
+raidz_reflow_sync(void *arg, dmu_tx_t *tx)
+{
+	spa_t *spa = arg;
+	int txgoff = dmu_tx_get_txg(tx) & TXG_MASK;
+	vdev_raidz_expand_t *vre = spa->spa_raidz_expand;
+
+	/*
+	 * Ensure there are no i/os to the range that is being committed.
+	 */
+	uint64_t old_offset = RRSS_GET_OFFSET(&spa->spa_uberblock);
+	ASSERT3U(vre->vre_offset_pertxg[txgoff], >=, old_offset);
+
+	mutex_enter(&vre->vre_lock);
+	uint64_t new_offset =
+	    MIN(vre->vre_offset_pertxg[txgoff], vre->vre_failed_offset);
+	/*
+	 * We should not have committed anything that failed.
+	 */
+	VERIFY3U(vre->vre_failed_offset, >=, old_offset);
+	mutex_exit(&vre->vre_lock);
+
+	zfs_locked_range_t *lr = zfs_rangelock_enter(&vre->vre_rangelock,
+	    old_offset, new_offset - old_offset,
+	    RL_WRITER);
+
+	/*
+	 * Update the uberblock that will be written when this txg completes.
+	 */
+	RAIDZ_REFLOW_SET(&spa->spa_uberblock,
+	    RRSS_SCRATCH_INVALID_SYNCED_REFLOW, new_offset);
+	vre->vre_offset_pertxg[txgoff] = 0;
+	zfs_rangelock_exit(lr);
+
+	mutex_enter(&vre->vre_lock);
+	vre->vre_bytes_copied += vre->vre_bytes_copied_pertxg[txgoff];
+	vre->vre_bytes_copied_pertxg[txgoff] = 0;
+	mutex_exit(&vre->vre_lock);
+
+	vdev_t *vd = vdev_lookup_top(spa, vre->vre_vdev_id);
+	VERIFY0(zap_update(spa->spa_meta_objset,
+	    vd->vdev_top_zap, VDEV_TOP_ZAP_RAIDZ_EXPAND_BYTES_COPIED,
+	    sizeof (vre->vre_bytes_copied), 1, &vre->vre_bytes_copied, tx));
+}
+
+static void
+raidz_reflow_complete_sync(void *arg, dmu_tx_t *tx)
+{
+	spa_t *spa = arg;
+	vdev_raidz_expand_t *vre = spa->spa_raidz_expand;
+	vdev_t *raidvd = vdev_lookup_top(spa, vre->vre_vdev_id);
+	vdev_raidz_t *vdrz = raidvd->vdev_tsd;
+
+	for (int i = 0; i < TXG_SIZE; i++)
+		VERIFY0(vre->vre_offset_pertxg[i]);
+
+	reflow_node_t *re = kmem_zalloc(sizeof (*re), KM_SLEEP);
+	re->re_txg = tx->tx_txg + TXG_CONCURRENT_STATES;
+	re->re_logical_width = vdrz->vd_physical_width;
+	mutex_enter(&vdrz->vd_expand_lock);
+	avl_add(&vdrz->vd_expand_txgs, re);
+	mutex_exit(&vdrz->vd_expand_lock);
+
+	vdev_t *vd = vdev_lookup_top(spa, vre->vre_vdev_id);
+
+	/*
+	 * Dirty the config so that the updated ZPOOL_CONFIG_RAIDZ_EXPAND_TXGS
+	 * will get written (based on vd_expand_txgs).
+	 */
+	vdev_config_dirty(vd);
+
+	/*
+	 * Before we change vre_state, the on-disk state must reflect that we
+	 * have completed all copying, so that vdev_raidz_io_start() can use
+	 * vre_state to determine if the reflow is in progress.  See also the
+	 * end of spa_raidz_expand_thread().
+	 */
+	VERIFY3U(RRSS_GET_OFFSET(&spa->spa_ubsync), ==,
+	    raidvd->vdev_ms_count << raidvd->vdev_ms_shift);
+
+	vre->vre_end_time = gethrestime_sec();
+	vre->vre_state = DSS_FINISHED;
+
+	uint64_t state = vre->vre_state;
+	VERIFY0(zap_update(spa->spa_meta_objset,
+	    vd->vdev_top_zap, VDEV_TOP_ZAP_RAIDZ_EXPAND_STATE,
+	    sizeof (state), 1, &state, tx));
+
+	uint64_t end_time = vre->vre_end_time;
+	VERIFY0(zap_update(spa->spa_meta_objset,
+	    vd->vdev_top_zap, VDEV_TOP_ZAP_RAIDZ_EXPAND_END_TIME,
+	    sizeof (end_time), 1, &end_time, tx));
+
+	spa->spa_uberblock.ub_raidz_reflow_info = 0;
+
+	spa_history_log_internal(spa, "raidz vdev expansion completed",  tx,
+	    "%s vdev %llu new width %llu", spa_name(spa),
+	    (unsigned long long)vd->vdev_id,
+	    (unsigned long long)vd->vdev_children);
+
+	spa->spa_raidz_expand = NULL;
+	raidvd->vdev_rz_expanding = B_FALSE;
+
+	spa_async_request(spa, SPA_ASYNC_INITIALIZE_RESTART);
+	spa_async_request(spa, SPA_ASYNC_TRIM_RESTART);
+	spa_async_request(spa, SPA_ASYNC_AUTOTRIM_RESTART);
+
+	spa_notify_waiters(spa);
+
+	/*
+	 * While we're in syncing context take the opportunity to
+	 * setup a scrub. All the data has been sucessfully copied
+	 * but we have not validated any checksums.
+	 */
+	pool_scan_func_t func = POOL_SCAN_SCRUB;
+	if (zfs_scrub_after_expand && dsl_scan_setup_check(&func, tx) == 0)
+		dsl_scan_setup_sync(&func, tx);
+}
+
+/*
+ * Struct for one copy zio.
+ */
+typedef struct raidz_reflow_arg {
+	vdev_raidz_expand_t *rra_vre;
+	zfs_locked_range_t *rra_lr;
+	uint64_t rra_txg;
+} raidz_reflow_arg_t;
+
+/*
+ * The write of the new location is done.
+ */
+static void
+raidz_reflow_write_done(zio_t *zio)
+{
+	raidz_reflow_arg_t *rra = zio->io_private;
+	vdev_raidz_expand_t *vre = rra->rra_vre;
+
+	abd_free(zio->io_abd);
+
+	mutex_enter(&vre->vre_lock);
+	if (zio->io_error != 0) {
+		/* Force a reflow pause on errors */
+		vre->vre_failed_offset =
+		    MIN(vre->vre_failed_offset, rra->rra_lr->lr_offset);
+	}
+	ASSERT3U(vre->vre_outstanding_bytes, >=, zio->io_size);
+	vre->vre_outstanding_bytes -= zio->io_size;
+	if (rra->rra_lr->lr_offset + rra->rra_lr->lr_length <
+	    vre->vre_failed_offset) {
+		vre->vre_bytes_copied_pertxg[rra->rra_txg & TXG_MASK] +=
+		    zio->io_size;
+	}
+	cv_signal(&vre->vre_cv);
+	mutex_exit(&vre->vre_lock);
+
+	zfs_rangelock_exit(rra->rra_lr);
+
+	kmem_free(rra, sizeof (*rra));
+	spa_config_exit(zio->io_spa, SCL_STATE, zio->io_spa);
+}
+
+/*
+ * The read of the old location is done.  The parent zio is the write to
+ * the new location.  Allow it to start.
+ */
+static void
+raidz_reflow_read_done(zio_t *zio)
+{
+	raidz_reflow_arg_t *rra = zio->io_private;
+	vdev_raidz_expand_t *vre = rra->rra_vre;
+
+	/*
+	 * If the read failed, or if it was done on a vdev that is not fully
+	 * healthy (e.g. a child that has a resilver in progress), we may not
+	 * have the correct data.  Note that it's OK if the write proceeds.
+	 * It may write garbage but the location is otherwise unused and we
+	 * will retry later due to vre_failed_offset.
+	 */
+	if (zio->io_error != 0 || !vdev_dtl_empty(zio->io_vd, DTL_MISSING)) {
+		zfs_dbgmsg("reflow read failed off=%llu size=%llu txg=%llu "
+		    "err=%u partial_dtl_empty=%u missing_dtl_empty=%u",
+		    (long long)rra->rra_lr->lr_offset,
+		    (long long)rra->rra_lr->lr_length,
+		    (long long)rra->rra_txg,
+		    zio->io_error,
+		    vdev_dtl_empty(zio->io_vd, DTL_PARTIAL),
+		    vdev_dtl_empty(zio->io_vd, DTL_MISSING));
+		mutex_enter(&vre->vre_lock);
+		/* Force a reflow pause on errors */
+		vre->vre_failed_offset =
+		    MIN(vre->vre_failed_offset, rra->rra_lr->lr_offset);
+		mutex_exit(&vre->vre_lock);
+	}
+
+	zio_nowait(zio_unique_parent(zio));
+}
+
+static void
+raidz_reflow_record_progress(vdev_raidz_expand_t *vre, uint64_t offset,
+    dmu_tx_t *tx)
+{
+	int txgoff = dmu_tx_get_txg(tx) & TXG_MASK;
+	spa_t *spa = dmu_tx_pool(tx)->dp_spa;
+
+	if (offset == 0)
+		return;
+
+	mutex_enter(&vre->vre_lock);
+	ASSERT3U(vre->vre_offset, <=, offset);
+	vre->vre_offset = offset;
+	mutex_exit(&vre->vre_lock);
+
+	if (vre->vre_offset_pertxg[txgoff] == 0) {
+		dsl_sync_task_nowait(dmu_tx_pool(tx), raidz_reflow_sync,
+		    spa, tx);
+	}
+	vre->vre_offset_pertxg[txgoff] = offset;
+}
+
+static boolean_t
+vdev_raidz_expand_child_replacing(vdev_t *raidz_vd)
+{
+	for (int i = 0; i < raidz_vd->vdev_children; i++) {
+		/* Quick check if a child is being replaced */
+		if (!raidz_vd->vdev_child[i]->vdev_ops->vdev_op_leaf)
+			return (B_TRUE);
+	}
+	return (B_FALSE);
+}
+
+static boolean_t
+raidz_reflow_impl(vdev_t *vd, vdev_raidz_expand_t *vre, range_tree_t *rt,
+    dmu_tx_t *tx)
+{
+	spa_t *spa = vd->vdev_spa;
+	int ashift = vd->vdev_top->vdev_ashift;
+	uint64_t offset, size;
+
+	if (!range_tree_find_in(rt, 0, vd->vdev_top->vdev_asize,
+	    &offset, &size)) {
+		return (B_FALSE);
+	}
+	ASSERT(IS_P2ALIGNED(offset, 1 << ashift));
+	ASSERT3U(size, >=, 1 << ashift);
+	uint64_t length = 1 << ashift;
+	int txgoff = dmu_tx_get_txg(tx) & TXG_MASK;
+
+	uint64_t blkid = offset >> ashift;
+
+	int old_children = vd->vdev_children - 1;
+
+	/*
+	 * We can only progress to the point that writes will not overlap
+	 * with blocks whose progress has not yet been recorded on disk.
+	 * Since partially-copied rows are still read from the old location,
+	 * we need to stop one row before the sector-wise overlap, to prevent
+	 * row-wise overlap.
+	 *
+	 * Note that even if we are skipping over a large unallocated region,
+	 * we can't move the on-disk progress to `offset`, because concurrent
+	 * writes/allocations could still use the currently-unallocated
+	 * region.
+	 */
+	uint64_t ubsync_blkid =
+	    RRSS_GET_OFFSET(&spa->spa_ubsync) >> ashift;
+	uint64_t next_overwrite_blkid = ubsync_blkid +
+	    ubsync_blkid / old_children - old_children;
+	VERIFY3U(next_overwrite_blkid, >, ubsync_blkid);
+
+	if (blkid >= next_overwrite_blkid) {
+		raidz_reflow_record_progress(vre,
+		    next_overwrite_blkid << ashift, tx);
+		return (B_TRUE);
+	}
+
+	range_tree_remove(rt, offset, length);
+
+	raidz_reflow_arg_t *rra = kmem_zalloc(sizeof (*rra), KM_SLEEP);
+	rra->rra_vre = vre;
+	rra->rra_lr = zfs_rangelock_enter(&vre->vre_rangelock,
+	    offset, length, RL_WRITER);
+	rra->rra_txg = dmu_tx_get_txg(tx);
+
+	raidz_reflow_record_progress(vre, offset + length, tx);
+
+	mutex_enter(&vre->vre_lock);
+	vre->vre_outstanding_bytes += length;
+	mutex_exit(&vre->vre_lock);
+
+	/*
+	 * SCL_STATE will be released when the read and write are done,
+	 * by raidz_reflow_write_done().
+	 */
+	spa_config_enter(spa, SCL_STATE, spa, RW_READER);
+
+	/* check if a replacing vdev was added, if so treat it as an error */
+	if (vdev_raidz_expand_child_replacing(vd)) {
+		zfs_dbgmsg("replacing vdev encountered, reflow paused at "
+		    "offset=%llu txg=%llu",
+		    (long long)rra->rra_lr->lr_offset,
+		    (long long)rra->rra_txg);
+
+		mutex_enter(&vre->vre_lock);
+		vre->vre_failed_offset =
+		    MIN(vre->vre_failed_offset, rra->rra_lr->lr_offset);
+		cv_signal(&vre->vre_cv);
+		mutex_exit(&vre->vre_lock);
+
+		/* drop everything we acquired */
+		zfs_rangelock_exit(rra->rra_lr);
+		kmem_free(rra, sizeof (*rra));
+		spa_config_exit(spa, SCL_STATE, spa);
+		return (B_TRUE);
+	}
+
+	zio_t *pio = spa->spa_txg_zio[txgoff];
+	abd_t *abd = abd_alloc_for_io(length, B_FALSE);
+	zio_t *write_zio = zio_vdev_child_io(pio, NULL,
+	    vd->vdev_child[blkid % vd->vdev_children],
+	    (blkid / vd->vdev_children) << ashift,
+	    abd, length,
+	    ZIO_TYPE_WRITE, ZIO_PRIORITY_REMOVAL,
+	    ZIO_FLAG_CANFAIL,
+	    raidz_reflow_write_done, rra);
+
+	zio_nowait(zio_vdev_child_io(write_zio, NULL,
+	    vd->vdev_child[blkid % old_children],
+	    (blkid / old_children) << ashift,
+	    abd, length,
+	    ZIO_TYPE_READ, ZIO_PRIORITY_REMOVAL,
+	    ZIO_FLAG_CANFAIL,
+	    raidz_reflow_read_done, rra));
+
+	return (B_FALSE);
+}
+
+/*
+ * For testing (ztest specific)
+ */
+static void
+raidz_expand_pause(uint_t pause_point)
+{
+	while (raidz_expand_pause_point != 0 &&
+	    raidz_expand_pause_point <= pause_point)
+		delay(hz);
+}
+
+static void
+raidz_scratch_child_done(zio_t *zio)
+{
+	zio_t *pio = zio->io_private;
+
+	mutex_enter(&pio->io_lock);
+	pio->io_error = zio_worst_error(pio->io_error, zio->io_error);
+	mutex_exit(&pio->io_lock);
+}
+
+/*
+ * Reflow the beginning portion of the vdev into an intermediate scratch area
+ * in memory and on disk. This operation must be persisted on disk before we
+ * proceed to overwrite the beginning portion with the reflowed data.
+ *
+ * This multi-step task can fail to complete if disk errors are encountered
+ * and we can return here after a pause (waiting for disk to become healthy).
+ */
+static void
+raidz_reflow_scratch_sync(void *arg, dmu_tx_t *tx)
+{
+	vdev_raidz_expand_t *vre = arg;
+	spa_t *spa = dmu_tx_pool(tx)->dp_spa;
+	zio_t *pio;
+	int error;
+
+	spa_config_enter(spa, SCL_STATE, FTAG, RW_READER);
+	vdev_t *raidvd = vdev_lookup_top(spa, vre->vre_vdev_id);
+	int ashift = raidvd->vdev_ashift;
+	uint64_t write_size = P2ALIGN(VDEV_BOOT_SIZE, 1 << ashift);
+	uint64_t logical_size = write_size * raidvd->vdev_children;
+	uint64_t read_size =
+	    P2ROUNDUP(DIV_ROUND_UP(logical_size, (raidvd->vdev_children - 1)),
+	    1 << ashift);
+
+	/*
+	 * The scratch space must be large enough to get us to the point
+	 * that one row does not overlap itself when moved.  This is checked
+	 * by vdev_raidz_attach_check().
+	 */
+	VERIFY3U(write_size, >=, raidvd->vdev_children << ashift);
+	VERIFY3U(write_size, <=, VDEV_BOOT_SIZE);
+	VERIFY3U(write_size, <=, read_size);
+
+	zfs_locked_range_t *lr = zfs_rangelock_enter(&vre->vre_rangelock,
+	    0, logical_size, RL_WRITER);
+
+	abd_t **abds = kmem_alloc(raidvd->vdev_children * sizeof (abd_t *),
