Multi-modifier protection (MMP)

Add multihost=on|off pool property to control MMP.  When enabled
a new thread writes uberblocks to the last slot in each label, at a
set frequency, to indicate to other hosts the pool is actively imported.
These uberblocks are the last synced uberblock with an updated
timestamp.  Property defaults to off.

During tryimport, find the "best" uberblock (newest txg and timestamp)
repeatedly, checking for change in the found uberblock.  Include the
results of the activity test in the config returned by tryimport.
These results are reported to user in "zpool import".

Allow the user to control the period between MMP writes, and the
duration of the activity test on import, via a new module parameter
zfs_multihost_interval.  The period is specified in milliseconds.  The
activity test duration is calculated from this value, and from the
mmp_delay in the "best" uberblock found initially.

Add a kstat interface to export statistics about Multiple Modifier
Protection (MMP) updates. Include the last synced txg number, the
timestamp, the delay since the last MMP update, the VDEV GUID, the VDEV
label that received the last MMP update, and the VDEV path.  Abbreviated
output below.

$ cat /proc/spl/kstat/zfs/mypool/multihost
31 0 0x01 10 880 105092382393521 105144180101111
txg   timestamp  mmp_delay   vdev_guid   vdev_label vdev_path
20468    261337  250274925   68396651780       3    /dev/sda
20468    261339  252023374   6267402363293     1    /dev/sdc
20468    261340  252000858   6698080955233     1    /dev/sdx
20468    261341  251980635   783892869810      2    /dev/sdy
20468    261342  253385953   8923255792467     3    /dev/sdd
20468    261344  253336622   042125143176      0    /dev/sdab
20468    261345  253310522   1200778101278     2    /dev/sde
20468    261346  253286429   0950576198362     2    /dev/sdt
20468    261347  253261545   96209817917       3    /dev/sds
20468    261349  253238188   8555725937673     3    /dev/sdb

Add a new tunable zfs_multihost_history to specify the number of MMP
updates to store history for. By default it is set to zero meaning that
no MMP statistics are stored.

When using ztest to generate activity, for automated tests of the MMP
function, some test functions interfere with the test.  For example, the
pool is exported to run zdb and then imported again.  Add a new ztest
function, "-M", to alter ztest behavior to prevent this.

Add new tests to verify the new functionality.  Tests provided by
Giuseppe Di Natale.

Reviewed by: Matthew Ahrens <[email protected]>
Reviewed-by: Giuseppe Di Natale <[email protected]>
Reviewed-by: Ned Bass <[email protected]>
Reviewed-by: Andreas Dilger <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Olaf Faaland <[email protected]>
Closes #745
Closes #6279

10 files changed, 998 insertions, 60 deletions

diff --git a/module/zfs/Makefile.in b/module/zfs/Makefile.in
index f8d54f4dd..d6336f314 100644
--- a/module/zfs/Makefile.in
+++ b/module/zfs/Makefile.in
@@ -43,6 +43,7 @@ $(MODULE)-objs += gzip.o
 $(MODULE)-objs += lzjb.o
 $(MODULE)-objs += lz4.o
 $(MODULE)-objs += metaslab.o
+$(MODULE)-objs += mmp.o
 $(MODULE)-objs += multilist.o
 $(MODULE)-objs += pathname.o
 $(MODULE)-objs += policy.o
diff --git a/module/zfs/dsl_pool.c b/module/zfs/dsl_pool.c
index 97eb0cced..c16708048 100644
--- a/module/zfs/dsl_pool.c
+++ b/module/zfs/dsl_pool.c
@@ -48,6 +48,7 @@
 #include <sys/zil_impl.h>
 #include <sys/dsl_userhold.h>
 #include <sys/trace_txg.h>
+#include <sys/mmp.h>
 
 /*
  * ZFS Write Throttle
@@ -160,6 +161,7 @@ dsl_pool_open_impl(spa_t *spa, uint64_t txg)
 	dp->dp_meta_rootbp = *bp;
 	rrw_init(&dp->dp_config_rwlock, B_TRUE);
 	txg_init(dp, txg);
+	mmp_init(spa);
 
 	txg_list_create(&dp->dp_dirty_datasets, spa,
 	    offsetof(dsl_dataset_t, ds_dirty_link));
@@ -342,6 +344,7 @@ dsl_pool_close(dsl_pool_t *dp)
 	 */
 	arc_flush(dp->dp_spa, FALSE);
 
