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 <mahrens@delphix.com>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Ned Bass <bass6@llnl.gov>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #745
Closes #6279

cmd/zdb/zdb.c

@@ -2165,6 +2165,13 @@ dump_uberblock(uberblock_t *ub, const char *header, const char *footer)
 	(void) printf("\tguid_sum = %llu\n", (u_longlong_t)ub->ub_guid_sum);
 	(void) printf("\ttimestamp = %llu UTC = %s",
 	    (u_longlong_t)ub->ub_timestamp, asctime(localtime(&timestamp)));
+
+	(void) printf("\tmmp_magic = %016llx\n",
+	    (u_longlong_t)ub->ub_mmp_magic);
+	if (ub->ub_mmp_magic == MMP_MAGIC)
+		(void) printf("\tmmp_delay = %0llu\n",
+		    (u_longlong_t)ub->ub_mmp_delay);
+
 	if (dump_opt['u'] >= 4) {
 		char blkbuf[BP_SPRINTF_LEN];
 		snprintf_blkptr(blkbuf, sizeof (blkbuf), &ub->ub_rootbp);
@@ -2529,6 +2536,11 @@ dump_label_uberblocks(label_t *label, uint64_t ashift, int label_num)
 		if ((dump_opt['u'] < 3) && (first_label(rec) != label_num))
 			continue;
 
+		if ((dump_opt['u'] < 4) &&
+		    (ub->ub_mmp_magic == MMP_MAGIC) && ub->ub_mmp_delay &&
+		    (i >= VDEV_UBERBLOCK_COUNT(&vd) - MMP_BLOCKS_PER_LABEL))
+			continue;
+
 		print_label_header(label, label_num);
 		(void) snprintf(header, ZDB_MAX_UB_HEADER_SIZE,
 		    "    Uberblock[%d]\n", i);
@@ -4125,89 +4137,6 @@ zdb_embedded_block(char *thing)
 	zdb_dump_block_raw(buf, BPE_GET_LSIZE(&bp), 0);
 }
 
-static boolean_t
-pool_match(nvlist_t *cfg, char *tgt)
-{
-	uint64_t v, guid = strtoull(tgt, NULL, 0);
-	char *s;
-
-	if (guid != 0) {
-		if (nvlist_lookup_uint64(cfg, ZPOOL_CONFIG_POOL_GUID, &v) == 0)
-			return (v == guid);
-	} else {
-		if (nvlist_lookup_string(cfg, ZPOOL_CONFIG_POOL_NAME, &s) == 0)
-			return (strcmp(s, tgt) == 0);
-	}
-	return (B_FALSE);
-}
-
-static char *
-find_zpool(char **target, nvlist_t **configp, int dirc, char **dirv)
-{
-	nvlist_t *pools;
-	nvlist_t *match = NULL;
-	char *name = NULL;
-	char *sepp = NULL;
-	char sep = '\0';
-	int count = 0;
-	importargs_t args = { 0 };
-
-	args.paths = dirc;
-	args.path = dirv;
-	args.can_be_active = B_TRUE;
-
-	if ((sepp = strpbrk(*target, "/@")) != NULL) {
-		sep = *sepp;
-		*sepp = '\0';
-	}
-
-	pools = zpool_search_import(g_zfs, &args);
-
-	if (pools != NULL) {
-		nvpair_t *elem = NULL;
-		while ((elem = nvlist_next_nvpair(pools, elem)) != NULL) {
-			verify(nvpair_value_nvlist(elem, configp) == 0);
-			if (pool_match(*configp, *target)) {
-				count++;
-				if (match != NULL) {
-					/* print previously found config */
-					if (name != NULL) {
-						(void) printf("%s\n", name);
-						dump_nvlist(match, 8);
-						name = NULL;
-					}
-					(void) printf("%s\n",
-					    nvpair_name(elem));
-					dump_nvlist(*configp, 8);
-				} else {
-					match = *configp;
-					name = nvpair_name(elem);
-				}
-			}
-		}
-	}
-	if (count > 1)
-		(void) fatal("\tMatched %d pools - use pool GUID "
-		    "instead of pool name or \n"
-		    "\tpool name part of a dataset name to select pool", count);
-
-	if (sepp)
-		*sepp = sep;
-	/*
-	 * If pool GUID was specified for pool id, replace it with pool name
-	 */
-	if (name && (strstr(*target, name) != *target)) {
-		int sz = 1 + strlen(name) + ((sepp) ? strlen(sepp) : 0);
-
-		*target = umem_alloc(sz, UMEM_NOFAIL);
-		(void) snprintf(*target, sz, "%s%s", name, sepp ? sepp : "");
-	}
-
-	*configp = name ? match : NULL;
-
-	return (name);
-}
-
 int
 main(int argc, char **argv)
 {
@@ -4424,21 +4353,31 @@ main(int argc, char **argv)
 	target = argv[0];
 
 	if (dump_opt['e']) {
+		importargs_t args = { 0 };
 		nvlist_t *cfg = NULL;
-		char *name = find_zpool(&target, &cfg, nsearch, searchdirs);
 
-		error = ENOENT;
+		args.paths = nsearch;
-		if (name) {
+		args.path = searchdirs;
-			if (dump_opt['C'] > 1) {
+		args.can_be_active = B_TRUE;
-				(void) printf("\nConfiguration for import:\n");
+
-				dump_nvlist(cfg, 8);
+		error = zpool_tryimport(g_zfs, target, &cfg, &args);
-			}
+		if (error == 0) {
 			if (nvlist_add_nvlist(cfg,
 			    ZPOOL_REWIND_POLICY, policy) != 0) {
 				fatal("can't open '%s': %s",
 				    target, strerror(ENOMEM));
 			}
-			error = spa_import(name, cfg, NULL, flags);
+
+			/*
+			 * Disable the activity check to allow examination of
+			 * active pools.
+			 */
+			if (dump_opt['C'] > 1) {
+				(void) printf("\nConfiguration for import:\n");
+				dump_nvlist(cfg, 8);
+			}
+			error = spa_import(target, cfg, NULL,
+			    flags | ZFS_IMPORT_SKIP_MMP);
 		}
 	}
 