+	    KM_SLEEP);
+	for (int i = 0; i < raidvd->vdev_children; i++) {
+		abds[i] = abd_alloc_linear(read_size, B_FALSE);
+	}
+
+	raidz_expand_pause(RAIDZ_EXPAND_PAUSE_PRE_SCRATCH_1);
+
+	/*
+	 * If we have already written the scratch area then we must read from
+	 * there, since new writes were redirected there while we were paused
+	 * or the original location may have been partially overwritten with
+	 * reflowed data.
+	 */
+	if (RRSS_GET_STATE(&spa->spa_ubsync) == RRSS_SCRATCH_VALID) {
+		VERIFY3U(RRSS_GET_OFFSET(&spa->spa_ubsync), ==, logical_size);
+		/*
+		 * Read from scratch space.
+		 */
+		pio = zio_root(spa, NULL, NULL, ZIO_FLAG_CANFAIL);
+		for (int i = 0; i < raidvd->vdev_children; i++) {
+			/*
+			 * Note: zio_vdev_child_io() adds VDEV_LABEL_START_SIZE
+			 * to the offset to calculate the physical offset to
+			 * write to.  Passing in a negative offset makes us
+			 * access the scratch area.
+			 */
+			zio_nowait(zio_vdev_child_io(pio, NULL,
+			    raidvd->vdev_child[i],
+			    VDEV_BOOT_OFFSET - VDEV_LABEL_START_SIZE, abds[i],
+			    write_size, ZIO_TYPE_READ, ZIO_PRIORITY_ASYNC_READ,
+			    ZIO_FLAG_CANFAIL, raidz_scratch_child_done, pio));
+		}
+		error = zio_wait(pio);
+		if (error != 0) {
+			zfs_dbgmsg("reflow: error %d reading scratch location",
+			    error);
+			goto io_error_exit;
+		}
+		goto overwrite;
+	}
+
+	/*
+	 * Read from original location.
+	 */
+	pio = zio_root(spa, NULL, NULL, ZIO_FLAG_CANFAIL);
+	for (int i = 0; i < raidvd->vdev_children - 1; i++) {
+		ASSERT0(vdev_is_dead(raidvd->vdev_child[i]));
+		zio_nowait(zio_vdev_child_io(pio, NULL, raidvd->vdev_child[i],
+		    0, abds[i], read_size, ZIO_TYPE_READ,
+		    ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_CANFAIL,
+		    raidz_scratch_child_done, pio));
+	}
+	error = zio_wait(pio);
+	if (error != 0) {
+		zfs_dbgmsg("reflow: error %d reading original location", error);
+io_error_exit:
+		for (int i = 0; i < raidvd->vdev_children; i++)
+			abd_free(abds[i]);
+		kmem_free(abds, raidvd->vdev_children * sizeof (abd_t *));
+		zfs_rangelock_exit(lr);
+		spa_config_exit(spa, SCL_STATE, FTAG);
+		return;
+	}
+
+	raidz_expand_pause(RAIDZ_EXPAND_PAUSE_PRE_SCRATCH_2);
+
+	/*
+	 * Reflow in memory.
+	 */
+	uint64_t logical_sectors = logical_size >> ashift;
+	for (int i = raidvd->vdev_children - 1; i < logical_sectors; i++) {
+		int oldchild = i % (raidvd->vdev_children - 1);
+		uint64_t oldoff = (i / (raidvd->vdev_children - 1)) << ashift;
+
+		int newchild = i % raidvd->vdev_children;
+		uint64_t newoff = (i / raidvd->vdev_children) << ashift;
+
+		/* a single sector should not be copying over itself */
+		ASSERT(!(newchild == oldchild && newoff == oldoff));
+
+		abd_copy_off(abds[newchild], abds[oldchild],
+		    newoff, oldoff, 1 << ashift);
+	}
+
+	/*
+	 * Verify that we filled in everything we intended to (write_size on
+	 * each child).
+	 */
+	VERIFY0(logical_sectors % raidvd->vdev_children);
+	VERIFY3U((logical_sectors / raidvd->vdev_children) << ashift, ==,
+	    write_size);
+
+	/*
+	 * Write to scratch location (boot area).
+	 */
+	pio = zio_root(spa, NULL, NULL, ZIO_FLAG_CANFAIL);
+	for (int i = 0; i < raidvd->vdev_children; i++) {
+		/*
+		 * Note: zio_vdev_child_io() adds VDEV_LABEL_START_SIZE to
+		 * the offset to calculate the physical offset to write to.
+		 * Passing in a negative offset lets us access the boot area.
+		 */
+		zio_nowait(zio_vdev_child_io(pio, NULL, raidvd->vdev_child[i],
+		    VDEV_BOOT_OFFSET - VDEV_LABEL_START_SIZE, abds[i],
+		    write_size, ZIO_TYPE_WRITE, ZIO_PRIORITY_ASYNC_WRITE,
+		    ZIO_FLAG_CANFAIL, raidz_scratch_child_done, pio));
+	}
+	error = zio_wait(pio);
+	if (error != 0) {
+		zfs_dbgmsg("reflow: error %d writing scratch location", error);
+		goto io_error_exit;
+	}
+	pio = zio_root(spa, NULL, NULL, 0);
+	zio_flush(pio, raidvd);
+	zio_wait(pio);
+
+	zfs_dbgmsg("reflow: wrote %llu bytes (logical) to scratch area",
+	    (long long)logical_size);
+
+	raidz_expand_pause(RAIDZ_EXPAND_PAUSE_PRE_SCRATCH_3);
+
+	/*
+	 * Update uberblock to indicate that scratch space is valid.  This is
+	 * needed because after this point, the real location may be
+	 * overwritten.  If we crash, we need to get the data from the
+	 * scratch space, rather than the real location.
+	 *
+	 * Note: ub_timestamp is bumped so that vdev_uberblock_compare()
+	 * will prefer this uberblock.
+	 */
+	RAIDZ_REFLOW_SET(&spa->spa_ubsync, RRSS_SCRATCH_VALID, logical_size);
+	spa->spa_ubsync.ub_timestamp++;
+	ASSERT0(vdev_uberblock_sync_list(&spa->spa_root_vdev, 1,
+	    &spa->spa_ubsync, ZIO_FLAG_CONFIG_WRITER));
+	if (spa_multihost(spa))
+		mmp_update_uberblock(spa, &spa->spa_ubsync);
+
+	zfs_dbgmsg("reflow: uberblock updated "
+	    "(txg %llu, SCRATCH_VALID, size %llu, ts %llu)",
+	    (long long)spa->spa_ubsync.ub_txg,
+	    (long long)logical_size,
+	    (long long)spa->spa_ubsync.ub_timestamp);
+
+	raidz_expand_pause(RAIDZ_EXPAND_PAUSE_SCRATCH_VALID);
+
+	/*
+	 * Overwrite with reflow'ed data.
+	 */
+overwrite:
+	pio = zio_root(spa, NULL, NULL, ZIO_FLAG_CANFAIL);
+	for (int i = 0; i < raidvd->vdev_children; i++) {
+		zio_nowait(zio_vdev_child_io(pio, NULL, raidvd->vdev_child[i],
+		    0, abds[i], write_size, ZIO_TYPE_WRITE,
+		    ZIO_PRIORITY_ASYNC_WRITE, ZIO_FLAG_CANFAIL,
+		    raidz_scratch_child_done, pio));
+	}
+	error = zio_wait(pio);
+	if (error != 0) {
+		/*
+		 * When we exit early here and drop the range lock, new
+		 * writes will go into the scratch area so we'll need to
+		 * read from there when we return after pausing.
+		 */
+		zfs_dbgmsg("reflow: error %d writing real location", error);
+		/*
+		 * Update the uberblock that is written when this txg completes.
+		 */
+		RAIDZ_REFLOW_SET(&spa->spa_uberblock, RRSS_SCRATCH_VALID,
+		    logical_size);
+		goto io_error_exit;
+	}
+	pio = zio_root(spa, NULL, NULL, 0);
+	zio_flush(pio, raidvd);
+	zio_wait(pio);
+
+	zfs_dbgmsg("reflow: overwrote %llu bytes (logical) to real location",
+	    (long long)logical_size);
+	for (int i = 0; i < raidvd->vdev_children; i++)
+		abd_free(abds[i]);
+	kmem_free(abds, raidvd->vdev_children * sizeof (abd_t *));
+
+	raidz_expand_pause(RAIDZ_EXPAND_PAUSE_SCRATCH_REFLOWED);
+
+	/*
+	 * Update uberblock to indicate that the initial part has been
+	 * reflow'ed.  This is needed because after this point (when we exit
+	 * the rangelock), we allow regular writes to this region, which will
+	 * be written to the new location only (because reflow_offset_next ==
+	 * reflow_offset_synced).  If we crashed and re-copied from the
+	 * scratch space, we would lose the regular writes.
+	 */
+	RAIDZ_REFLOW_SET(&spa->spa_ubsync, RRSS_SCRATCH_INVALID_SYNCED,
+	    logical_size);
+	spa->spa_ubsync.ub_timestamp++;
+	ASSERT0(vdev_uberblock_sync_list(&spa->spa_root_vdev, 1,
+	    &spa->spa_ubsync, ZIO_FLAG_CONFIG_WRITER));
+	if (spa_multihost(spa))
+		mmp_update_uberblock(spa, &spa->spa_ubsync);
+
+	zfs_dbgmsg("reflow: uberblock updated "
+	    "(txg %llu, SCRATCH_NOT_IN_USE, size %llu, ts %llu)",
+	    (long long)spa->spa_ubsync.ub_txg,
+	    (long long)logical_size,
+	    (long long)spa->spa_ubsync.ub_timestamp);
+
+	raidz_expand_pause(RAIDZ_EXPAND_PAUSE_SCRATCH_POST_REFLOW_1);
+
+	/*
+	 * Update progress.
+	 */
+	vre->vre_offset = logical_size;
+	zfs_rangelock_exit(lr);
+	spa_config_exit(spa, SCL_STATE, FTAG);
+
+	int txgoff = dmu_tx_get_txg(tx) & TXG_MASK;
+	vre->vre_offset_pertxg[txgoff] = vre->vre_offset;
+	vre->vre_bytes_copied_pertxg[txgoff] = vre->vre_bytes_copied;
+	/*
+	 * Note - raidz_reflow_sync() will update the uberblock state to
+	 * RRSS_SCRATCH_INVALID_SYNCED_REFLOW
+	 */
+	raidz_reflow_sync(spa, tx);
+
+	raidz_expand_pause(RAIDZ_EXPAND_PAUSE_SCRATCH_POST_REFLOW_2);
+}
+
+/*
+ * We crashed in the middle of raidz_reflow_scratch_sync(); complete its work
+ * here.  No other i/o can be in progress, so we don't need the vre_rangelock.
+ */
+void
+vdev_raidz_reflow_copy_scratch(spa_t *spa)
+{
+	vdev_raidz_expand_t *vre = spa->spa_raidz_expand;
+	uint64_t logical_size = RRSS_GET_OFFSET(&spa->spa_uberblock);
+	ASSERT3U(RRSS_GET_STATE(&spa->spa_uberblock), ==, RRSS_SCRATCH_VALID);
+
+	spa_config_enter(spa, SCL_STATE, FTAG, RW_READER);
+	vdev_t *raidvd = vdev_lookup_top(spa, vre->vre_vdev_id);
+	ASSERT0(logical_size % raidvd->vdev_children);
+	uint64_t write_size = logical_size / raidvd->vdev_children;
+
+	zio_t *pio;
+
+	/*
+	 * Read from scratch space.
+	 */
+	abd_t **abds = kmem_alloc(raidvd->vdev_children * sizeof (abd_t *),
+	    KM_SLEEP);
+	for (int i = 0; i < raidvd->vdev_children; i++) {
+		abds[i] = abd_alloc_linear(write_size, B_FALSE);
+	}
+
+	pio = zio_root(spa, NULL, NULL, 0);
+	for (int i = 0; i < raidvd->vdev_children; i++) {
+		/*
+		 * Note: zio_vdev_child_io() adds VDEV_LABEL_START_SIZE to
+		 * the offset to calculate the physical offset to write to.
+		 * Passing in a negative offset lets us access the boot area.
+		 */
+		zio_nowait(zio_vdev_child_io(pio, NULL, raidvd->vdev_child[i],
+		    VDEV_BOOT_OFFSET - VDEV_LABEL_START_SIZE, abds[i],
+		    write_size, ZIO_TYPE_READ,
+		    ZIO_PRIORITY_ASYNC_READ, 0,
+		    raidz_scratch_child_done, pio));
+	}
+	zio_wait(pio);
+
+	/*
+	 * Overwrite real location with reflow'ed data.
+	 */
+	pio = zio_root(spa, NULL, NULL, 0);
+	for (int i = 0; i < raidvd->vdev_children; i++) {
+		zio_nowait(zio_vdev_child_io(pio, NULL, raidvd->vdev_child[i],
+		    0, abds[i], write_size, ZIO_TYPE_WRITE,
+		    ZIO_PRIORITY_ASYNC_WRITE, 0,
+		    raidz_scratch_child_done, pio));
+	}
+	zio_wait(pio);
+	pio = zio_root(spa, NULL, NULL, 0);
+	zio_flush(pio, raidvd);
+	zio_wait(pio);
+
+	zfs_dbgmsg("reflow recovery: overwrote %llu bytes (logical) "
+	    "to real location", (long long)logical_size);
+
+	for (int i = 0; i < raidvd->vdev_children; i++)
+		abd_free(abds[i]);
+	kmem_free(abds, raidvd->vdev_children * sizeof (abd_t *));
+
+	/*
+	 * Update uberblock.
+	 */
+	RAIDZ_REFLOW_SET(&spa->spa_ubsync,
+	    RRSS_SCRATCH_INVALID_SYNCED_ON_IMPORT, logical_size);
+	spa->spa_ubsync.ub_timestamp++;
+	VERIFY0(vdev_uberblock_sync_list(&spa->spa_root_vdev, 1,
+	    &spa->spa_ubsync, ZIO_FLAG_CONFIG_WRITER));
+	if (spa_multihost(spa))
+		mmp_update_uberblock(spa, &spa->spa_ubsync);
+
+	zfs_dbgmsg("reflow recovery: uberblock updated "
+	    "(txg %llu, SCRATCH_NOT_IN_USE, size %llu, ts %llu)",
+	    (long long)spa->spa_ubsync.ub_txg,
+	    (long long)logical_size,
+	    (long long)spa->spa_ubsync.ub_timestamp);
+
+	dmu_tx_t *tx = dmu_tx_create_assigned(spa->spa_dsl_pool,
+	    spa_first_txg(spa));
+	int txgoff = dmu_tx_get_txg(tx) & TXG_MASK;
+	vre->vre_offset = logical_size;
+	vre->vre_offset_pertxg[txgoff] = vre->vre_offset;
+	vre->vre_bytes_copied_pertxg[txgoff] = vre->vre_bytes_copied;
+	/*
+	 * Note that raidz_reflow_sync() will update the uberblock once more
+	 */
+	raidz_reflow_sync(spa, tx);
+
+	dmu_tx_commit(tx);
+
+	spa_config_exit(spa, SCL_STATE, FTAG);
+}
+
+static boolean_t
+spa_raidz_expand_thread_check(void *arg, zthr_t *zthr)
+{
+	(void) zthr;
+	spa_t *spa = arg;
+
+	return (spa->spa_raidz_expand != NULL &&
+	    !spa->spa_raidz_expand->vre_waiting_for_resilver);
+}
+
+/*
+ * RAIDZ expansion background thread
+ *
+ * Can be called multiple times if the reflow is paused
+ */
+static void
+spa_raidz_expand_thread(void *arg, zthr_t *zthr)
+{
+	spa_t *spa = arg;
+	vdev_raidz_expand_t *vre = spa->spa_raidz_expand;
+
+	if (RRSS_GET_STATE(&spa->spa_ubsync) == RRSS_SCRATCH_VALID)
+		vre->vre_offset = 0;
+	else
+		vre->vre_offset = RRSS_GET_OFFSET(&spa->spa_ubsync);
+
+	/* Reflow the begining portion using the scratch area */
+	if (vre->vre_offset == 0) {
+		VERIFY0(dsl_sync_task(spa_name(spa),
+		    NULL, raidz_reflow_scratch_sync,
+		    vre, 0, ZFS_SPACE_CHECK_NONE));
+
+		/* if we encountered errors then pause */
+		if (vre->vre_offset == 0) {
+			mutex_enter(&vre->vre_lock);
+			vre->vre_waiting_for_resilver = B_TRUE;
+			mutex_exit(&vre->vre_lock);
+			return;
+		}
+	}
+
+	spa_config_enter(spa, SCL_CONFIG, FTAG, RW_READER);
+	vdev_t *raidvd = vdev_lookup_top(spa, vre->vre_vdev_id);
+
+	uint64_t guid = raidvd->vdev_guid;
+
+	/* Iterate over all the remaining metaslabs */
+	for (uint64_t i = vre->vre_offset >> raidvd->vdev_ms_shift;
+	    i < raidvd->vdev_ms_count &&
+	    !zthr_iscancelled(zthr) &&