+	mmp_fini(dp->dp_spa);
 	txg_fini(dp);
 	dsl_scan_fini(dp);
 	dmu_buf_user_evict_wait();
diff --git a/module/zfs/mmp.c b/module/zfs/mmp.c
new file mode 100644
index 000000000..35348f8b4
--- /dev/null
+++ b/module/zfs/mmp.c
@@ -0,0 +1,475 @@
+/*
+ * 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 http://www.opensolaris.org/os/licensing.
+ * 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) 2017 by Lawrence Livermore National Security, LLC.
+ */
+
+#include <sys/abd.h>
+#include <sys/mmp.h>
+#include <sys/spa.h>
+#include <sys/spa_impl.h>
+#include <sys/vdev.h>
+#include <sys/vdev_impl.h>
+#include <sys/zfs_context.h>
+#include <sys/callb.h>
+
+/*
+ * Multi-Modifier Protection (MMP) attempts to prevent a user from importing
+ * or opening a pool on more than one host at a time.  In particular, it
+ * prevents "zpool import -f" on a host from succeeding while the pool is
+ * already imported on another host.  There are many other ways in which a
+ * device could be used by two hosts for different purposes at the same time
+ * resulting in pool damage.  This implementation does not attempt to detect
+ * those cases.
+ *
+ * MMP operates by ensuring there are frequent visible changes on disk (a
+ * "heartbeat") at all times.  And by altering the import process to check
+ * for these changes and failing the import when they are detected.  This
+ * functionality is enabled by setting the 'multihost' pool property to on.
+ *
+ * Uberblocks written by the txg_sync thread always go into the first
+ * (N-MMP_BLOCKS_PER_LABEL) slots, the remaining slots are reserved for MMP.
+ * They are used to hold uberblocks which are exactly the same as the last
+ * synced uberblock except that the ub_timestamp is frequently updated.
+ * Like all other uberblocks, the slot is written with an embedded checksum,
+ * and slots with invalid checksums are ignored.  This provides the
+ * "heartbeat", with no risk of overwriting good uberblocks that must be
+ * preserved, e.g. previous txgs and associated block pointers.
+ *
+ * Two optional fields are added to uberblock structure: ub_mmp_magic and
+ * ub_mmp_delay.  The magic field allows zfs to tell whether ub_mmp_delay is
+ * valid.  The delay field is a decaying average of the amount of time between
+ * completion of successive MMP writes, in nanoseconds.  It is used to predict
+ * how long the import must wait to detect activity in the pool, before
+ * concluding it is not in use.
+ *
+ * During import an activity test may now be performed to determine if
+ * the pool is in use.  The activity test is typically required if the
+ * ZPOOL_CONFIG_HOSTID does not match the system hostid, the pool state is
+ * POOL_STATE_ACTIVE, and the pool is not a root pool.
+ *
+ * The activity test finds the "best" uberblock (highest txg & timestamp),
+ * waits some time, and then finds the "best" uberblock again.  If the txg
+ * and timestamp in both "best" uberblocks do not match, the pool is in use
+ * by another host and the import fails.  Since the granularity of the
+ * timestamp is in seconds this activity test must take a bare minimum of one
+ * second.  In order to assure the accuracy of the activity test, the default
+ * values result in an activity test duration of 10x the mmp write interval.
+ *
+ * The "zpool import"  activity test can be expected to take a minimum time of
+ * zfs_multihost_import_intervals * zfs_multihost_interval milliseconds.  If the
+ * "best" uberblock has a valid ub_mmp_delay field, then the duration of the
+ * test may take longer if MMP writes were occurring less frequently than
+ * expected.  Additionally, the duration is then extended by a random 25% to
+ * attempt to to detect simultaneous imports.  For example, if both partner
+ * hosts are rebooted at the same time and automatically attempt to import the
+ * pool.
+ */
+
+/*
+ * Used to control the frequency of mmp writes which are performed when the
+ * 'multihost' pool property is on.  This is one factor used to determine the
+ * length of the activity check during import.
+ *
+ * The mmp write period is zfs_multihost_interval / leaf-vdevs milliseconds.
+ * This means that on average an mmp write will be issued for each leaf vdev
+ * every zfs_multihost_interval milliseconds.  In practice, the observed period
+ * can vary with the I/O load and this observed value is the delay which is
+ * stored in the uberblock.  The minimum allowed value is 100 ms.
+ */
+ulong_t zfs_multihost_interval = MMP_DEFAULT_INTERVAL;
+
+/*
+ * Used to control the duration of the activity test on import.  Smaller values
+ * of zfs_multihost_import_intervals will reduce the import time but increase
+ * the risk of failing to detect an active pool.  The total activity check time
+ * is never allowed to drop below one second.  A value of 0 is ignored and
+ * treated as if it was set to 1.
+ */
+uint_t zfs_multihost_import_intervals = MMP_DEFAULT_IMPORT_INTERVALS;
+
+/*
+ * Controls the behavior of the pool when mmp write failures are detected.
+ *
+ * When zfs_multihost_fail_intervals = 0 then mmp write failures are ignored.
+ * The failures will still be reported to the ZED which depending on its
+ * configuration may take action such as suspending the pool or taking a
+ * device offline.
+ *
+ * When zfs_multihost_fail_intervals > 0 then sequential mmp write failures will
+ * cause the pool to be suspended.  This occurs when
+ * zfs_multihost_fail_intervals * zfs_multihost_interval milliseconds have
+ * passed since the last successful mmp write.  This guarantees the activity
+ * test will see mmp writes if the
+ * pool is imported.
+ */
+uint_t zfs_multihost_fail_intervals = MMP_DEFAULT_FAIL_INTERVALS;
+
+static void mmp_thread(spa_t *spa);
+
+void
+mmp_init(spa_t *spa)
+{
+	mmp_thread_t *mmp = &spa->spa_mmp;
+
+	mutex_init(&mmp->mmp_thread_lock, NULL, MUTEX_DEFAULT, NULL);
+	cv_init(&mmp->mmp_thread_cv, NULL, CV_DEFAULT, NULL);
+	mutex_init(&mmp->mmp_io_lock, NULL, MUTEX_DEFAULT, NULL);
+}
+
+void
+mmp_fini(spa_t *spa)
+{
+	mmp_thread_t *mmp = &spa->spa_mmp;
+
+	mutex_destroy(&mmp->mmp_thread_lock);
+	cv_destroy(&mmp->mmp_thread_cv);
+	mutex_destroy(&mmp->mmp_io_lock);
+}
+
+static void
+mmp_thread_enter(mmp_thread_t *mmp, callb_cpr_t *cpr)
+{
+	CALLB_CPR_INIT(cpr, &mmp->mmp_thread_lock, callb_generic_cpr, FTAG);
+	mutex_enter(&mmp->mmp_thread_lock);
+}
+
+static void
+mmp_thread_exit(mmp_thread_t *mmp, kthread_t **mpp, callb_cpr_t *cpr)
+{
+	ASSERT(*mpp != NULL);
+	*mpp = NULL;
+	cv_broadcast(&mmp->mmp_thread_cv);
+	CALLB_CPR_EXIT(cpr);		/* drops &mmp->mmp_thread_lock */
+	thread_exit();
+}
+
+void
+mmp_thread_start(spa_t *spa)
+{
+	mmp_thread_t *mmp = &spa->spa_mmp;
+
+	if (spa_writeable(spa)) {
+		mutex_enter(&mmp->mmp_thread_lock);
+		if (!mmp->mmp_thread) {
+			dprintf("mmp_thread_start pool %s\n",
+			    spa->spa_name);
+			mmp->mmp_thread = thread_create(NULL, 0, mmp_thread,
+			    spa, 0, &p0, TS_RUN, defclsyspri);
+		}
+		mutex_exit(&mmp->mmp_thread_lock);
+	}
+}
+
+void
+mmp_thread_stop(spa_t *spa)
+{
+	mmp_thread_t *mmp = &spa->spa_mmp;
+
+	mutex_enter(&mmp->mmp_thread_lock);
+	mmp->mmp_thread_exiting = 1;
+	cv_broadcast(&mmp->mmp_thread_cv);
+
+	while (mmp->mmp_thread) {
+		cv_wait(&mmp->mmp_thread_cv, &mmp->mmp_thread_lock);
+	}
+	mutex_exit(&mmp->mmp_thread_lock);
+
+	ASSERT(mmp->mmp_thread == NULL);
+	mmp->mmp_thread_exiting = 0;
+}
+
+/*
+ * Randomly choose a leaf vdev, to write an MMP block to.  It must be
+ * writable.  It must not have an outstanding mmp write (if so then
+ * there is a problem, and a new write will also block).
+ *
+ * We try 10 times to pick a random leaf without an outstanding write.
+ * If 90% of the leaves have pending writes, this gives us a >65%
+ * chance of finding one we can write to.  There will be at least
+ * (zfs_multihost_fail_intervals) tries before the inability to write an MMP
+ * block causes serious problems.
+ */
+static vdev_t *
+vdev_random_leaf(spa_t *spa)
+{
+	vdev_t *vd, *child;
+	int pending_writes = 10;
+
+	ASSERT(spa);
+	ASSERT(spa_config_held(spa, SCL_STATE, RW_READER) == SCL_STATE);
+