@@ -4453,6 +4392,16 @@ main(int argc, char **argv)
 
 	if (error == 0) {
 		if (target_is_spa || dump_opt['R']) {
+			/*
+			 * Disable the activity check to allow examination of
+			 * active pools.
+			 */
+			mutex_enter(&spa_namespace_lock);
+			if ((spa = spa_lookup(target)) != NULL) {
+				spa->spa_import_flags |= ZFS_IMPORT_SKIP_MMP;
+			}
+			mutex_exit(&spa_namespace_lock);
+
 			error = spa_open_rewind(target, &spa, FTAG, policy,
 			    NULL);
 			if (error) {

cmd/zhack/zhack.c

@@ -121,16 +121,11 @@ space_delta_cb(dmu_object_type_t bonustype, void *data,
  * Target is the dataset whose pool we want to open.
  */
 static void
-import_pool(const char *target, boolean_t readonly)
+zhack_import(char *target, boolean_t readonly)
 {
 	nvlist_t *config;
-	nvlist_t *pools;
-	int error;
-	char *sepp;
-	spa_t *spa;
-	nvpair_t *elem;
 	nvlist_t *props;
-	char *name;
+	int error;
 
 	kernel_init(readonly ? FREAD : (FREAD | FWRITE));
 	g_zfs = libzfs_init();
@@ -139,43 +134,14 @@ import_pool(const char *target, boolean_t readonly)
 	dmu_objset_register_type(DMU_OST_ZFS, space_delta_cb);
 
 	g_readonly = readonly;
-
-	/*
-	 * If we only want readonly access, it's OK if we find
-	 * a potentially-active (ie, imported into the kernel) pool from the
-	 * default cachefile.
-	 */
-	if (readonly && spa_open(target, &spa, FTAG) == 0) {
-		spa_close(spa, FTAG);
-		return;
-	}
-
 	g_importargs.unique = B_TRUE;
 	g_importargs.can_be_active = readonly;
 	g_pool = strdup(target);
-	if ((sepp = strpbrk(g_pool, "/@")) != NULL)
-		*sepp = '\0';
-	g_importargs.poolname = g_pool;
-	pools = zpool_search_import(g_zfs, &g_importargs);
 
-	if (nvlist_empty(pools)) {
+	error = zpool_tryimport(g_zfs, target, &config, &g_importargs);
-		if (!g_importargs.can_be_active) {
+	if (error)
-			g_importargs.can_be_active = B_TRUE;
+		fatal(NULL, FTAG, "cannot import '%s': %s", target,
-			if (zpool_search_import(g_zfs, &g_importargs) != NULL ||
+		    libzfs_error_description(g_zfs));
-			    spa_open(target, &spa, FTAG) == 0) {
-				fatal(spa, FTAG, "cannot import '%s': pool is "
-				    "active; run " "\"zpool export %s\" "
-				    "first\n", g_pool, g_pool);
-			}
-		}
-
-		fatal(NULL, FTAG, "cannot import '%s': no such pool "
-		    "available\n", g_pool);
-	}
-
-	elem = nvlist_next_nvpair(pools, NULL);
-	name = nvpair_name(elem);
-	VERIFY(nvpair_value_nvlist(elem, &config) == 0);
 
 	props = NULL;
 	if (readonly) {
@@ -185,22 +151,22 @@ import_pool(const char *target, boolean_t readonly)
 	}
 
 	zfeature_checks_disable = B_TRUE;
-	error = spa_import(name, config, props, ZFS_IMPORT_NORMAL);
+	error = spa_import(target, config, props, ZFS_IMPORT_NORMAL);
 	zfeature_checks_disable = B_FALSE;
 	if (error == EEXIST)
 		error = 0;
 
 	if (error)
-		fatal(NULL, FTAG, "can't import '%s': %s", name,
+		fatal(NULL, FTAG, "can't import '%s': %s", target,
 		    strerror(error));
 }
 
 static void
-zhack_spa_open(const char *target, boolean_t readonly, void *tag, spa_t **spa)
+zhack_spa_open(char *target, boolean_t readonly, void *tag, spa_t **spa)
 {
 	int err;
 
-	import_pool(target, readonly);
+	zhack_import(target, readonly);
 
 	zfeature_checks_disable = B_TRUE;
 	err = spa_open(target, spa, tag);

cmd/zpool/zpool_main.c

@@ -1743,6 +1743,9 @@ print_status_config(zpool_handle_t *zhp, status_cbdata_t *cb, const char *name,
 			(void) printf(gettext("split into new pool"));
 			break;
 
+		case VDEV_AUX_ACTIVE:
+			(void) printf(gettext("currently in use"));
+
 		default:
 			(void) printf(gettext("corrupted data"));
 			break;
@@ -1838,6 +1841,10 @@ print_import_config(status_cbdata_t *cb, const char *name, nvlist_t *nv,
 			(void) printf(gettext("too many errors"));
 			break;
 
+		case VDEV_AUX_ACTIVE:
+			(void) printf(gettext("currently in use"));
+			break;
+
 		default:
 			(void) printf(gettext("corrupted data"));
 			break;
@@ -1935,8 +1942,10 @@ show_import(nvlist_t *config)
 	vdev_stat_t *vs;
 	char *name;
 	uint64_t guid;
+	uint64_t hostid = 0;
 	char *msgid;
-	nvlist_t *nvroot;
+	char *hostname = "unknown";
+	nvlist_t *nvroot, *nvinfo;
 	zpool_status_t reason;
 	zpool_errata_t errata;
 	const char *health;
@@ -2024,6 +2033,17 @@ show_import(nvlist_t *config)
 		zpool_print_unsup_feat(config);
 		break;
 
+	case ZPOOL_STATUS_HOSTID_ACTIVE:
+		(void) printf(gettext(" status: The pool is currently "
+		    "imported by another system.\n"));
+		break;
+
+	case ZPOOL_STATUS_HOSTID_REQUIRED:
+		(void) printf(gettext(" status: The pool has the "
+		    "multihost property on.  It cannot\n\tbe safely imported "
+		    "when the system hostid is not set.\n"));
+		break;
+
 	case ZPOOL_STATUS_HOSTID_MISMATCH:
 		(void) printf(gettext(" status: The pool was last accessed by "
 		    "another system.\n"));
@@ -2137,6 +2157,27 @@ show_import(nvlist_t *config)
 			    "imported. Attach the missing\n\tdevices and try "
 			    "again.\n"));
 			break;
+		case ZPOOL_STATUS_HOSTID_ACTIVE:
+			VERIFY0(nvlist_lookup_nvlist(config,
+			    ZPOOL_CONFIG_LOAD_INFO, &nvinfo));
+
+			if (nvlist_exists(nvinfo, ZPOOL_CONFIG_MMP_HOSTNAME))
+				hostname = fnvlist_lookup_string(nvinfo,
+				    ZPOOL_CONFIG_MMP_HOSTNAME);
+
+			if (nvlist_exists(nvinfo, ZPOOL_CONFIG_MMP_HOSTID))
+				hostid = fnvlist_lookup_uint64(nvinfo,
+				    ZPOOL_CONFIG_MMP_HOSTID);
+
+			(void) printf(gettext(" action: The pool must be "
+			    "exported from %s (hostid=%lx)\n\tbefore it "
+			    "can be safely imported.\n"), hostname,
+			    (unsigned long) hostid);
+			break;
+		case ZPOOL_STATUS_HOSTID_REQUIRED:
+			(void) printf(gettext(" action: Set a unique system "
+			    "hostid with the genhostid(1) command.\n"));
+			break;
 		default:
 			(void) printf(gettext(" action: The pool cannot be "
 			    "imported due to damaged devices or data.\n"));
@@ -2184,6 +2225,31 @@ show_import(nvlist_t *config)
 	}
 }
 