+	    vre->vre_failed_offset == UINT64_MAX; i++) {
+		metaslab_t *msp = raidvd->vdev_ms[i];
+
+		metaslab_disable(msp);
+		mutex_enter(&msp->ms_lock);
+
+		/*
+		 * The metaslab may be newly created (for the expanded
+		 * space), in which case its trees won't exist yet,
+		 * so we need to bail out early.
+		 */
+		if (msp->ms_new) {
+			mutex_exit(&msp->ms_lock);
+			metaslab_enable(msp, B_FALSE, B_FALSE);
+			continue;
+		}
+
+		VERIFY0(metaslab_load(msp));
+
+		/*
+		 * We want to copy everything except the free (allocatable)
+		 * space.  Note that there may be a little bit more free
+		 * space (e.g. in ms_defer), and it's fine to copy that too.
+		 */
+		range_tree_t *rt = range_tree_create(NULL, RANGE_SEG64,
+		    NULL, 0, 0);
+		range_tree_add(rt, msp->ms_start, msp->ms_size);
+		range_tree_walk(msp->ms_allocatable, range_tree_remove, rt);
+		mutex_exit(&msp->ms_lock);
+
+		/*
+		 * Force the last sector of each metaslab to be copied.  This
+		 * ensures that we advance the on-disk progress to the end of
+		 * this metaslab while the metaslab is disabled.  Otherwise, we
+		 * could move past this metaslab without advancing the on-disk
+		 * progress, and then an allocation to this metaslab would not
+		 * be copied.
+		 */
+		int sectorsz = 1 << raidvd->vdev_ashift;
+		uint64_t ms_last_offset = msp->ms_start +
+		    msp->ms_size - sectorsz;
+		if (!range_tree_contains(rt, ms_last_offset, sectorsz)) {
+			range_tree_add(rt, ms_last_offset, sectorsz);
+		}
+
+		/*
+		 * When we are resuming from a paused expansion (i.e.
+		 * when importing a pool with a expansion in progress),
+		 * discard any state that we have already processed.
+		 */
+		range_tree_clear(rt, 0, vre->vre_offset);
+
+		while (!zthr_iscancelled(zthr) &&
+		    !range_tree_is_empty(rt) &&
+		    vre->vre_failed_offset == UINT64_MAX) {
+
+			/*
+			 * We need to periodically drop the config lock so that
+			 * writers can get in.  Additionally, we can't wait
+			 * for a txg to sync while holding a config lock
+			 * (since a waiting writer could cause a 3-way deadlock
+			 * with the sync thread, which also gets a config
+			 * lock for reader).  So we can't hold the config lock
+			 * while calling dmu_tx_assign().
+			 */
+			spa_config_exit(spa, SCL_CONFIG, FTAG);
+
+			/*
+			 * If requested, pause the reflow when the amount
+			 * specified by raidz_expand_max_reflow_bytes is reached
+			 *
+			 * This pause is only used during testing or debugging.
+			 */
+			while (raidz_expand_max_reflow_bytes != 0 &&
+			    raidz_expand_max_reflow_bytes <=
+			    vre->vre_bytes_copied && !zthr_iscancelled(zthr)) {
+				delay(hz);
+			}
+
+			mutex_enter(&vre->vre_lock);
+			while (vre->vre_outstanding_bytes >
+			    raidz_expand_max_copy_bytes) {
+				cv_wait(&vre->vre_cv, &vre->vre_lock);
+			}
+			mutex_exit(&vre->vre_lock);
+
+			dmu_tx_t *tx =
+			    dmu_tx_create_dd(spa_get_dsl(spa)->dp_mos_dir);
+
+			VERIFY0(dmu_tx_assign(tx, TXG_WAIT));
+			uint64_t txg = dmu_tx_get_txg(tx);
+
+			/*
+			 * Reacquire the vdev_config lock.  Theoretically, the
+			 * vdev_t that we're expanding may have changed.
+			 */
+			spa_config_enter(spa, SCL_CONFIG, FTAG, RW_READER);
+			raidvd = vdev_lookup_top(spa, vre->vre_vdev_id);
+
+			boolean_t needsync =
+			    raidz_reflow_impl(raidvd, vre, rt, tx);
+
+			dmu_tx_commit(tx);
+
+			if (needsync) {
+				spa_config_exit(spa, SCL_CONFIG, FTAG);
+				txg_wait_synced(spa->spa_dsl_pool, txg);
+				spa_config_enter(spa, SCL_CONFIG, FTAG,
+				    RW_READER);
+			}
+		}
+
+		spa_config_exit(spa, SCL_CONFIG, FTAG);
+
+		metaslab_enable(msp, B_FALSE, B_FALSE);
+		range_tree_vacate(rt, NULL, NULL);
+		range_tree_destroy(rt);
+
+		spa_config_enter(spa, SCL_CONFIG, FTAG, RW_READER);
+		raidvd = vdev_lookup_top(spa, vre->vre_vdev_id);
+	}
+
+	spa_config_exit(spa, SCL_CONFIG, FTAG);
+
+	/*
+	 * The txg_wait_synced() here ensures that all reflow zio's have
+	 * completed, and vre_failed_offset has been set if necessary.  It
+	 * also ensures that the progress of the last raidz_reflow_sync() is
+	 * written to disk before raidz_reflow_complete_sync() changes the
+	 * in-memory vre_state.  vdev_raidz_io_start() uses vre_state to
+	 * determine if a reflow is in progress, in which case we may need to
+	 * write to both old and new locations.  Therefore we can only change
+	 * vre_state once this is not necessary, which is once the on-disk
+	 * progress (in spa_ubsync) has been set past any possible writes (to
+	 * the end of the last metaslab).
+	 */
+	txg_wait_synced(spa->spa_dsl_pool, 0);
+
+	if (!zthr_iscancelled(zthr) &&
+	    vre->vre_offset == raidvd->vdev_ms_count << raidvd->vdev_ms_shift) {
+		/*
+		 * We are not being canceled or paused, so the reflow must be
+		 * complete. In that case also mark it as completed on disk.
+		 */
+		ASSERT3U(vre->vre_failed_offset, ==, UINT64_MAX);
+		VERIFY0(dsl_sync_task(spa_name(spa), NULL,
+		    raidz_reflow_complete_sync, spa,
+		    0, ZFS_SPACE_CHECK_NONE));
+		(void) vdev_online(spa, guid, ZFS_ONLINE_EXPAND, NULL);
+	} else {
+		/*
+		 * Wait for all copy zio's to complete and for all the
+		 * raidz_reflow_sync() synctasks to be run.
+		 */
+		spa_history_log_internal(spa, "reflow pause",
+		    NULL, "offset=%llu failed_offset=%lld",
+		    (long long)vre->vre_offset,
+		    (long long)vre->vre_failed_offset);
+		mutex_enter(&vre->vre_lock);
+		if (vre->vre_failed_offset != UINT64_MAX) {
+			/*
+			 * Reset progress so that we will retry everything
+			 * after the point that something failed.
+			 */
+			vre->vre_offset = vre->vre_failed_offset;
+			vre->vre_failed_offset = UINT64_MAX;
+			vre->vre_waiting_for_resilver = B_TRUE;
+		}
+		mutex_exit(&vre->vre_lock);
+	}
+}
+
+void
+spa_start_raidz_expansion_thread(spa_t *spa)
+{
+	ASSERT3P(spa->spa_raidz_expand_zthr, ==, NULL);
+	spa->spa_raidz_expand_zthr = zthr_create("raidz_expand",
+	    spa_raidz_expand_thread_check, spa_raidz_expand_thread,
+	    spa, defclsyspri);
+}
+
+void
+raidz_dtl_reassessed(vdev_t *vd)
+{
+	spa_t *spa = vd->vdev_spa;
+	if (spa->spa_raidz_expand != NULL) {
+		vdev_raidz_expand_t *vre = spa->spa_raidz_expand;
+		/*
+		 * we get called often from vdev_dtl_reassess() so make
+		 * sure it's our vdev and any replacing is complete
+		 */
+		if (vd->vdev_top->vdev_id == vre->vre_vdev_id &&
+		    !vdev_raidz_expand_child_replacing(vd->vdev_top)) {
+			mutex_enter(&vre->vre_lock);
+			if (vre->vre_waiting_for_resilver) {
+				vdev_dbgmsg(vd, "DTL reassessed, "
+				    "continuing raidz expansion");
+				vre->vre_waiting_for_resilver = B_FALSE;
+				zthr_wakeup(spa->spa_raidz_expand_zthr);
+			}
+			mutex_exit(&vre->vre_lock);
+		}
+	}
+}
+
+int
+vdev_raidz_attach_check(vdev_t *new_child)
+{
+	vdev_t *raidvd = new_child->vdev_parent;
+	uint64_t new_children = raidvd->vdev_children;
+
+	/*
+	 * We use the "boot" space as scratch space to handle overwriting the
+	 * initial part of the vdev.  If it is too small, then this expansion
+	 * is not allowed.  This would be very unusual (e.g. ashift > 13 and
+	 * >200 children).
+	 */
+	if (new_children << raidvd->vdev_ashift > VDEV_BOOT_SIZE) {
+		return (EINVAL);
+	}
+	return (0);
+}
+
+void
+vdev_raidz_attach_sync(void *arg, dmu_tx_t *tx)
+{
+	vdev_t *new_child = arg;
+	spa_t *spa = new_child->vdev_spa;
+	vdev_t *raidvd = new_child->vdev_parent;
+	vdev_raidz_t *vdrz = raidvd->vdev_tsd;
+	ASSERT3P(raidvd->vdev_ops, ==, &vdev_raidz_ops);
+	ASSERT3P(raidvd->vdev_top, ==, raidvd);
+	ASSERT3U(raidvd->vdev_children, >, vdrz->vd_original_width);
+	ASSERT3U(raidvd->vdev_children, ==, vdrz->vd_physical_width + 1);
+	ASSERT3P(raidvd->vdev_child[raidvd->vdev_children - 1], ==,
+	    new_child);
+
+	spa_feature_incr(spa, SPA_FEATURE_RAIDZ_EXPANSION, tx);
+
+	vdrz->vd_physical_width++;
+
+	VERIFY0(spa->spa_uberblock.ub_raidz_reflow_info);
+	vdrz->vn_vre.vre_vdev_id = raidvd->vdev_id;
+	vdrz->vn_vre.vre_offset = 0;
+	vdrz->vn_vre.vre_failed_offset = UINT64_MAX;
+	spa->spa_raidz_expand = &vdrz->vn_vre;
+	zthr_wakeup(spa->spa_raidz_expand_zthr);
+
+	/*
+	 * Dirty the config so that ZPOOL_CONFIG_RAIDZ_EXPANDING will get
+	 * written to the config.
+	 */
+	vdev_config_dirty(raidvd);
+
+	vdrz->vn_vre.vre_start_time = gethrestime_sec();
+	vdrz->vn_vre.vre_end_time = 0;
+	vdrz->vn_vre.vre_state = DSS_SCANNING;
+	vdrz->vn_vre.vre_bytes_copied = 0;
+
+	uint64_t state = vdrz->vn_vre.vre_state;
+	VERIFY0(zap_update(spa->spa_meta_objset,
+	    raidvd->vdev_top_zap, VDEV_TOP_ZAP_RAIDZ_EXPAND_STATE,
+	    sizeof (state), 1, &state, tx));
+
+	uint64_t start_time = vdrz->vn_vre.vre_start_time;
+	VERIFY0(zap_update(spa->spa_meta_objset,
+	    raidvd->vdev_top_zap, VDEV_TOP_ZAP_RAIDZ_EXPAND_START_TIME,
+	    sizeof (start_time), 1, &start_time, tx));
+
+	(void) zap_remove(spa->spa_meta_objset,
+	    raidvd->vdev_top_zap, VDEV_TOP_ZAP_RAIDZ_EXPAND_END_TIME, tx);
+	(void) zap_remove(spa->spa_meta_objset,
+	    raidvd->vdev_top_zap, VDEV_TOP_ZAP_RAIDZ_EXPAND_BYTES_COPIED, tx);
+
+	spa_history_log_internal(spa, "raidz vdev expansion started",  tx,
+	    "%s vdev %llu new width %llu", spa_name(spa),
+	    (unsigned long long)raidvd->vdev_id,
+	    (unsigned long long)raidvd->vdev_children);
+}
+
+int
+vdev_raidz_load(vdev_t *vd)
+{
+	vdev_raidz_t *vdrz = vd->vdev_tsd;
+	int err;
+
+	uint64_t state = DSS_NONE;
+	uint64_t start_time = 0;
+	uint64_t end_time = 0;
+	uint64_t bytes_copied = 0;
+
+	if (vd->vdev_top_zap != 0) {
+		err = zap_lookup(vd->vdev_spa->spa_meta_objset,
+		    vd->vdev_top_zap, VDEV_TOP_ZAP_RAIDZ_EXPAND_STATE,
+		    sizeof (state), 1, &state);
+		if (err != 0 && err != ENOENT)
+			return (err);
+
+		err = zap_lookup(vd->vdev_spa->spa_meta_objset,
+		    vd->vdev_top_zap, VDEV_TOP_ZAP_RAIDZ_EXPAND_START_TIME,
+		    sizeof (start_time), 1, &start_time);
+		if (err != 0 && err != ENOENT)
+			return (err);
+
+		err = zap_lookup(vd->vdev_spa->spa_meta_objset,
+		    vd->vdev_top_zap, VDEV_TOP_ZAP_RAIDZ_EXPAND_END_TIME,
+		    sizeof (end_time), 1, &end_time);
+		if (err != 0 && err != ENOENT)
+			return (err);
+
+		err = zap_lookup(vd->vdev_spa->spa_meta_objset,
+		    vd->vdev_top_zap, VDEV_TOP_ZAP_RAIDZ_EXPAND_BYTES_COPIED,
+		    sizeof (bytes_copied), 1, &bytes_copied);
+		if (err != 0 && err != ENOENT)
+			return (err);
+	}
+
+	/*
+	 * If we are in the middle of expansion, vre_state should have
+	 * already been set by vdev_raidz_init().
+	 */
+	EQUIV(vdrz->vn_vre.vre_state == DSS_SCANNING, state == DSS_SCANNING);
+	vdrz->vn_vre.vre_state = (dsl_scan_state_t)state;
+	vdrz->vn_vre.vre_start_time = start_time;
+	vdrz->vn_vre.vre_end_time = end_time;
+	vdrz->vn_vre.vre_bytes_copied = bytes_copied;
+
+	return (0);
+}
+
+int
+spa_raidz_expand_get_stats(spa_t *spa, pool_raidz_expand_stat_t *pres)
+{
+	vdev_raidz_expand_t *vre = spa->spa_raidz_expand;
+
+	if (vre == NULL) {
+		/* no removal in progress; find most recent completed */
+		for (int c = 0; c < spa->spa_root_vdev->vdev_children; c++) {
+			vdev_t *vd = spa->spa_root_vdev->vdev_child[c];
+			if (vd->vdev_ops == &vdev_raidz_ops) {
+				vdev_raidz_t *vdrz = vd->vdev_tsd;
+
+				if (vdrz->vn_vre.vre_end_time != 0 &&
+				    (vre == NULL ||
+				    vdrz->vn_vre.vre_end_time >
+				    vre->vre_end_time)) {
+					vre = &vdrz->vn_vre;
+				}
+			}
+		}
+	}
+
+	if (vre == NULL) {
+		return (SET_ERROR(ENOENT));
+	}
+
+	pres->pres_state = vre->vre_state;
+	pres->pres_expanding_vdev = vre->vre_vdev_id;
+
+	vdev_t *vd = vdev_lookup_top(spa, vre->vre_vdev_id);
+	pres->pres_to_reflow = vd->vdev_stat.vs_alloc;
+
+	mutex_enter(&vre->vre_lock);
+	pres->pres_reflowed = vre->vre_bytes_copied;
+	for (int i = 0; i < TXG_SIZE; i++)
+		pres->pres_reflowed += vre->vre_bytes_copied_pertxg[i];
+	mutex_exit(&vre->vre_lock);
+
+	pres->pres_start_time = vre->vre_start_time;
+	pres->pres_end_time = vre->vre_end_time;
+	pres->pres_waiting_for_resilver = vre->vre_waiting_for_resilver;
+
+	return (0);
+}
+
 /*
  * Initialize private RAIDZ specific fields from the nvlist.
  */
 static int
 vdev_raidz_init(spa_t *spa, nvlist_t *nv, void **tsd)
 {
-	vdev_raidz_t *vdrz;
-	uint64_t nparity;
-
 	uint_t children;
 	nvlist_t **child;
 	int error = nvlist_lookup_nvlist_array(nv,
@@ -2566,6 +4819,7 @@ vdev_raidz_init(spa_t *spa, nvlist_t *nv, void **tsd)
 	if (error != 0)
 		return (SET_ERROR(EINVAL));
 
