11 files changed, 1001 insertions, 60 deletions

diff --git a/module/zcommon/zpool_prop.c b/module/zcommon/zpool_prop.c
index 77b4d62ff..fd21f3117 100644
--- a/module/zcommon/zpool_prop.c
+++ b/module/zcommon/zpool_prop.c
@@ -120,6 +120,9 @@ zpool_prop_init(void)
 	    PROP_DEFAULT, ZFS_TYPE_POOL, "on | off", "EXPAND", boolean_table);
 	zprop_register_index(ZPOOL_PROP_READONLY, "readonly", 0,
 	    PROP_DEFAULT, ZFS_TYPE_POOL, "on | off", "RDONLY", boolean_table);
+	zprop_register_index(ZPOOL_PROP_MULTIHOST, "multihost", 0,
+	    PROP_DEFAULT, ZFS_TYPE_POOL, "on | off", "MULTIHOST",
+	    boolean_table);
 
 	/* default index properties */
 	zprop_register_index(ZPOOL_PROP_FAILUREMODE, "failmode",
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,