+static boolean_t
+zfs_force_import_required(nvlist_t *config)
+{
+	uint64_t state;
+	uint64_t hostid = 0;
+	nvlist_t *nvinfo;
+
+	state = fnvlist_lookup_uint64(config, ZPOOL_CONFIG_POOL_STATE);
+	(void) nvlist_lookup_uint64(config, ZPOOL_CONFIG_HOSTID, &hostid);
+
+	if (state != POOL_STATE_EXPORTED && hostid != get_system_hostid())
+		return (B_TRUE);
+
+	nvinfo = fnvlist_lookup_nvlist(config, ZPOOL_CONFIG_LOAD_INFO);
+	if (nvlist_exists(nvinfo, ZPOOL_CONFIG_MMP_STATE)) {
+		mmp_state_t mmp_state = fnvlist_lookup_uint64(nvinfo,
+		    ZPOOL_CONFIG_MMP_STATE);
+
+		if (mmp_state != MMP_STATE_INACTIVE)
+			return (B_TRUE);
+	}
+
+	return (B_FALSE);
+}
+
 /*
  * Perform the import for the given configuration.  This passes the heavy
  * lifting off to zpool_import_props(), and then mounts the datasets contained
@@ -2198,45 +2264,72 @@ do_import(nvlist_t *config, const char *newname, const char *mntopts,
 	uint64_t state;
 	uint64_t version;
 
-	verify(nvlist_lookup_string(config, ZPOOL_CONFIG_POOL_NAME,
+	name = fnvlist_lookup_string(config, ZPOOL_CONFIG_POOL_NAME);
-	    &name) == 0);
+	state = fnvlist_lookup_uint64(config, ZPOOL_CONFIG_POOL_STATE);
+	version = fnvlist_lookup_uint64(config, ZPOOL_CONFIG_VERSION);
 
-	verify(nvlist_lookup_uint64(config,
-	    ZPOOL_CONFIG_POOL_STATE, &state) == 0);
-	verify(nvlist_lookup_uint64(config,
-	    ZPOOL_CONFIG_VERSION, &version) == 0);
 	if (!SPA_VERSION_IS_SUPPORTED(version)) {
 		(void) fprintf(stderr, gettext("cannot import '%s': pool "
 		    "is formatted using an unsupported ZFS version\n"), name);
 		return (1);
-	} else if (state != POOL_STATE_EXPORTED &&
+	} else if (zfs_force_import_required(config) &&
 	    !(flags & ZFS_IMPORT_ANY_HOST)) {
-		uint64_t hostid = 0;
+		mmp_state_t mmp_state = MMP_STATE_INACTIVE;
-		unsigned long system_hostid = get_system_hostid();
+		nvlist_t *nvinfo;
 
-		(void) nvlist_lookup_uint64(config, ZPOOL_CONFIG_HOSTID,
+		nvinfo = fnvlist_lookup_nvlist(config, ZPOOL_CONFIG_LOAD_INFO);
-		    &hostid);
+		if (nvlist_exists(nvinfo, ZPOOL_CONFIG_MMP_STATE))
+			mmp_state = fnvlist_lookup_uint64(nvinfo,
+			    ZPOOL_CONFIG_MMP_STATE);
 
-		if (hostid != 0 && (unsigned long)hostid != system_hostid) {
+		if (mmp_state == MMP_STATE_ACTIVE) {
-			char *hostname;
+			char *hostname = "<unknown>";
-			uint64_t timestamp;
+			uint64_t hostid = 0;
-			time_t t;
 
-			verify(nvlist_lookup_string(config,
+			if (nvlist_exists(nvinfo, ZPOOL_CONFIG_MMP_HOSTNAME))
-			    ZPOOL_CONFIG_HOSTNAME, &hostname) == 0);
+				hostname = fnvlist_lookup_string(nvinfo,
-			verify(nvlist_lookup_uint64(config,
+				    ZPOOL_CONFIG_MMP_HOSTNAME);
-			    ZPOOL_CONFIG_TIMESTAMP, &timestamp) == 0);
+
-			t = timestamp;
+			if (nvlist_exists(nvinfo, ZPOOL_CONFIG_MMP_HOSTID))
-			(void) fprintf(stderr, gettext("cannot import "
+				hostid = fnvlist_lookup_uint64(nvinfo,
-			    "'%s': pool may be in use from other "
+				    ZPOOL_CONFIG_MMP_HOSTID);
-			    "system, it was last accessed by %s "
+
-			    "(hostid: 0x%lx) on %s"), name, hostname,
+			(void) fprintf(stderr, gettext("cannot import '%s': "
-			    (unsigned long)hostid,
+			    "pool is imported on %s (hostid: "
-			    asctime(localtime(&t)));
+			    "0x%lx)\nExport the pool on the other system, "
-			(void) fprintf(stderr, gettext("use '-f' to "
+			    "then run 'zpool import'.\n"),
-			    "import anyway\n"));
+			    name, hostname, (unsigned long) hostid);
-			return (1);
+		} else if (mmp_state == MMP_STATE_NO_HOSTID) {
+			(void) fprintf(stderr, gettext("Cannot import '%s': "
+			    "pool has the multihost property on and the\n"
+			    "system's hostid is not set. Set a unique hostid "
+			    "with the genhostid(1) command.\n"), name);
+		} else {
+			char *hostname = "<unknown>";
+			uint64_t timestamp = 0;
+			uint64_t hostid = 0;
+
+			if (nvlist_exists(config, ZPOOL_CONFIG_HOSTNAME))
+				hostname = fnvlist_lookup_string(config,
+				    ZPOOL_CONFIG_HOSTNAME);
+
+			if (nvlist_exists(config, ZPOOL_CONFIG_TIMESTAMP))
+				timestamp = fnvlist_lookup_uint64(config,
+				    ZPOOL_CONFIG_TIMESTAMP);
+
+			if (nvlist_exists(config, ZPOOL_CONFIG_HOSTID))
+				hostid = fnvlist_lookup_uint64(config,
+				    ZPOOL_CONFIG_HOSTID);
+
+			(void) fprintf(stderr, gettext("cannot import '%s': "
+			    "pool was previously in use from another system.\n"
+			    "Last accessed by %s (hostid=%lx) at %s"
+			    "The pool can be imported, use 'zpool import -f' "
+			    "to import the pool.\n"), name, hostname,
+			    (unsigned long)hostid, ctime((time_t *)&timestamp));
 		}
+
+		return (1);
 	}
 
 	if (zpool_import_props(g_zfs, config, newname, props, flags) != 0)

cmd/ztest/ztest.c

@@ -126,6 +126,7 @@
 #include <sys/fs/zfs.h>
 #include <zfs_fletcher.h>
 #include <libnvpair.h>
+#include <libzfs.h>
 #ifdef __GLIBC__
 #include <execinfo.h> /* for backtrace() */
 #endif
@@ -166,6 +167,7 @@ typedef struct ztest_shared_opts {
 	uint64_t zo_time;
 	uint64_t zo_maxloops;
 	uint64_t zo_metaslab_gang_bang;
+	int zo_mmp_test;
 } ztest_shared_opts_t;
 