+	uint64_t nparity;
 	if (nvlist_lookup_uint64(nv, ZPOOL_CONFIG_NPARITY, &nparity) == 0) {
 		if (nparity == 0 || nparity > VDEV_RAIDZ_MAXPARITY)
 			return (SET_ERROR(EINVAL));
@@ -2592,10 +4846,56 @@ vdev_raidz_init(spa_t *spa, nvlist_t *nv, void **tsd)
 		nparity = 1;
 	}
 
-	vdrz = kmem_zalloc(sizeof (*vdrz), KM_SLEEP);
-	vdrz->vd_logical_width = children;
+	vdev_raidz_t *vdrz = kmem_zalloc(sizeof (*vdrz), KM_SLEEP);
+	vdrz->vn_vre.vre_vdev_id = -1;
+	vdrz->vn_vre.vre_offset = UINT64_MAX;
+	vdrz->vn_vre.vre_failed_offset = UINT64_MAX;
+	mutex_init(&vdrz->vn_vre.vre_lock, NULL, MUTEX_DEFAULT, NULL);
+	cv_init(&vdrz->vn_vre.vre_cv, NULL, CV_DEFAULT, NULL);
+	zfs_rangelock_init(&vdrz->vn_vre.vre_rangelock, NULL, NULL);
+	mutex_init(&vdrz->vd_expand_lock, NULL, MUTEX_DEFAULT, NULL);
+	avl_create(&vdrz->vd_expand_txgs, vdev_raidz_reflow_compare,
+	    sizeof (reflow_node_t), offsetof(reflow_node_t, re_link));
+
+	vdrz->vd_physical_width = children;
 	vdrz->vd_nparity = nparity;
 