+	/*
+	 * Since we hold SCL_STATE, neither pool nor vdev state can
+	 * change.  Therefore, if the root is not dead, there is a
+	 * child that is not dead, and so on down to a leaf.
+	 */
+	if (!vdev_writeable(spa->spa_root_vdev))
+		return (NULL);
+
+	vd = spa->spa_root_vdev;
+	while (!vd->vdev_ops->vdev_op_leaf) {
+		child = vd->vdev_child[spa_get_random(vd->vdev_children)];
+
+		if (!vdev_writeable(child))
+			continue;
+
+		if (child->vdev_ops->vdev_op_leaf && child->vdev_mmp_pending) {
+			if (pending_writes-- > 0)
+				continue;
+			else
+				return (NULL);
+		}
+
+		vd = child;
+	}
+	return (vd);
+}
+
+static void
+mmp_write_done(zio_t *zio)
+{
+	spa_t *spa = zio->io_spa;
+	vdev_t *vd = zio->io_vd;
+	mmp_thread_t *mts = zio->io_private;
+
+	mutex_enter(&mts->mmp_io_lock);
+	vd->vdev_mmp_pending = 0;
+
+	if (zio->io_error)
+		goto unlock;
+
+	/*
+	 * Mmp writes are queued on a fixed schedule, but under many
+	 * circumstances, such as a busy device or faulty hardware,
+	 * the writes will complete at variable, much longer,
+	 * intervals.  In these cases, another node checking for
+	 * activity must wait longer to account for these delays.
+	 *
+	 * The mmp_delay is calculated as a decaying average of the interval
+	 * between completed mmp writes.  This is used to predict how long
+	 * the import must wait to detect activity in the pool, before
+	 * concluding it is not in use.
+	 *
+	 * Do not set mmp_delay if the multihost property is not on,
+	 * so as not to trigger an activity check on import.
+	 */
+	if (spa_multihost(spa)) {
+		hrtime_t delay = gethrtime() - mts->mmp_last_write;
+
+		if (delay > mts->mmp_delay)
+			mts->mmp_delay = delay;
+		else
+			mts->mmp_delay = (delay + mts->mmp_delay * 127) /
+			    128;
+	} else {
+		mts->mmp_delay = 0;
+	}
+	mts->mmp_last_write = gethrtime();
+
+unlock:
+	mutex_exit(&mts->mmp_io_lock);
+
+	abd_free(zio->io_abd);
+}
+
+/*
+ * When the uberblock on-disk is updated by a spa_sync,
+ * creating a new "best" uberblock, update the one stored
+ * in the mmp thread state, used for mmp writes.
+ */
+void
+mmp_update_uberblock(spa_t *spa, uberblock_t *ub)
+{
+	mmp_thread_t *mmp = &spa->spa_mmp;
+
+	mutex_enter(&mmp->mmp_io_lock);
+	mmp->mmp_ub = *ub;
+	mmp->mmp_ub.ub_timestamp = gethrestime_sec();
+	mutex_exit(&mmp->mmp_io_lock);
+}
+
+/*
+ * Choose a random vdev, label, and MMP block, and write over it
+ * with a copy of the last-synced uberblock, whose timestamp
+ * has been updated to reflect that the pool is in use.
+ */
+static void
+mmp_write_uberblock(spa_t *spa)
+{
+	int flags = ZIO_FLAG_CONFIG_WRITER | ZIO_FLAG_CANFAIL;
+	mmp_thread_t *mmp = &spa->spa_mmp;
+	uberblock_t *ub;
+	vdev_t *vd;
+	int label;
+	uint64_t offset;
+
+	vd = vdev_random_leaf(spa);
+	if (vd == NULL || !vdev_writeable(vd))
+		return;
+
+	mutex_enter(&mmp->mmp_io_lock);
+
+	if (mmp->mmp_zio_root == NULL)
+		mmp->mmp_zio_root = zio_root(spa, NULL, NULL,
+		    flags | ZIO_FLAG_GODFATHER);
+
+	ub = &mmp->mmp_ub;
+	ub->ub_timestamp = gethrestime_sec();
+	ub->ub_mmp_magic = MMP_MAGIC;
+	ub->ub_mmp_delay = mmp->mmp_delay;
+	vd->vdev_mmp_pending = gethrtime();
+
+	zio_t *zio  = zio_null(mmp->mmp_zio_root, spa, NULL, NULL, NULL, flags);
+	abd_t *ub_abd = abd_alloc_for_io(VDEV_UBERBLOCK_SIZE(vd), B_TRUE);
+	abd_zero(ub_abd, VDEV_UBERBLOCK_SIZE(vd));
+	abd_copy_from_buf(ub_abd, ub, sizeof (uberblock_t));
+
+	mutex_exit(&mmp->mmp_io_lock);
+
+	offset = VDEV_UBERBLOCK_OFFSET(vd, VDEV_UBERBLOCK_COUNT(vd) -
+	    MMP_BLOCKS_PER_LABEL + spa_get_random(MMP_BLOCKS_PER_LABEL));
+
+	label = spa_get_random(VDEV_LABELS);
+	vdev_label_write(zio, vd, label, ub_abd, offset,
+	    VDEV_UBERBLOCK_SIZE(vd), mmp_write_done, mmp,
+	    flags | ZIO_FLAG_DONT_PROPAGATE);
+
+	spa_mmp_history_add(ub->ub_txg, ub->ub_timestamp, ub->ub_mmp_delay, vd,
+	    label);
+
+	zio_nowait(zio);
+}
+
+static void
+mmp_thread(spa_t *spa)
+{
+	mmp_thread_t *mmp = &spa->spa_mmp;
+	boolean_t last_spa_suspended = spa_suspended(spa);
+	boolean_t last_spa_multihost = spa_multihost(spa);
+	callb_cpr_t cpr;
+	hrtime_t max_fail_ns = zfs_multihost_fail_intervals *
+	    MSEC2NSEC(MAX(zfs_multihost_interval, MMP_MIN_INTERVAL));
+
+	mmp_thread_enter(mmp, &cpr);
+
+	/*
+	 * The mmp_write_done() function calculates mmp_delay based on the
+	 * prior value of mmp_delay and the elapsed time since the last write.
+	 * For the first mmp write, there is no "last write", so we start
+	 * with fake, but reasonable, default non-zero values.
+	 */
+	mmp->mmp_delay = MSEC2NSEC(MAX(zfs_multihost_interval,
+	    MMP_MIN_INTERVAL)) / vdev_count_leaves(spa);
+	mmp->mmp_last_write = gethrtime() - mmp->mmp_delay;
+
+	while (!mmp->mmp_thread_exiting) {
+		uint64_t mmp_fail_intervals = zfs_multihost_fail_intervals;
+		uint64_t mmp_interval = MSEC2NSEC(
+		    MAX(zfs_multihost_interval, MMP_MIN_INTERVAL));
+		boolean_t suspended = spa_suspended(spa);
+		boolean_t multihost = spa_multihost(spa);
+		hrtime_t start, next_time;
+
+		start = gethrtime();
+		if (multihost) {
+			next_time = start + mmp_interval /
+			    vdev_count_leaves(spa);
+		} else {
+			next_time = start + MSEC2NSEC(MMP_DEFAULT_INTERVAL);
+		}
+
+		/*
+		 * When MMP goes off => on, or spa goes suspended =>
+		 * !suspended, we know no writes occurred recently.  We
+		 * update mmp_last_write to give us some time to try.
+		 */
+		if ((!last_spa_multihost && multihost) ||
+		    (last_spa_suspended && !suspended)) {
+			mutex_enter(&mmp->mmp_io_lock);
+			mmp->mmp_last_write = gethrtime();
+			mutex_exit(&mmp->mmp_io_lock);
+		} else if (last_spa_multihost && !multihost) {
+			mutex_enter(&mmp->mmp_io_lock);
+			mmp->mmp_delay = 0;
+			mutex_exit(&mmp->mmp_io_lock);
+		}
+		last_spa_multihost = multihost;
+		last_spa_suspended = suspended;
+
+		/*
+		 * Smooth max_fail_ns when its factors are decreased, because
+		 * making (max_fail_ns < mmp_interval) results in the pool being
+		 * immediately suspended before writes can occur at the new
+		 * higher frequency.
+		 */
+		if ((mmp_interval * mmp_fail_intervals) < max_fail_ns) {
+			max_fail_ns = ((31 * max_fail_ns) + (mmp_interval *
+			    mmp_fail_intervals)) / 32;
+		} else {
+			max_fail_ns = mmp_interval * mmp_fail_intervals;
+		}
+
+		/*
+		 * Suspend the pool if no MMP write has succeeded in over
+		 * mmp_interval * mmp_fail_intervals nanoseconds.
+		 */
+		if (!suspended && mmp_fail_intervals && multihost &&
+		    (start - mmp->mmp_last_write) > max_fail_ns) {
+			zio_suspend(spa, NULL);
+		}
+
+		if (multihost) {
+			spa_config_enter(spa, SCL_STATE, FTAG, RW_READER);
+			mmp_write_uberblock(spa);
+			spa_config_exit(spa, SCL_STATE, FTAG);
+		}
+
+		CALLB_CPR_SAFE_BEGIN(&cpr);
+		(void) cv_timedwait_sig(&mmp->mmp_thread_cv,
+		    &mmp->mmp_thread_lock, ddi_get_lbolt() +
+		    ((next_time - gethrtime()) / (NANOSEC / HZ)));
+		CALLB_CPR_SAFE_END(&cpr, &mmp->mmp_thread_lock);
+	}
+
+	/* Outstanding writes are allowed to complete. */
+	if (mmp->mmp_zio_root)
+		zio_wait(mmp->mmp_zio_root);
+
+	mmp->mmp_zio_root = NULL;
+	mmp_thread_exit(mmp, &mmp->mmp_thread, &cpr);
+}
+
+#if defined(_KERNEL) && defined(HAVE_SPL)
+/* BEGIN CSTYLED */
+module_param(zfs_multihost_fail_intervals, uint, 0644);
+MODULE_PARM_DESC(zfs_multihost_fail_intervals,
+	"Max allowed period without a successful mmp write");
+
+module_param(zfs_multihost_interval, ulong, 0644);
+MODULE_PARM_DESC(zfs_multihost_interval,
+	"Milliseconds between mmp writes to each leaf");
+
+module_param(zfs_multihost_import_intervals, uint, 0644);
+MODULE_PARM_DESC(zfs_multihost_import_intervals,
+	"Number of zfs_multihost_interval periods to wait for activity");
+/* END CSTYLED */
+#endif
diff --git a/module/zfs/spa.c b/module/zfs/spa.c
index 52420194c..7edf0459c 100644
--- a/module/zfs/spa.c
+++ b/module/zfs/spa.c
@@ -55,6 +55,7 @@
 #include <sys/vdev_disk.h>
 #include <sys/metaslab.h>
 #include <sys/metaslab_impl.h>
+#include <sys/mmp.h>
 #include <sys/uberblock_impl.h>
 #include <sys/txg.h>
 #include <sys/avl.h>
@@ -491,6 +492,16 @@ spa_prop_validate(spa_t *spa, nvlist_t *props)
 				error = SET_ERROR(EINVAL);
 			break;
 