 static const ztest_shared_opts_t ztest_opts_defaults = {
@@ -184,6 +186,7 @@ static const ztest_shared_opts_t ztest_opts_defaults = {
 	.zo_passtime = 60,		/* 60 seconds */
 	.zo_killrate = 70,		/* 70% kill rate */
 	.zo_verbose = 0,
+	.zo_mmp_test = 0,
 	.zo_init = 1,
 	.zo_time = 300,			/* 5 minutes */
 	.zo_maxloops = 50,		/* max loops during spa_freeze() */
@@ -623,6 +626,7 @@ usage(boolean_t requested)
 	    "\t[-k kill_percentage (default: %llu%%)]\n"
 	    "\t[-p pool_name (default: %s)]\n"
 	    "\t[-f dir (default: %s)] file directory for vdev files\n"
+	    "\t[-M] Multi-host simulate pool imported on remote host\n"
 	    "\t[-V] verbose (use multiple times for ever more blather)\n"
 	    "\t[-E] use existing pool instead of creating new one\n"
 	    "\t[-T time (default: %llu sec)] total run time\n"
@@ -666,7 +670,7 @@ process_options(int argc, char **argv)
 	bcopy(&ztest_opts_defaults, zo, sizeof (*zo));
 
 	while ((opt = getopt(argc, argv,
-	    "v:s:a:m:r:R:d:t:g:i:k:p:f:VET:P:hF:B:o:")) != EOF) {
+	    "v:s:a:m:r:R:d:t:g:i:k:p:f:MVET:P:hF:B:o:")) != EOF) {
 		value = 0;
 		switch (opt) {
 		case 'v':
@@ -736,6 +740,9 @@ process_options(int argc, char **argv)
 				free(path);
 			}
 			break;
+		case 'M':
+			zo->zo_mmp_test = 1;
+			break;
 		case 'V':
 			zo->zo_verbose++;
 			break;
@@ -2619,6 +2626,9 @@ ztest_spa_create_destroy(ztest_ds_t *zd, uint64_t id)
 	spa_t *spa;
 	nvlist_t *nvroot;
 
+	if (zo->zo_mmp_test)
+		return;
+
 	/*
 	 * Attempt to create using a bad file.
 	 */
@@ -2660,6 +2670,9 @@ ztest_spa_upgrade(ztest_ds_t *zd, uint64_t id)
 	nvlist_t *nvroot, *props;
 	char *name;
 
+	if (ztest_opts.zo_mmp_test)
+		return;
+
 	mutex_enter(&ztest_vdev_lock);
 	name = kmem_asprintf("%s_upgrade", ztest_opts.zo_pool);
 
@@ -2773,6 +2786,9 @@ ztest_vdev_add_remove(ztest_ds_t *zd, uint64_t id)
 	nvlist_t *nvroot;
 	int error;
 
+	if (ztest_opts.zo_mmp_test)
+		return;
+
 	mutex_enter(&ztest_vdev_lock);
 	leaves = MAX(zs->zs_mirrors + zs->zs_splits, 1) * ztest_opts.zo_raidz;
 
@@ -2844,6 +2860,9 @@ ztest_vdev_aux_add_remove(ztest_ds_t *zd, uint64_t id)
 	uint64_t guid = 0;
 	int error;
 
+	if (ztest_opts.zo_mmp_test)
+		return;
+
 	path = umem_alloc(MAXPATHLEN, UMEM_NOFAIL);
 
 	if (ztest_random(2) == 0) {
@@ -2929,6 +2948,9 @@ ztest_split_pool(ztest_ds_t *zd, uint64_t id)
 	uint_t c, children, schildren = 0, lastlogid = 0;
 	int error = 0;
 
+	if (ztest_opts.zo_mmp_test)
+		return;
+
 	mutex_enter(&ztest_vdev_lock);
 
 	/* ensure we have a useable config; mirrors of raidz aren't supported */
@@ -3036,6 +3058,9 @@ ztest_vdev_attach_detach(ztest_ds_t *zd, uint64_t id)
 	int oldvd_is_log;
 	int error, expected_error;
 
+	if (ztest_opts.zo_mmp_test)
+		return;
+
 	oldpath = umem_alloc(MAXPATHLEN, UMEM_NOFAIL);
 	newpath = umem_alloc(MAXPATHLEN, UMEM_NOFAIL);
 
@@ -5624,6 +5649,9 @@ ztest_spa_rename(ztest_ds_t *zd, uint64_t id)
 	char *oldname, *newname;
 	spa_t *spa;
 
+	if (ztest_opts.zo_mmp_test)
+		return;
+
 	(void) rw_wrlock(&ztest_name_lock);
 
 	oldname = ztest_opts.zo_pool;
@@ -6414,7 +6442,7 @@ ztest_run(ztest_shared_t *zs)
 	 * Verify that we can export the pool and reimport it under a
 	 * different name.
 	 */
-	if (ztest_random(2) == 0) {
+	if ((ztest_random(2) == 0) && !ztest_opts.zo_mmp_test) {
 		char name[ZFS_MAX_DATASET_NAME_LEN];
 		(void) snprintf(name, sizeof (name), "%s_import",
 		    ztest_opts.zo_pool);
@@ -6561,6 +6589,56 @@ make_random_props(void)
 	return (props);
 }
 
+/*
+ * Import a storage pool with the given name.
+ */
+static void
+ztest_import(ztest_shared_t *zs)
+{
+	libzfs_handle_t *hdl;
+	importargs_t args = { 0 };
+	spa_t *spa;
+	nvlist_t *cfg = NULL;
+	int nsearch = 1;
+	char *searchdirs[nsearch];
+	char *name = ztest_opts.zo_pool;
+	int flags = ZFS_IMPORT_MISSING_LOG;
+	int error;
+
+	mutex_init(&ztest_vdev_lock, NULL, MUTEX_DEFAULT, NULL);
+	VERIFY(rwlock_init(&ztest_name_lock, USYNC_THREAD, NULL) == 0);
+
+	kernel_init(FREAD | FWRITE);
+	hdl = libzfs_init();
+
+	searchdirs[0] = ztest_opts.zo_dir;
+	args.paths = nsearch;
+	args.path = searchdirs;
+	args.can_be_active = B_FALSE;
+
+	error = zpool_tryimport(hdl, name, &cfg, &args);
+	if (error)
+		(void) fatal(0, "No pools found\n");
+
+	VERIFY0(spa_import(name, cfg, NULL, flags));
+	VERIFY0(spa_open(name, &spa, FTAG));
+	zs->zs_metaslab_sz =
+	    1ULL << spa->spa_root_vdev->vdev_child[0]->vdev_ms_shift;
+	spa_close(spa, FTAG);
+
+	libzfs_fini(hdl);
+	kernel_fini();
+
+	if (!ztest_opts.zo_mmp_test) {
+		ztest_run_zdb(ztest_opts.zo_pool);
+		ztest_freeze();
+		ztest_run_zdb(ztest_opts.zo_pool);
+	}
+
+	(void) rwlock_destroy(&ztest_name_lock);
+	mutex_destroy(&ztest_vdev_lock);
+}
+
 /*
  * Create a storage pool with the given name and initial vdev size.
  * Then test spa_freeze() functionality.
@@ -6605,11 +6683,11 @@ ztest_init(ztest_shared_t *zs)
 
 	kernel_fini();
 