+	/* note, the ID does not exist when creating a pool */
+	(void) nvlist_lookup_uint64(nv, ZPOOL_CONFIG_ID,
+	    &vdrz->vn_vre.vre_vdev_id);
+
+	boolean_t reflow_in_progress =
+	    nvlist_exists(nv, ZPOOL_CONFIG_RAIDZ_EXPANDING);
+	if (reflow_in_progress) {
+		spa->spa_raidz_expand = &vdrz->vn_vre;
+		vdrz->vn_vre.vre_state = DSS_SCANNING;
+	}
+
+	vdrz->vd_original_width = children;
+	uint64_t *txgs;
+	unsigned int txgs_size = 0;
+	error = nvlist_lookup_uint64_array(nv, ZPOOL_CONFIG_RAIDZ_EXPAND_TXGS,
+	    &txgs, &txgs_size);
+	if (error == 0) {
+		for (int i = 0; i < txgs_size; i++) {
+			reflow_node_t *re = kmem_zalloc(sizeof (*re), KM_SLEEP);
+			re->re_txg = txgs[txgs_size - i - 1];
+			re->re_logical_width = vdrz->vd_physical_width - i;
+
+			if (reflow_in_progress)
+				re->re_logical_width--;
+
+			avl_add(&vdrz->vd_expand_txgs, re);
+		}
+
+		vdrz->vd_original_width = vdrz->vd_physical_width - txgs_size;
+	}
+	if (reflow_in_progress) {
+		vdrz->vd_original_width--;
+		zfs_dbgmsg("reflow_in_progress, %u wide, %d prior expansions",
+		    children, txgs_size);
+	}
+
 	*tsd = vdrz;
 