+		case ZPOOL_PROP_MULTIHOST:
+			error = nvpair_value_uint64(elem, &intval);
+			if (!error && intval > 1)
+				error = SET_ERROR(EINVAL);
+
+			if (!error && !spa_get_hostid())
+				error = SET_ERROR(ENOTSUP);
+
+			break;
+
 		case ZPOOL_PROP_BOOTFS:
 			/*
 			 * If the pool version is less than SPA_VERSION_BOOTFS,
@@ -1346,6 +1357,9 @@ spa_unload(spa_t *spa)
 		spa_config_exit(spa, SCL_ALL, FTAG);
 	}
 
+	if (spa->spa_mmp.mmp_thread)
+		mmp_thread_stop(spa);
+
 	/*
 	 * Wait for any outstanding async I/O to complete.
 	 */
@@ -2324,6 +2338,197 @@ vdev_count_verify_zaps(vdev_t *vd)
 #endif
 
 /*
+ * Determine whether the activity check is required.
+ */
+static boolean_t
+spa_activity_check_required(spa_t *spa, uberblock_t *ub, nvlist_t *config)
+{
+	uint64_t state = 0;
+	uint64_t hostid = 0;
+	uint64_t tryconfig_txg = 0;
+	uint64_t tryconfig_timestamp = 0;
+	nvlist_t *nvinfo;
+
+	if (nvlist_exists(config, ZPOOL_CONFIG_LOAD_INFO)) {
+		nvinfo = fnvlist_lookup_nvlist(config, ZPOOL_CONFIG_LOAD_INFO);
+		(void) nvlist_lookup_uint64(nvinfo, ZPOOL_CONFIG_MMP_TXG,
+		    &tryconfig_txg);
+		(void) nvlist_lookup_uint64(config, ZPOOL_CONFIG_TIMESTAMP,
+		    &tryconfig_timestamp);
+	}
+
+	(void) nvlist_lookup_uint64(config, ZPOOL_CONFIG_POOL_STATE, &state);
+	(void) nvlist_lookup_uint64(config, ZPOOL_CONFIG_HOSTID, &hostid);
+
+	/*
+	 * Disable the MMP activity check - This is used by zdb which
+	 * is intended to be used on potentially active pools.
+	 */
+	if (spa->spa_import_flags & ZFS_IMPORT_SKIP_MMP)
+		return (B_FALSE);
+
+	/*
+	 * Skip the activity check when the MMP feature is disabled.
+	 */
+	if (ub->ub_mmp_magic == MMP_MAGIC && ub->ub_mmp_delay == 0)
+		return (B_FALSE);
+	/*
+	 * If the tryconfig_* values are nonzero, they are the results of an
+	 * earlier tryimport.  If they match the uberblock we just found, then
+	 * the pool has not changed and we return false so we do not test a
+	 * second time.
+	 */
+	if (tryconfig_txg && tryconfig_txg == ub->ub_txg &&
+	    tryconfig_timestamp && tryconfig_timestamp == ub->ub_timestamp)
+		return (B_FALSE);
+
+	/*
+	 * Allow the activity check to be skipped when importing the pool
+	 * on the same host which last imported it.
+	 */
+	if (hostid == spa_get_hostid())
+		return (B_FALSE);
+
+	/*
+	 * Skip the activity test when the pool was cleanly exported.
+	 */
+	if (state != POOL_STATE_ACTIVE)
+		return (B_FALSE);
+
+	return (B_TRUE);
+}
+
+/*
+ * Perform the import activity check.  If the user canceled the import or
+ * we detected activity then fail.
+ */
+static int
+spa_activity_check(spa_t *spa, uberblock_t *ub, nvlist_t *config)
+{
+	uint64_t import_intervals = MAX(zfs_multihost_import_intervals, 1);
+	uint64_t txg = ub->ub_txg;
+	uint64_t timestamp = ub->ub_timestamp;
+	uint64_t import_delay = NANOSEC;
+	hrtime_t import_expire;
+	nvlist_t *mmp_label = NULL;
+	vdev_t *rvd = spa->spa_root_vdev;
+	kcondvar_t cv;
+	kmutex_t mtx;
+	int error = 0;
+
+	cv_init(&cv, NULL, CV_DEFAULT, NULL);
+	mutex_init(&mtx, NULL, MUTEX_DEFAULT, NULL);
+	mutex_enter(&mtx);
+
+	/*
+	 * If ZPOOL_CONFIG_MMP_TXG is present an activity check was performed
+	 * during the earlier tryimport.  If the txg recorded there is 0 then
+	 * the pool is known to be active on another host.
+	 *
+	 * Otherwise, the pool might be in use on another node.  Check for
+	 * changes in the uberblocks on disk if necessary.
+	 */
+	if (nvlist_exists(config, ZPOOL_CONFIG_LOAD_INFO)) {
+		nvlist_t *nvinfo = fnvlist_lookup_nvlist(config,
+		    ZPOOL_CONFIG_LOAD_INFO);
+
+		if (nvlist_exists(nvinfo, ZPOOL_CONFIG_MMP_TXG) &&
+		    fnvlist_lookup_uint64(nvinfo, ZPOOL_CONFIG_MMP_TXG) == 0) {
+			vdev_uberblock_load(rvd, ub, &mmp_label);
+			error = SET_ERROR(EREMOTEIO);
+			goto out;
+		}
+	}
+
+	/*
+	 * Preferentially use the zfs_multihost_interval from the node which
+	 * last imported the pool.  This value is stored in an MMP uberblock as.
+	 *
+	 * ub_mmp_delay * vdev_count_leaves() == zfs_multihost_interval
+	 */
+	if (ub->ub_mmp_magic == MMP_MAGIC && ub->ub_mmp_delay)
+		import_delay = MAX(import_delay, import_intervals *
+		    ub->ub_mmp_delay * vdev_count_leaves(spa));
+
+	/* Apply a floor using the local default values. */
+	import_delay = MAX(import_delay, import_intervals *
+	    MSEC2NSEC(MAX(zfs_multihost_interval, MMP_MIN_INTERVAL)));
+
+	/* Add a small random factor in case of simultaneous imports (0-25%) */
+	import_expire = gethrtime() + import_delay +
+	    (import_delay * spa_get_random(250) / 1000);
+
+	while (gethrtime() < import_expire) {
+		vdev_uberblock_load(rvd, ub, &mmp_label);
+
+		if (txg != ub->ub_txg || timestamp != ub->ub_timestamp) {
+			error = SET_ERROR(EREMOTEIO);
+			break;
+		}
+
+		if (mmp_label) {
+			nvlist_free(mmp_label);
+			mmp_label = NULL;
+		}
+
+		error = cv_timedwait_sig(&cv, &mtx, ddi_get_lbolt() + hz);
+		if (error != -1) {
+			error = SET_ERROR(EINTR);
+			break;
+		}
+		error = 0;
+	}
+
+out:
+	mutex_exit(&mtx);
+	mutex_destroy(&mtx);
+	cv_destroy(&cv);
+
+	/*
+	 * If the pool is determined to be active store the status in the
+	 * spa->spa_load_info nvlist.  If the remote hostname or hostid are
+	 * available from configuration read from disk store them as well.
+	 * This allows 'zpool import' to generate a more useful message.
+	 *
+	 * ZPOOL_CONFIG_MMP_STATE    - observed pool status (mandatory)
+	 * ZPOOL_CONFIG_MMP_HOSTNAME - hostname from the active pool
+	 * ZPOOL_CONFIG_MMP_HOSTID   - hostid from the active pool
+	 */
+	if (error == EREMOTEIO) {
+		char *hostname = "<unknown>";
+		uint64_t hostid = 0;
+
+		if (mmp_label) {
+			if (nvlist_exists(mmp_label, ZPOOL_CONFIG_HOSTNAME)) {
+				hostname = fnvlist_lookup_string(mmp_label,
+				    ZPOOL_CONFIG_HOSTNAME);
+				fnvlist_add_string(spa->spa_load_info,
+				    ZPOOL_CONFIG_MMP_HOSTNAME, hostname);
+			}
+
+			if (nvlist_exists(mmp_label, ZPOOL_CONFIG_HOSTID)) {
+				hostid = fnvlist_lookup_uint64(mmp_label,
+				    ZPOOL_CONFIG_HOSTID);
+				fnvlist_add_uint64(spa->spa_load_info,
+				    ZPOOL_CONFIG_MMP_HOSTID, hostid);
+			}
+		}
+
+		fnvlist_add_uint64(spa->spa_load_info,
+		    ZPOOL_CONFIG_MMP_STATE, MMP_STATE_ACTIVE);
+		fnvlist_add_uint64(spa->spa_load_info,
+		    ZPOOL_CONFIG_MMP_TXG, 0);
+
+		error = spa_vdev_err(rvd, VDEV_AUX_ACTIVE, EREMOTEIO);
+	}
+
+	if (mmp_label)
+		nvlist_free(mmp_label);
+
+	return (error);
+}
+
+/*
  * Load an existing storage pool, using the pool's builtin spa_config as a
  * source of configuration information.
  */
@@ -2343,6 +2548,7 @@ spa_load_impl(spa_t *spa, uint64_t pool_guid, nvlist_t *config,
 	int parse, i;
 	uint64_t obj;
 	boolean_t missing_feat_write = B_FALSE;
+	boolean_t activity_check = B_FALSE;
 	nvlist_t *mos_config;
 