-	ztest_run_zdb(ztest_opts.zo_pool);
+	if (!ztest_opts.zo_mmp_test) {
-
+		ztest_run_zdb(ztest_opts.zo_pool);
-	ztest_freeze();
+		ztest_freeze();
-
+		ztest_run_zdb(ztest_opts.zo_pool);
-	ztest_run_zdb(ztest_opts.zo_pool);
+	}
 
 	(void) rwlock_destroy(&ztest_name_lock);
 	mutex_destroy(&ztest_vdev_lock);
@@ -6769,13 +6847,19 @@ ztest_run_init(void)
 
 	ztest_shared_t *zs = ztest_shared;
 
-	ASSERT(ztest_opts.zo_init != 0);
-
 	/*
 	 * Blow away any existing copy of zpool.cache
 	 */
 	(void) remove(spa_config_path);
 
+	if (ztest_opts.zo_init == 0) {
+		if (ztest_opts.zo_verbose >= 1)
+			(void) printf("Importing pool %s\n",
+			    ztest_opts.zo_pool);
+		ztest_import(zs);
+		return;
+	}
+
 	/*
 	 * Create and initialize our storage pool.
 	 */
@@ -7002,7 +7086,8 @@ main(int argc, char **argv)
 		}
 		kernel_fini();
 
-		ztest_run_zdb(ztest_opts.zo_pool);
+		if (!ztest_opts.zo_mmp_test)
+			ztest_run_zdb(ztest_opts.zo_pool);
 	}
 
 	if (ztest_opts.zo_verbose >= 1) {

configure.ac

@@ -253,6 +253,7 @@ AC_CONFIG_FILES([
 	tests/zfs-tests/tests/functional/libzfs/Makefile
 	tests/zfs-tests/tests/functional/migration/Makefile
 	tests/zfs-tests/tests/functional/mmap/Makefile
+	tests/zfs-tests/tests/functional/mmp/Makefile
 	tests/zfs-tests/tests/functional/mount/Makefile
 	tests/zfs-tests/tests/functional/mv_files/Makefile
 	tests/zfs-tests/tests/functional/nestedfs/Makefile

include/libzfs.h

@@ -148,6 +148,7 @@ typedef enum zfs_error {
 	EZFS_DIFFDATA,		/* bad zfs diff data */
 	EZFS_POOLREADONLY,	/* pool is in read-only mode */
 	EZFS_SCRUB_PAUSED,	/* scrub currently paused */
+	EZFS_ACTIVE_POOL,	/* pool is imported on a different system */
 	EZFS_UNKNOWN
 } zfs_error_t;
 
@@ -326,6 +327,8 @@ typedef enum {
 	ZPOOL_STATUS_FAILING_DEV,	/* device experiencing errors */
 	ZPOOL_STATUS_VERSION_NEWER,	/* newer on-disk version */
 	ZPOOL_STATUS_HOSTID_MISMATCH,	/* last accessed by another system */
+	ZPOOL_STATUS_HOSTID_ACTIVE,	/* currently active on another system */
+	ZPOOL_STATUS_HOSTID_REQUIRED,	/* multihost=on and hostid=0 */
 	ZPOOL_STATUS_IO_FAILURE_WAIT,	/* failed I/O, failmode 'wait' */
 	ZPOOL_STATUS_IO_FAILURE_CONTINUE, /* failed I/O, failmode 'continue' */
 	ZPOOL_STATUS_BAD_LOG,		/* cannot read log chain(s) */
@@ -408,6 +411,8 @@ typedef struct importargs {
 } importargs_t;
 
 extern nvlist_t *zpool_search_import(libzfs_handle_t *, importargs_t *);
+extern int zpool_tryimport(libzfs_handle_t *hdl, char *target,
+    nvlist_t **configp, importargs_t *args);
 
 /* legacy pool search routines */
 extern nvlist_t *zpool_find_import(libzfs_handle_t *, int, char **);

include/libzfs_impl.h

@@ -131,7 +131,7 @@ typedef enum {
 	SHARED_SMB = 0x4
 } zfs_share_type_t;
 
-#define	CONFIG_BUF_MINSIZE	65536
+#define	CONFIG_BUF_MINSIZE	262144
 
 int zfs_error(libzfs_handle_t *, int, const char *);
 int zfs_error_fmt(libzfs_handle_t *, int, const char *, ...);

include/sys/Makefile.am

@@ -36,6 +36,7 @@ COMMON_H = \
 	$(top_srcdir)/include/sys/efi_partition.h \
 	$(top_srcdir)/include/sys/metaslab.h \
 	$(top_srcdir)/include/sys/metaslab_impl.h \
+	$(top_srcdir)/include/sys/mmp.h \
 	$(top_srcdir)/include/sys/mntent.h \
 	$(top_srcdir)/include/sys/multilist.h \
 	$(top_srcdir)/include/sys/nvpair.h \

include/sys/dsl_pool.h

@@ -38,6 +38,7 @@
 #include <sys/bpobj.h>
 #include <sys/bptree.h>
 #include <sys/rrwlock.h>
+#include <sys/mmp.h>
 
 #ifdef	__cplusplus
 extern "C" {

include/sys/fs/zfs.h

@@ -223,6 +223,7 @@ typedef enum {
 	ZPOOL_PROP_MAXBLOCKSIZE,
 	ZPOOL_PROP_TNAME,
 	ZPOOL_PROP_MAXDNODESIZE,
+	ZPOOL_PROP_MULTIHOST,
 	ZPOOL_NUM_PROPS
 } zpool_prop_t;
 
@@ -651,6 +652,11 @@ typedef struct zpool_rewind_policy {
 #define	ZPOOL_CONFIG_VDEV_TOP_ZAP	"com.delphix:vdev_zap_top"
 #define	ZPOOL_CONFIG_VDEV_LEAF_ZAP	"com.delphix:vdev_zap_leaf"
 #define	ZPOOL_CONFIG_HAS_PER_VDEV_ZAPS	"com.delphix:has_per_vdev_zaps"
+#define	ZPOOL_CONFIG_MMP_STATE		"mmp_state"	/* not stored on disk */
+#define	ZPOOL_CONFIG_MMP_TXG		"mmp_txg"	/* not stored on disk */
+#define	ZPOOL_CONFIG_MMP_HOSTNAME	"mmp_hostname"	/* not stored on disk */
+#define	ZPOOL_CONFIG_MMP_HOSTID		"mmp_hostid"	/* not stored on disk */
+
 /*
  * The persistent vdev state is stored as separate values rather than a single
  * 'vdev_state' entry.  This is because a device can be in multiple states, such
@@ -744,7 +750,8 @@ typedef enum vdev_aux {
 	VDEV_AUX_EXTERNAL,	/* external diagnosis or forced fault	*/
 	VDEV_AUX_SPLIT_POOL,	/* vdev was split off into another pool	*/
 	VDEV_AUX_BAD_ASHIFT,	/* vdev ashift is invalid		*/
-	VDEV_AUX_EXTERNAL_PERSIST	/* persistent forced fault	*/
+	VDEV_AUX_EXTERNAL_PERSIST,	/* persistent forced fault	*/
+	VDEV_AUX_ACTIVE,	/* vdev active on a different host	*/
 } vdev_aux_t;
 