 	return (0);
@@ -2604,7 +4904,20 @@ vdev_raidz_init(spa_t *spa, nvlist_t *nv, void **tsd)
 static void
 vdev_raidz_fini(vdev_t *vd)
 {
-	kmem_free(vd->vdev_tsd, sizeof (vdev_raidz_t));
+	vdev_raidz_t *vdrz = vd->vdev_tsd;
+	if (vd->vdev_spa->spa_raidz_expand == &vdrz->vn_vre)
+		vd->vdev_spa->spa_raidz_expand = NULL;
+	reflow_node_t *re;
+	void *cookie = NULL;
+	avl_tree_t *tree = &vdrz->vd_expand_txgs;
+	while ((re = avl_destroy_nodes(tree, &cookie)) != NULL)
+		kmem_free(re, sizeof (*re));
+	avl_destroy(&vdrz->vd_expand_txgs);
+	mutex_destroy(&vdrz->vd_expand_lock);
+	mutex_destroy(&vdrz->vn_vre.vre_lock);
+	cv_destroy(&vdrz->vn_vre.vre_cv);
+	zfs_rangelock_fini(&vdrz->vn_vre.vre_rangelock);
+	kmem_free(vdrz, sizeof (*vdrz));
 }
 
 /*
@@ -2632,6 +4945,29 @@ vdev_raidz_config_generate(vdev_t *vd, nvlist_t *nv)
 	 * it.
 	 */
 	fnvlist_add_uint64(nv, ZPOOL_CONFIG_NPARITY, vdrz->vd_nparity);
+
+	if (vdrz->vn_vre.vre_state == DSS_SCANNING) {
+		fnvlist_add_boolean(nv, ZPOOL_CONFIG_RAIDZ_EXPANDING);
+	}
+
+	mutex_enter(&vdrz->vd_expand_lock);
+	if (!avl_is_empty(&vdrz->vd_expand_txgs)) {
+		uint64_t count = avl_numnodes(&vdrz->vd_expand_txgs);
+		uint64_t *txgs = kmem_alloc(sizeof (uint64_t) * count,
+		    KM_SLEEP);
+		uint64_t i = 0;
+
+		for (reflow_node_t *re = avl_first(&vdrz->vd_expand_txgs);
+		    re != NULL; re = AVL_NEXT(&vdrz->vd_expand_txgs, re)) {
+			txgs[i++] = re->re_txg;
+		}
+
+		fnvlist_add_uint64_array(nv, ZPOOL_CONFIG_RAIDZ_EXPAND_TXGS,
+		    txgs, count);
+
+		kmem_free(txgs, sizeof (uint64_t) * count);
+	}
+	mutex_exit(&vdrz->vd_expand_lock);
 }
 