 	/*
@@ -2441,6 +2647,33 @@ spa_load_impl(spa_t *spa, uint64_t pool_guid, nvlist_t *config,
 	}
 
 	/*
+	 * For pools which have the multihost property on determine if the
+	 * pool is truly inactive and can be safely imported.  Prevent
+	 * hosts which don't have a hostid set from importing the pool.
+	 */
+	activity_check = spa_activity_check_required(spa, ub, config);
+	if (activity_check) {
+		error = spa_activity_check(spa, ub, config);
+		if (error) {
+			nvlist_free(label);
+			return (error);
+		}
+
+		if (ub->ub_mmp_magic == MMP_MAGIC && ub->ub_mmp_delay &&
+		    spa_get_hostid() == 0) {
+			nvlist_free(label);
+			fnvlist_add_uint64(spa->spa_load_info,
+			    ZPOOL_CONFIG_MMP_STATE, MMP_STATE_NO_HOSTID);
+			return (spa_vdev_err(rvd, VDEV_AUX_ACTIVE, EREMOTEIO));
+		}
+
+		fnvlist_add_uint64(spa->spa_load_info,
+		    ZPOOL_CONFIG_MMP_STATE, MMP_STATE_INACTIVE);
+		fnvlist_add_uint64(spa->spa_load_info,
+		    ZPOOL_CONFIG_MMP_TXG, ub->ub_txg);
+	}
+
+	/*
 	 * If the pool has an unsupported version we can't open it.
 	 */
 	if (!SPA_VERSION_IS_SUPPORTED(ub->ub_version)) {
@@ -2667,24 +2900,9 @@ spa_load_impl(spa_t *spa, uint64_t pool_guid, nvlist_t *config,
 			VERIFY(nvlist_lookup_string(nvconfig,
 			    ZPOOL_CONFIG_HOSTNAME, &hostname) == 0);
 
-#ifdef	_KERNEL
-			myhostid = zone_get_hostid(NULL);
-#else	/* _KERNEL */
-			/*
-			 * We're emulating the system's hostid in userland, so
-			 * we can't use zone_get_hostid().
-			 */
-			(void) ddi_strtoul(hw_serial, NULL, 10, &myhostid);
-#endif	/* _KERNEL */
-			if (hostid != 0 && myhostid != 0 &&
-			    hostid != myhostid) {
+			myhostid = spa_get_hostid();
+			if (hostid && myhostid && hostid != myhostid) {
 				nvlist_free(nvconfig);
-				cmn_err(CE_WARN, "pool '%s' could not be "
-				    "loaded as it was last accessed by another "
-				    "system (host: %s hostid: 0x%lx). See: "
-				    "http://zfsonlinux.org/msg/ZFS-8000-EY",
-				    spa_name(spa), hostname,
-				    (unsigned long)hostid);
 				return (SET_ERROR(EBADF));
 			}
 		}
@@ -2850,6 +3068,7 @@ spa_load_impl(spa_t *spa, uint64_t pool_guid, nvlist_t *config,
 		spa_prop_find(spa, ZPOOL_PROP_DELEGATION, &spa->spa_delegation);
 		spa_prop_find(spa, ZPOOL_PROP_FAILUREMODE, &spa->spa_failmode);
 		spa_prop_find(spa, ZPOOL_PROP_AUTOEXPAND, &spa->spa_autoexpand);
+		spa_prop_find(spa, ZPOOL_PROP_MULTIHOST, &spa->spa_multihost);
 		spa_prop_find(spa, ZPOOL_PROP_DEDUPDITTO,
 		    &spa->spa_dedup_ditto);
 