 /*
@@ -764,6 +771,16 @@ typedef enum pool_state {
 	POOL_STATE_POTENTIALLY_ACTIVE	/* Internal libzfs state	*/
 } pool_state_t;
 
+/*
+ * mmp state. The following states provide additional detail describing
+ * why a pool couldn't be safely imported.
+ */
+typedef enum mmp_state {
+	MMP_STATE_ACTIVE = 0,		/* In active use		*/
+	MMP_STATE_INACTIVE,		/* Inactive and safe to import	*/
+	MMP_STATE_NO_HOSTID		/* System hostid is not set	*/
+} mmp_state_t;
+
 /*
  * Scan Functions.
  */
@@ -1126,6 +1143,7 @@ typedef enum {
 #define	ZFS_IMPORT_MISSING_LOG	0x4
 #define	ZFS_IMPORT_ONLY		0x8
 #define	ZFS_IMPORT_TEMP_NAME	0x10
+#define	ZFS_IMPORT_SKIP_MMP	0x20
 
 /*
  * Sysevent payload members.  ZFS will generate the following sysevents with the

include/sys/mmp.h

@@ -0,0 +1,63 @@
+/*
+ * CDDL HEADER START
+ *
+ * This file and its contents are supplied under the terms of the
+ * Common Development and Distribution License ("CDDL"), version 1.0.
+ * You may only use this file in accordance with the terms of version
+ * 1.0 of the CDDL.
+ *
+ * A full copy of the text of the CDDL should have accompanied this
+ * source.  A copy of the CDDL is also available via the Internet at
+ * http://www.illumos.org/license/CDDL.
+ *
+ * CDDL HEADER END
+ */
+/*
+ * Copyright (C) 2017 by Lawrence Livermore National Security, LLC.
+ */
+
+#ifndef _SYS_MMP_H
+#define	_SYS_MMP_H
+
+#include <sys/spa.h>
+#include <sys/zfs_context.h>
+#include <sys/uberblock_impl.h>
+
+#ifdef	__cplusplus
+extern "C" {
+#endif
+
+#define	MMP_MIN_INTERVAL		100	/* ms */
+#define	MMP_DEFAULT_INTERVAL		1000	/* ms */
+#define	MMP_DEFAULT_IMPORT_INTERVALS	10
+#define	MMP_DEFAULT_FAIL_INTERVALS	5
+
+typedef struct mmp_thread {
+	kmutex_t	mmp_thread_lock; /* protect thread mgmt fields */
+	kcondvar_t	mmp_thread_cv;
+	kthread_t	*mmp_thread;
+	uint8_t		mmp_thread_exiting;
+	kmutex_t	mmp_io_lock;	/* protect below */
+	hrtime_t	mmp_last_write;	/* last successful MMP write */
+	uint64_t	mmp_delay;	/* decaying avg ns between MMP writes */
+	uberblock_t	mmp_ub;		/* last ub written by sync */
+	zio_t		*mmp_zio_root;	/* root of mmp write zios */
+} mmp_thread_t;
+
+
+extern void mmp_init(struct spa *spa);
+extern void mmp_fini(struct spa *spa);
+extern void mmp_thread_start(struct spa *spa);
+extern void mmp_thread_stop(struct spa *spa);
+extern void mmp_update_uberblock(struct spa *spa, struct uberblock *ub);
+
+/* Global tuning */
+extern ulong_t zfs_multihost_interval;
+extern uint_t zfs_multihost_fail_intervals;
+extern uint_t zfs_multihost_import_intervals;
+
+#ifdef	__cplusplus
+}
+#endif
+
+#endif	/* _SYS_MMP_H */

include/sys/spa.h

@@ -729,6 +729,7 @@ typedef struct spa_stats {
 	spa_stats_history_t	txg_history;
 	spa_stats_history_t	tx_assign_histogram;
 	spa_stats_history_t	io_history;
+	spa_stats_history_t	mmp_history;
 } spa_stats_t;
 
 typedef enum txg_state {
@@ -758,6 +759,8 @@ extern txg_stat_t *spa_txg_history_init_io(spa_t *, uint64_t,
     struct dsl_pool *);
 extern void spa_txg_history_fini_io(spa_t *, txg_stat_t *);
 extern void spa_tx_assign_add_nsecs(spa_t *spa, uint64_t nsecs);
+extern void spa_mmp_history_add(uint64_t txg, uint64_t timestamp,
+    uint64_t mmp_delay, vdev_t *vd, int label);
 
 /* Pool configuration locks */
 extern int spa_config_tryenter(spa_t *spa, int locks, void *tag, krw_t rw);
@@ -860,6 +863,8 @@ extern boolean_t spa_has_pending_synctask(spa_t *spa);
 extern int spa_maxblocksize(spa_t *spa);
 extern int spa_maxdnodesize(spa_t *spa);
 extern void zfs_blkptr_verify(spa_t *spa, const blkptr_t *bp);
+extern boolean_t spa_multihost(spa_t *spa);
+extern unsigned long spa_get_hostid(void);
 
 extern int spa_mode(spa_t *spa);
 extern uint64_t zfs_strtonum(const char *str, char **nptr);

include/sys/spa_impl.h

@@ -275,6 +275,8 @@ struct spa {
 	spa_stats_t	spa_stats;		/* assorted spa statistics */
 	hrtime_t	spa_ccw_fail_time;	/* Conf cache write fail time */
 	taskq_t		*spa_zvol_taskq;	/* Taskq for minor management */
+	uint64_t	spa_multihost;		/* multihost aware (mmp) */
+	mmp_thread_t	spa_mmp;		/* multihost mmp thread */
 
 	/*
 	 * spa_refcount & spa_config_lock must be the last elements

include/sys/uberblock.h

@@ -40,7 +40,8 @@ extern "C" {
 typedef struct uberblock uberblock_t;
 
 extern int uberblock_verify(uberblock_t *);
-extern boolean_t uberblock_update(uberblock_t *, vdev_t *, uint64_t);
+extern boolean_t uberblock_update(uberblock_t *ub, vdev_t *rvd, uint64_t txg,
+    uint64_t mmp_delay);
 