 static uint64_t
@@ -2671,3 +5007,15 @@ vdev_ops_t vdev_raidz_ops = {
 	.vdev_op_type = VDEV_TYPE_RAIDZ,	/* name of this vdev type */
 	.vdev_op_leaf = B_FALSE			/* not a leaf vdev */
 };
+
+/* BEGIN CSTYLED */
+ZFS_MODULE_PARAM(zfs_vdev, raidz_, expand_max_reflow_bytes, ULONG, ZMOD_RW,
+	"For testing, pause RAIDZ expansion after reflowing this many bytes");
+ZFS_MODULE_PARAM(zfs_vdev, raidz_, expand_max_copy_bytes, ULONG, ZMOD_RW,
+	"Max amount of concurrent i/o for RAIDZ expansion");
+ZFS_MODULE_PARAM(zfs_vdev, raidz_, io_aggregate_rows, ULONG, ZMOD_RW,
+	"For expanded RAIDZ, aggregate reads that have more rows than this");
+ZFS_MODULE_PARAM(zfs, zfs_, scrub_after_expand, INT, ZMOD_RW,
+	"For expanded RAIDZ, automatically start a pool scrub when expansion "
+	"completes");
+/* END CSTYLED */
diff --git a/module/zfs/vdev_trim.c b/module/zfs/vdev_trim.c
index 03e17db02..1c54eae40 100644
--- a/module/zfs/vdev_trim.c
+++ b/module/zfs/vdev_trim.c
@@ -169,7 +169,8 @@ static boolean_t
 vdev_trim_should_stop(vdev_t *vd)
 {
 	return (vd->vdev_trim_exit_wanted || !vdev_writeable(vd) ||
-	    vd->vdev_detached || vd->vdev_top->vdev_removing);
+	    vd->vdev_detached || vd->vdev_top->vdev_removing ||
+	    vd->vdev_top->vdev_rz_expanding);
 }
 