@@ -2857,6 +3076,18 @@ spa_load_impl(spa_t *spa, uint64_t pool_guid, nvlist_t *config,
 	}
 
 	/*
+	 * If the 'multihost' property is set, then never allow a pool to
+	 * be imported when the system hostid is zero.  The exception to
+	 * this rule is zdb which is always allowed to access pools.
+	 */
+	if (spa_multihost(spa) && spa_get_hostid() == 0 &&
+	    (spa->spa_import_flags & ZFS_IMPORT_SKIP_MMP) == 0) {
+		fnvlist_add_uint64(spa->spa_load_info,
+		    ZPOOL_CONFIG_MMP_STATE, MMP_STATE_NO_HOSTID);
+		return (spa_vdev_err(rvd, VDEV_AUX_ACTIVE, EREMOTEIO));
+	}
+
+	/*
 	 * If the 'autoreplace' property is set, then post a resource notifying
 	 * the ZFS DE that it should not issue any faults for unopenable
 	 * devices.  We also iterate over the vdevs, and post a sysevent for any
@@ -2980,6 +3211,7 @@ spa_load_impl(spa_t *spa, uint64_t pool_guid, nvlist_t *config,
 		spa_set_log_state(spa, SPA_LOG_GOOD);
 		spa->spa_sync_on = B_TRUE;
 		txg_sync_start(spa->spa_dsl_pool);
+		mmp_thread_start(spa);
 
 		/*
 		 * Wait for all claims to sync.  We sync up to the highest
@@ -3632,18 +3864,6 @@ spa_validate_aux_devs(spa_t *spa, nvlist_t *nvroot, uint64_t crtxg, int mode,
 			goto out;
 		}
 
-		/*
-		 * The L2ARC currently only supports disk devices in
-		 * kernel context.  For user-level testing, we allow it.
-		 */
-#ifdef _KERNEL
-		if ((strcmp(config, ZPOOL_CONFIG_L2CACHE) == 0) &&
-		    strcmp(vd->vdev_ops->vdev_op_type, VDEV_TYPE_DISK) != 0) {
-			error = SET_ERROR(ENOTBLK);
-			vdev_free(vd);
-			goto out;
-		}
-#endif
 		vd->vdev_top = vd;
 
 		if ((error = vdev_open(vd)) == 0 &&
@@ -3986,6 +4206,7 @@ spa_create(const char *pool, nvlist_t *nvroot, nvlist_t *props,
 	spa->spa_delegation = zpool_prop_default_numeric(ZPOOL_PROP_DELEGATION);
 	spa->spa_failmode = zpool_prop_default_numeric(ZPOOL_PROP_FAILUREMODE);
 	spa->spa_autoexpand = zpool_prop_default_numeric(ZPOOL_PROP_AUTOEXPAND);
+	spa->spa_multihost = zpool_prop_default_numeric(ZPOOL_PROP_MULTIHOST);
 
 	if (props != NULL) {
 		spa_configfile_set(spa, props, B_FALSE);
@@ -3996,6 +4217,7 @@ spa_create(const char *pool, nvlist_t *nvroot, nvlist_t *props,
 
 	spa->spa_sync_on = B_TRUE;
 	txg_sync_start(spa->spa_dsl_pool);
+	mmp_thread_start(spa);
 
 	/*
 	 * We explicitly wait for the first transaction to complete so that our
@@ -6405,6 +6627,9 @@ spa_sync_props(void *arg, dmu_tx_t *tx)
 					spa_async_request(spa,
 					    SPA_ASYNC_AUTOEXPAND);
 				break;
+			case ZPOOL_PROP_MULTIHOST:
+				spa->spa_multihost = intval;
+				break;
 			case ZPOOL_PROP_DEDUPDITTO:
 				spa->spa_dedup_ditto = intval;
 				break;
diff --git a/module/zfs/spa_config.c b/module/zfs/spa_config.c
index 2715898cc..5b792b868 100644
--- a/module/zfs/spa_config.c
+++ b/module/zfs/spa_config.c
@@ -436,15 +436,7 @@ spa_config_generate(spa_t *spa, vdev_t *vd, uint64_t txg, int getstats)
 		fnvlist_add_string(config, ZPOOL_CONFIG_COMMENT,
 		    spa->spa_comment);
 
-#ifdef	_KERNEL
-	hostid = zone_get_hostid(NULL);
-#else	/* _KERNEL */
-	/*
-	 * We're emulating the system's hostid in userland, so we can't use
-	 * zone_get_hostid().
-	 */
-	(void) ddi_strtoul(hw_serial, NULL, 10, &hostid);
-#endif	/* _KERNEL */
+	hostid = spa_get_hostid();
 	if (hostid != 0)
 		fnvlist_add_uint64(config, ZPOOL_CONFIG_HOSTID, hostid);
 	fnvlist_add_string(config, ZPOOL_CONFIG_HOSTNAME, utsname()->nodename);
diff --git a/module/zfs/spa_misc.c b/module/zfs/spa_misc.c
index 09e5067a5..3787e010f 100644
--- a/module/zfs/spa_misc.c
+++ b/module/zfs/spa_misc.c
@@ -1384,6 +1384,9 @@ spa_get_random(uint64_t range)
 
 	ASSERT(range != 0);
 
+	if (range == 1)
+		return (0);
+
 	(void) random_get_pseudo_bytes((void *)&r, sizeof (uint64_t));
 
 	return (r % range);
@@ -2073,6 +2076,30 @@ spa_maxdnodesize(spa_t *spa)
 		return (DNODE_MIN_SIZE);
 }
 
+boolean_t
+spa_multihost(spa_t *spa)
+{
+	return (spa->spa_multihost ? B_TRUE : B_FALSE);
+}
+
+unsigned long
+spa_get_hostid(void)
+{
+	unsigned long myhostid;
+
+#ifdef	_KERNEL
+	myhostid = zone_get_hostid(NULL);
+#else	/* _KERNEL */
+	/*
+	 * We're emulating the system's hostid in userland, so
+	 * we can't use zone_get_hostid().
+	 */
+	(void) ddi_strtoul(hw_serial, NULL, 10, &myhostid);
+#endif	/* _KERNEL */
+
+	return (myhostid);
+}
+
 #if defined(_KERNEL) && defined(HAVE_SPL)
 /* Namespace manipulation */
 EXPORT_SYMBOL(spa_lookup);
diff --git a/module/zfs/spa_stats.c b/module/zfs/spa_stats.c
index ac1fb8c6f..7ca359806 100644
--- a/module/zfs/spa_stats.c
+++ b/module/zfs/spa_stats.c
@@ -21,6 +21,7 @@
 
 #include <sys/zfs_context.h>
 #include <sys/spa_impl.h>
+#include <sys/vdev_impl.h>
 
 /*
  * Keeps stats on last N reads per spa_t, disabled by default.
@@ -38,6 +39,11 @@ int zfs_read_history_hits = 0;
 int zfs_txg_history = 0;
 
 /*
+ * Keeps stats on the last N MMP updates, disabled by default.
+ */
+int zfs_multihost_history = 0;
+
+/*
  * ==========================================================================
  * SPA Read History Routines
  * ==========================================================================
@@ -701,6 +707,198 @@ spa_io_history_destroy(spa_t *spa)
 	mutex_destroy(&ssh->lock);
 }
 