 #ifdef	__cplusplus
 }

include/sys/uberblock_impl.h

@@ -43,6 +43,7 @@ extern "C" {
  */
 #define	UBERBLOCK_MAGIC		0x00bab10c		/* oo-ba-bloc!	*/
 #define	UBERBLOCK_SHIFT		10			/* up to 1K	*/
+#define	MMP_MAGIC		0xa11cea11		/* all-see-all  */
 
 struct uberblock {
 	uint64_t	ub_magic;	/* UBERBLOCK_MAGIC		*/
@@ -54,6 +55,11 @@ struct uberblock {
 
 	/* highest SPA_VERSION supported by software that wrote this txg */
 	uint64_t	ub_software_version;
+
+	/* Maybe missing in uberblocks we read, but always written */
+	uint64_t	ub_mmp_magic;	/* MMP_MAGIC			*/
+	uint64_t	ub_mmp_delay;	/* nanosec since last MMP write	*/
+	uint64_t	ub_mmp_seq;	/* reserved for sequence number	*/
 };
 
 #ifdef	__cplusplus

include/sys/vdev.h

@@ -155,6 +155,8 @@ extern int vdev_label_number(uint64_t psise, uint64_t offset);
 extern nvlist_t *vdev_label_read_config(vdev_t *vd, uint64_t txg);
 extern void vdev_uberblock_load(vdev_t *, struct uberblock *, nvlist_t **);
 extern void vdev_config_generate_stats(vdev_t *vd, nvlist_t *nv);
+extern 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);
 
 typedef enum {
 	VDEV_LABEL_CREATE,	/* create/add a new device */

include/sys/vdev_impl.h

@@ -238,6 +238,7 @@ struct vdev {
 	zio_t		*vdev_probe_zio; /* root of current probe	*/
 	vdev_aux_t	vdev_label_aux;	/* on-disk aux state		*/
 	uint64_t	vdev_leaf_zap;
+	hrtime_t	vdev_mmp_pending; /* 0 if write finished	*/
 
 	/*
 	 * For DTrace to work in userland (libzpool) context, these fields must
@@ -268,6 +269,12 @@ struct vdev {
 #define	VDEV_PHYS_SIZE		(112 << 10)
 #define	VDEV_UBERBLOCK_RING	(128 << 10)
 
+/*
+ * MMP blocks occupy the last MMP_BLOCKS_PER_LABEL slots in the uberblock
+ * ring when MMP is enabled.
+ */
+#define	MMP_BLOCKS_PER_LABEL	1
+
 /* The largest uberblock we support is 8k. */
 #define	MAX_UBERBLOCK_SHIFT (13)
 #define	VDEV_UBERBLOCK_SHIFT(vd)	\

lib/libzfs/libzfs_import.c

@@ -2166,6 +2166,80 @@ zpool_search_import(libzfs_handle_t *hdl, importargs_t *import)
 	return (zpool_find_import_impl(hdl, import));
 }
 
+static boolean_t
+pool_match(nvlist_t *cfg, char *tgt)
+{
+	uint64_t v, guid = strtoull(tgt, NULL, 0);
+	char *s;
+
+	if (guid != 0) {
+		if (nvlist_lookup_uint64(cfg, ZPOOL_CONFIG_POOL_GUID, &v) == 0)
+			return (v == guid);
+	} else {
+		if (nvlist_lookup_string(cfg, ZPOOL_CONFIG_POOL_NAME, &s) == 0)
+			return (strcmp(s, tgt) == 0);
+	}
+	return (B_FALSE);
+}
+
+int
+zpool_tryimport(libzfs_handle_t *hdl, char *target, nvlist_t **configp,
+    importargs_t *args)
+{
+	nvlist_t *pools;
+	nvlist_t *match = NULL;
+	nvlist_t *config = NULL;
+	char *name = NULL, *sepp = NULL;
+	char sep = '\0';
+	int count = 0;
+	char *targetdup = strdup(target);
+
+	*configp = NULL;
+
+	if ((sepp = strpbrk(targetdup, "/@")) != NULL) {
+		sep = *sepp;
+		*sepp = '\0';
+	}
+
+	pools = zpool_search_import(hdl, args);
+
+	if (pools != NULL) {
+		nvpair_t *elem = NULL;
+		while ((elem = nvlist_next_nvpair(pools, elem)) != NULL) {
+			VERIFY0(nvpair_value_nvlist(elem, &config));
+			if (pool_match(config, targetdup)) {
+				count++;
+				if (match != NULL) {
+					/* multiple matches found */
+					continue;
+				} else {
+					match = config;
+					name = nvpair_name(elem);
+				}
+			}
+		}
+	}
+
+	if (count == 0) {
+		(void) zfs_error_aux(hdl, dgettext(TEXT_DOMAIN,
+		    "no pools found"));
+		free(targetdup);
+		return (ENOENT);
+	}
+
+	if (count > 1) {
+		(void) zfs_error_aux(hdl, dgettext(TEXT_DOMAIN,
+		    "%d pools found, use pool GUID\n"), count);
+		free(targetdup);
+		return (EINVAL);
+	}
+
+	*configp = match;
+	free(targetdup);
+
+	return (0);
+}
+
 boolean_t
 find_guid(nvlist_t *nv, uint64_t guid)
 {

lib/libzfs/libzfs_pool.c

@@ -676,7 +676,14 @@ zpool_valid_proplist(libzfs_handle_t *hdl, const char *poolname,
 				goto error;
 			}
 			break;
-
+		case ZPOOL_PROP_MULTIHOST:
+			if (get_system_hostid() == 0) {
+				zfs_error_aux(hdl, dgettext(TEXT_DOMAIN,
+				    "requires a non-zero system hostid"));
+				(void) zfs_error(hdl, EZFS_BADPROP, errbuf);
+				goto error;
+			}
+			break;
 		default:
 			zfs_error_aux(hdl, dgettext(TEXT_DOMAIN,
 			    "property '%s'(%d) not defined"), propname, prop);
@@ -1779,6 +1786,7 @@ zpool_import_props(libzfs_handle_t *hdl, nvlist_t *config, const char *newname,
 
 	if (error) {
 		char desc[1024];
+		char aux[256];
 
 		/*
 		 * Dry-run failed, but we print out what success
@@ -1824,6 +1832,47 @@ zpool_import_props(libzfs_handle_t *hdl, nvlist_t *config, const char *newname,
 			(void) zfs_error(hdl, EZFS_BADVERSION, desc);
 			break;
 