 /*
@@ -180,6 +181,7 @@ vdev_autotrim_should_stop(vdev_t *tvd)
 {
 	return (tvd->vdev_autotrim_exit_wanted ||
 	    !vdev_writeable(tvd) || tvd->vdev_removing ||
+	    tvd->vdev_rz_expanding ||
 	    spa_get_autotrim(tvd->vdev_spa) == SPA_AUTOTRIM_OFF);
 }
 
@@ -222,7 +224,8 @@ vdev_trim_zap_update_sync(void *arg, dmu_tx_t *tx)
 	kmem_free(arg, sizeof (uint64_t));
 
 	vdev_t *vd = spa_lookup_by_guid(tx->tx_pool->dp_spa, guid, B_FALSE);
-	if (vd == NULL || vd->vdev_top->vdev_removing || !vdev_is_concrete(vd))
+	if (vd == NULL || vd->vdev_top->vdev_removing ||
+	    !vdev_is_concrete(vd) || vd->vdev_top->vdev_rz_expanding)
 		return;
 
 	uint64_t last_offset = vd->vdev_trim_offset[txg & TXG_MASK];
@@ -1005,6 +1008,7 @@ vdev_trim(vdev_t *vd, uint64_t rate, boolean_t partial, boolean_t secure)
 	ASSERT(!vd->vdev_detached);
 	ASSERT(!vd->vdev_trim_exit_wanted);
 	ASSERT(!vd->vdev_top->vdev_removing);
+	ASSERT(!vd->vdev_rz_expanding);
 
 	vdev_trim_change_state(vd, VDEV_TRIM_ACTIVE, rate, partial, secure);
 	vd->vdev_trim_thread = thread_create(NULL, 0,
@@ -1162,12 +1166,13 @@ vdev_trim_restart(vdev_t *vd)
 		ASSERT(err == 0 || err == ENOENT);
 		vd->vdev_trim_action_time = timestamp;
 
-		if (vd->vdev_trim_state == VDEV_TRIM_SUSPENDED ||
-		    vd->vdev_offline) {
+		if ((vd->vdev_trim_state == VDEV_TRIM_SUSPENDED ||
+		    vd->vdev_offline) && !vd->vdev_top->vdev_rz_expanding) {
 			/* load progress for reporting, but don't resume */
 			VERIFY0(vdev_trim_load(vd));
 		} else if (vd->vdev_trim_state == VDEV_TRIM_ACTIVE &&
 		    vdev_writeable(vd) && !vd->vdev_top->vdev_removing &&
+		    !vd->vdev_top->vdev_rz_expanding &&
 		    vd->vdev_trim_thread == NULL) {
 			VERIFY0(vdev_trim_load(vd));
 			vdev_trim(vd, vd->vdev_trim_rate,
@@ -1492,7 +1497,8 @@ vdev_autotrim(spa_t *spa)
 
 		mutex_enter(&tvd->vdev_autotrim_lock);
 		if (vdev_writeable(tvd) && !tvd->vdev_removing &&
-		    tvd->vdev_autotrim_thread == NULL) {
+		    tvd->vdev_autotrim_thread == NULL &&
+		    !tvd->vdev_rz_expanding) {
 			ASSERT3P(tvd->vdev_top, ==, tvd);
 
 			tvd->vdev_autotrim_thread = thread_create(NULL, 0,
@@ -1717,6 +1723,7 @@ vdev_trim_simple(vdev_t *vd, uint64_t start, uint64_t size)
 	ASSERT(vd->vdev_ops->vdev_op_leaf);
 	ASSERT(!vd->vdev_detached);
 	ASSERT(!vd->vdev_top->vdev_removing);
+	ASSERT(!vd->vdev_top->vdev_rz_expanding);
 
 	ta.trim_vdev = vd;
 	ta.trim_tree = range_tree_create(NULL, RANGE_SEG64, NULL, 0, 0);