+/*
+ * ==========================================================================
+ * SPA MMP History Routines
+ * ==========================================================================
+ */
+
+/*
+ * MMP statistics - Information exported regarding each MMP update
+ */
+
+typedef struct spa_mmp_history {
+	uint64_t	txg;		/* txg of last sync */
+	uint64_t	timestamp;	/* UTC time of of last sync */
+	uint64_t	mmp_delay;	/* nanosec since last MMP write */
+	uint64_t	vdev_guid;	/* unique ID of leaf vdev */
+	char		*vdev_path;
+	uint64_t	vdev_label;	/* vdev label */
+	list_node_t	smh_link;
+} spa_mmp_history_t;
+
+static int
+spa_mmp_history_headers(char *buf, size_t size)
+{
+	(void) snprintf(buf, size, "%-10s %-10s %-12s %-24s %-10s %s\n",
+	    "txg", "timestamp", "mmp_delay", "vdev_guid", "vdev_label",
+	    "vdev_path");
+	return (0);
+}
+
+static int
+spa_mmp_history_data(char *buf, size_t size, void *data)
+{
+	spa_mmp_history_t *smh = (spa_mmp_history_t *)data;
+
+	(void) snprintf(buf, size, "%-10llu %-10llu %-12llu %-24llu %-10llu "
+	    "%s\n",
+	    (u_longlong_t)smh->txg, (u_longlong_t)smh->timestamp,
+	    (u_longlong_t)smh->mmp_delay, (u_longlong_t)smh->vdev_guid,
+	    (u_longlong_t)smh->vdev_label,
+	    (smh->vdev_path ? smh->vdev_path : "-"));
+
+	return (0);
+}
+
+/*
+ * Calculate the address for the next spa_stats_history_t entry.  The
+ * ssh->lock will be held until ksp->ks_ndata entries are processed.
+ */
+static void *
+spa_mmp_history_addr(kstat_t *ksp, loff_t n)
+{
+	spa_t *spa = ksp->ks_private;
+	spa_stats_history_t *ssh = &spa->spa_stats.mmp_history;
+
+	ASSERT(MUTEX_HELD(&ssh->lock));
+
+	if (n == 0)
+		ssh->private = list_tail(&ssh->list);
+	else if (ssh->private)
+		ssh->private = list_prev(&ssh->list, ssh->private);
+
+	return (ssh->private);
+}
+
+/*
+ * When the kstat is written discard all spa_mmp_history_t entries.  The
+ * ssh->lock will be held until ksp->ks_ndata entries are processed.
+ */
+static int
+spa_mmp_history_update(kstat_t *ksp, int rw)
+{
+	spa_t *spa = ksp->ks_private;
+	spa_stats_history_t *ssh = &spa->spa_stats.mmp_history;
+
+	ASSERT(MUTEX_HELD(&ssh->lock));
+
+	if (rw == KSTAT_WRITE) {
+		spa_mmp_history_t *smh;
+
+		while ((smh = list_remove_head(&ssh->list))) {
+			ssh->size--;
+			if (smh->vdev_path)
+				strfree(smh->vdev_path);
+			kmem_free(smh, sizeof (spa_mmp_history_t));
+		}
+
+		ASSERT3U(ssh->size, ==, 0);
+	}
+
+	ksp->ks_ndata = ssh->size;
+	ksp->ks_data_size = ssh->size * sizeof (spa_mmp_history_t);
+
+	return (0);
+}
+
+static void
+spa_mmp_history_init(spa_t *spa)
+{
+	spa_stats_history_t *ssh = &spa->spa_stats.mmp_history;
+	char name[KSTAT_STRLEN];
+	kstat_t *ksp;
+
+	mutex_init(&ssh->lock, NULL, MUTEX_DEFAULT, NULL);
+	list_create(&ssh->list, sizeof (spa_mmp_history_t),
+	    offsetof(spa_mmp_history_t, smh_link));
+
+	ssh->count = 0;
+	ssh->size = 0;
+	ssh->private = NULL;
+
+	(void) snprintf(name, KSTAT_STRLEN, "zfs/%s", spa_name(spa));
+
+	ksp = kstat_create(name, 0, "multihost", "misc",
+	    KSTAT_TYPE_RAW, 0, KSTAT_FLAG_VIRTUAL);
+	ssh->kstat = ksp;
+
+	if (ksp) {
+		ksp->ks_lock = &ssh->lock;
+		ksp->ks_data = NULL;
+		ksp->ks_private = spa;
+		ksp->ks_update = spa_mmp_history_update;
+		kstat_set_raw_ops(ksp, spa_mmp_history_headers,
+		    spa_mmp_history_data, spa_mmp_history_addr);
+		kstat_install(ksp);
+	}
+}
+
+static void
+spa_mmp_history_destroy(spa_t *spa)
+{
+	spa_stats_history_t *ssh = &spa->spa_stats.mmp_history;
+	spa_mmp_history_t *smh;
+	kstat_t *ksp;
+
+	ksp = ssh->kstat;
+	if (ksp)
+		kstat_delete(ksp);
+
+	mutex_enter(&ssh->lock);
+	while ((smh = list_remove_head(&ssh->list))) {
+		ssh->size--;
+		if (smh->vdev_path)
+			strfree(smh->vdev_path);
+		kmem_free(smh, sizeof (spa_mmp_history_t));
+	}
+
+	ASSERT3U(ssh->size, ==, 0);
+	list_destroy(&ssh->list);
+	mutex_exit(&ssh->lock);
+
+	mutex_destroy(&ssh->lock);
+}
+
+/*
+ * Add a new MMP update to historical record.
+ */
+void
+spa_mmp_history_add(uint64_t txg, uint64_t timestamp, uint64_t mmp_delay,
+    vdev_t *vd, int label)
+{
+	spa_t *spa = vd->vdev_spa;
+	spa_stats_history_t *ssh = &spa->spa_stats.mmp_history;
+	spa_mmp_history_t *smh, *rm;
+
+	if (zfs_multihost_history == 0 && ssh->size == 0)
+		return;
+
+	smh = kmem_zalloc(sizeof (spa_mmp_history_t), KM_SLEEP);
+	smh->txg = txg;
+	smh->timestamp = timestamp;
+	smh->mmp_delay = mmp_delay;
+	smh->vdev_guid = vd->vdev_guid;
+	if (vd->vdev_path)
+		smh->vdev_path = strdup(vd->vdev_path);
+	smh->vdev_label = label;
+
+	mutex_enter(&ssh->lock);
+
+	list_insert_head(&ssh->list, smh);
+	ssh->size++;
+
+	while (ssh->size > zfs_multihost_history) {
+		ssh->size--;
+		rm = list_remove_tail(&ssh->list);
+		if (rm->vdev_path)
+			strfree(rm->vdev_path);
+		kmem_free(rm, sizeof (spa_mmp_history_t));
+	}
+
+	mutex_exit(&ssh->lock);
+}
+
 void
 spa_stats_init(spa_t *spa)
 {
@@ -708,6 +906,7 @@ spa_stats_init(spa_t *spa)
 	spa_txg_history_init(spa);
 	spa_tx_assign_init(spa);
 	spa_io_history_init(spa);
+	spa_mmp_history_init(spa);
 }
 
 void
@@ -717,15 +916,25 @@ spa_stats_destroy(spa_t *spa)
 	spa_txg_history_destroy(spa);
 	spa_read_history_destroy(spa);
 	spa_io_history_destroy(spa);
+	spa_mmp_history_destroy(spa);
 }
 
 #if defined(_KERNEL) && defined(HAVE_SPL)
+/* CSTYLED */
 module_param(zfs_read_history, int, 0644);
-MODULE_PARM_DESC(zfs_read_history, "Historic statistics for the last N reads");
+MODULE_PARM_DESC(zfs_read_history,
+	"Historical statistics for the last N reads");
 
 module_param(zfs_read_history_hits, int, 0644);
-MODULE_PARM_DESC(zfs_read_history_hits, "Include cache hits in read history");
+MODULE_PARM_DESC(zfs_read_history_hits,
+	"Include cache hits in read history");
 
 module_param(zfs_txg_history, int, 0644);
-MODULE_PARM_DESC(zfs_txg_history, "Historic statistics for the last N txgs");
+MODULE_PARM_DESC(zfs_txg_history,
+	"Historical statistics for the last N txgs");
+
+module_param(zfs_multihost_history, int, 0644);
+MODULE_PARM_DESC(zfs_multihost_history,
+	"Historical statistics for last N multihost writes");
+/* END CSTYLED */
 #endif
diff --git a/module/zfs/uberblock.c b/module/zfs/uberblock.c
index f8bdecdf5..c1e85bdce 100644
--- a/module/zfs/uberblock.c
+++ b/module/zfs/uberblock.c
@@ -44,7 +44,7 @@ uberblock_verify(uberblock_t *ub)
  * transaction group.
  */
 boolean_t
-uberblock_update(uberblock_t *ub, vdev_t *rvd, uint64_t txg)
+uberblock_update(uberblock_t *ub, vdev_t *rvd, uint64_t txg, uint64_t mmp_delay)
 {
 	ASSERT(ub->ub_txg < txg);
 
@@ -57,6 +57,9 @@ uberblock_update(uberblock_t *ub, vdev_t *rvd, uint64_t txg)
 	ub->ub_guid_sum = rvd->vdev_guid_sum;
 	ub->ub_timestamp = gethrestime_sec();
 	ub->ub_software_version = SPA_VERSION;
+	ub->ub_mmp_magic = MMP_MAGIC;
+	ub->ub_mmp_delay = spa_multihost(rvd->vdev_spa) ? mmp_delay : 0;
+	ub->ub_mmp_seq = 0;
 
 	return (ub->ub_rootbp.blk_birth == txg);
 }
diff --git a/module/zfs/vdev_label.c b/module/zfs/vdev_label.c
index 021f4774b..a0a02366e 100644
--- a/module/zfs/vdev_label.c
+++ b/module/zfs/vdev_label.c
@@ -193,7 +193,7 @@ vdev_label_read(zio_t *zio, vdev_t *vd, int l, abd_t *buf, uint64_t offset,
 	    ZIO_PRIORITY_SYNC_READ, flags, B_TRUE));
 }
 
-static void
+void
 vdev_label_write(zio_t *zio, vdev_t *vd, int l, abd_t *buf, uint64_t offset,
     uint64_t size, zio_done_func_t *done, void *private, int flags)
 {
@@ -1082,14 +1082,12 @@ static void
 vdev_uberblock_load_impl(zio_t *zio, vdev_t *vd, int flags,
     struct ubl_cbdata *cbp)
 {
-	int c, l, n;
-
-	for (c = 0; c < vd->vdev_children; c++)
+	for (int c = 0; c < vd->vdev_children; c++)
 		vdev_uberblock_load_impl(zio, vd->vdev_child[c], flags, cbp);
 
 	if (vd->vdev_ops->vdev_op_leaf && vdev_readable(vd)) {
-		for (l = 0; l < VDEV_LABELS; l++) {
-			for (n = 0; n < VDEV_UBERBLOCK_COUNT(vd); n++) {
+		for (int l = 0; l < VDEV_LABELS; l++) {
+			for (int n = 0; n < VDEV_UBERBLOCK_COUNT(vd); n++) {
 				vdev_label_read(zio, vd, l,
 				    abd_alloc_linear(VDEV_UBERBLOCK_SIZE(vd),
 				    B_TRUE), VDEV_UBERBLOCK_OFFSET(vd, n),
@@ -1213,10 +1211,7 @@ vdev_uberblock_sync_done(zio_t *zio)
 static void
 vdev_uberblock_sync(zio_t *zio, uberblock_t *ub, vdev_t *vd, int flags)
 {
-	abd_t *ub_abd;
-	int c, l, n;
-
-	for (c = 0; c < vd->vdev_children; c++)
+	for (int c = 0; c < vd->vdev_children; c++)
 		vdev_uberblock_sync(zio, ub, vd->vdev_child[c], flags);
 
 	if (!vd->vdev_ops->vdev_op_leaf)
@@ -1232,14 +1227,15 @@ vdev_uberblock_sync(zio_t *zio, uberblock_t *ub, vdev_t *vd, int flags)
 		vd->vdev_copy_uberblocks = B_FALSE;
 	}
 
-	n = ub->ub_txg & (VDEV_UBERBLOCK_COUNT(vd) - 1);
+	int m = spa_multihost(vd->vdev_spa) ? MMP_BLOCKS_PER_LABEL : 0;
+	int n = ub->ub_txg % (VDEV_UBERBLOCK_COUNT(vd) - m);
 
 	/* Copy the uberblock_t into the ABD */
-	ub_abd = abd_alloc_for_io(VDEV_UBERBLOCK_SIZE(vd), B_TRUE);
+	abd_t *ub_abd = abd_alloc_for_io(VDEV_UBERBLOCK_SIZE(vd), B_TRUE);
 	abd_zero(ub_abd, VDEV_UBERBLOCK_SIZE(vd));
 	abd_copy_from_buf(ub_abd, ub, sizeof (uberblock_t));
 
-	for (l = 0; l < VDEV_LABELS; l++)
+	for (int l = 0; l < VDEV_LABELS; l++)
 		vdev_label_write(zio, vd, l, ub_abd,
 		    VDEV_UBERBLOCK_OFFSET(vd, n), VDEV_UBERBLOCK_SIZE(vd),
 		    vdev_uberblock_sync_done, zio->io_private,
@@ -1448,10 +1444,13 @@ retry:
 	 * and the vdev configuration hasn't changed,
 	 * then there's nothing to do.
 	 */
-	if (ub->ub_txg < txg &&
-	    uberblock_update(ub, spa->spa_root_vdev, txg) == B_FALSE &&
-	    list_is_empty(&spa->spa_config_dirty_list))
-		return (0);
+	if (ub->ub_txg < txg) {
+		boolean_t changed = uberblock_update(ub, spa->spa_root_vdev,
+		    txg, spa->spa_mmp.mmp_delay);
+
+		if (!changed && list_is_empty(&spa->spa_config_dirty_list))
+			return (0);
+	}
 
 	if (txg > spa_freeze_txg(spa))
 		return (0);
@@ -1502,6 +1501,10 @@ retry:
 	if ((error = vdev_uberblock_sync_list(svd, svdcount, ub, flags)) != 0)
 		goto retry;
 
+
+	if (spa_multihost(spa))
+		mmp_update_uberblock(spa, ub);
+
 	/*
 	 * Sync out odd labels for every dirty vdev.  If the system dies
 	 * in the middle of this process, the even labels and the new
diff --git a/module/zfs/zfs_ioctl.c b/module/zfs/zfs_ioctl.c
index 728b02377..b2f5db584 100644
--- a/module/zfs/zfs_ioctl.c
+++ b/module/zfs/zfs_ioctl.c
@@ -1650,7 +1650,7 @@ zfs_ioc_pool_stats(zfs_cmd_t *zc)
 static int
 zfs_ioc_pool_tryimport(zfs_cmd_t *zc)
 {
-	nvlist_t *tryconfig, *config;
+	nvlist_t *tryconfig, *config = NULL;
 	int error;
 
 	if ((error = get_nvlist(zc->zc_nvlist_conf, zc->zc_nvlist_conf_size,