+		case EREMOTEIO:
+			if (nv != NULL && nvlist_lookup_nvlist(nv,
+			    ZPOOL_CONFIG_LOAD_INFO, &nvinfo) == 0) {
+				char *hostname = "<unknown>";
+				uint64_t hostid = 0;
+				mmp_state_t mmp_state;
+
+				mmp_state = fnvlist_lookup_uint64(nvinfo,
+				    ZPOOL_CONFIG_MMP_STATE);
+
+				if (nvlist_exists(nvinfo,
+				    ZPOOL_CONFIG_MMP_HOSTNAME))
+					hostname = fnvlist_lookup_string(nvinfo,
+					    ZPOOL_CONFIG_MMP_HOSTNAME);
+
+				if (nvlist_exists(nvinfo,
+				    ZPOOL_CONFIG_MMP_HOSTID))
+					hostid = fnvlist_lookup_uint64(nvinfo,
+					    ZPOOL_CONFIG_MMP_HOSTID);
+
+				if (mmp_state == MMP_STATE_ACTIVE) {
+					(void) snprintf(aux, sizeof (aux),
+					    dgettext(TEXT_DOMAIN, "pool is imp"
+					    "orted on host '%s' (hostid=%lx).\n"
+					    "Export the pool on the other "
+					    "system, then run 'zpool import'."),
+					    hostname, (unsigned long) hostid);
+				} else if (mmp_state == MMP_STATE_NO_HOSTID) {
+					(void) snprintf(aux, sizeof (aux),
+					    dgettext(TEXT_DOMAIN, "pool has "
+					    "the multihost property on and "
+					    "the\nsystem's hostid is not set. "
+					    "Set a unique system hostid with "
+					    "the genhostid(1) command.\n"));
+				}
+
+				(void) zfs_error_aux(hdl, aux);
+			}
+			(void) zfs_error(hdl, EZFS_ACTIVE_POOL, desc);
+			break;
+
 		case EINVAL:
 			(void) zfs_error(hdl, EZFS_INVALCONFIG, desc);
 			break;
@@ -2206,7 +2255,7 @@ zpool_find_vdev(zpool_handle_t *zhp, const char *path, boolean_t *avail_spare,
 }
 
 static int
-vdev_online(nvlist_t *nv)
+vdev_is_online(nvlist_t *nv)
 {
 	uint64_t ival;
 
@@ -2274,7 +2323,7 @@ vdev_get_physpaths(nvlist_t *nv, char *physpath, size_t phypath_size,
 				return (EZFS_INVALCONFIG);
 		}
 
-		if (vdev_online(nv)) {
+		if (vdev_is_online(nv)) {
 			if ((ret = vdev_get_one_physpath(nv, physpath,
 			    phypath_size, rsz)) != 0)
 				return (ret);

lib/libzfs/libzfs_status.c

@@ -64,6 +64,8 @@ static char *zfs_msgid_table[] = {
 	"ZFS-8000-9P",
 	"ZFS-8000-A5",
 	"ZFS-8000-EY",
+	"ZFS-8000-EY",
+	"ZFS-8000-EY",
 	"ZFS-8000-HC",
 	"ZFS-8000-JQ",
 	"ZFS-8000-K4",
@@ -215,6 +217,26 @@ check_status(nvlist_t *config, boolean_t isimport, zpool_errata_t *erratap)
 	    ps->pss_state == DSS_SCANNING)
 		return (ZPOOL_STATUS_RESILVERING);
 
+	/*
+	 * The multihost property is set and the pool may be active.
+	 */
+	if (vs->vs_state == VDEV_STATE_CANT_OPEN &&
+	    vs->vs_aux == VDEV_AUX_ACTIVE) {
+		mmp_state_t mmp_state;
+		nvlist_t *nvinfo;
+
+		nvinfo = fnvlist_lookup_nvlist(config, ZPOOL_CONFIG_LOAD_INFO);
+		mmp_state = fnvlist_lookup_uint64(nvinfo,
+		    ZPOOL_CONFIG_MMP_STATE);
+
+		if (mmp_state == MMP_STATE_ACTIVE)
+			return (ZPOOL_STATUS_HOSTID_ACTIVE);
+		else if (mmp_state == MMP_STATE_NO_HOSTID)
+			return (ZPOOL_STATUS_HOSTID_REQUIRED);
+		else
+			return (ZPOOL_STATUS_HOSTID_MISMATCH);
+	}
+
 	/*
 	 * Pool last accessed by another system.
 	 */
@@ -344,8 +366,9 @@ check_status(nvlist_t *config, boolean_t isimport, zpool_errata_t *erratap)
 		if (isimport) {
 			feat = fnvlist_lookup_nvlist(config,
 			    ZPOOL_CONFIG_LOAD_INFO);
-			feat = fnvlist_lookup_nvlist(feat,
+			if (nvlist_exists(feat, ZPOOL_CONFIG_ENABLED_FEAT))
-			    ZPOOL_CONFIG_ENABLED_FEAT);
+				feat = fnvlist_lookup_nvlist(feat,
+				    ZPOOL_CONFIG_ENABLED_FEAT);
 		} else {
 			feat = fnvlist_lookup_nvlist(config,
 			    ZPOOL_CONFIG_FEATURE_STATS);

lib/libzfs/libzfs_util.c

@@ -261,6 +261,9 @@ libzfs_error_description(libzfs_handle_t *hdl)
 		return (dgettext(TEXT_DOMAIN, "invalid diff data"));
 	case EZFS_POOLREADONLY:
 		return (dgettext(TEXT_DOMAIN, "pool is read-only"));
+	case EZFS_ACTIVE_POOL:
+		return (dgettext(TEXT_DOMAIN, "pool is imported on a "
+		    "different host"));
 	case EZFS_UNKNOWN:
 		return (dgettext(TEXT_DOMAIN, "unknown error"));
 	default:
@@ -423,6 +426,9 @@ zfs_standard_error_fmt(libzfs_handle_t *hdl, int error, const char *fmt, ...)
 		    "pool I/O is currently suspended"));
 		zfs_verror(hdl, EZFS_POOLUNAVAIL, fmt, ap);
 		break;
+	case EREMOTEIO:
+		zfs_verror(hdl, EZFS_ACTIVE_POOL, fmt, ap);
+		break;
 	default:
 		zfs_error_aux(hdl, strerror(error));
 		zfs_verror(hdl, EZFS_UNKNOWN, fmt, ap);
@@ -511,6 +517,9 @@ zpool_standard_error_fmt(libzfs_handle_t *hdl, int error, const char *fmt, ...)
 		    "block size out of range or does not match"));
 		zfs_verror(hdl, EZFS_BADPROP, fmt, ap);
 		break;
+	case EREMOTEIO:
+		zfs_verror(hdl, EZFS_ACTIVE_POOL, fmt, ap);
+		break;
 
 	default:
 		zfs_error_aux(hdl, strerror(error));

lib/libzpool/Makefile.am

@@ -71,6 +71,7 @@ KERNEL_C = \
 	lzjb.c \
 	lz4.c \
 	metaslab.c \
+	mmp.c \
 	multilist.c \
 	pathname.c \
 	range_tree.c \

man/man1/ztest.1

@@ -147,6 +147,12 @@ ztest -f / -V -T 120
 
 .SH "ENVIRONMENT VARIABLES"
 .TP
+.B "ZFS_HOSTID=id"
+Use \fBid\fR instead of the SPL hostid to identify this host.  Intended for use
+with ztest, but this environment variable will affect any utility which uses
+libzpool, including \fBzpool(8)\fR.  Since the kernel is unaware of this setting
+results with utilites other than ztest are undefined.
+.TP
 .B "ZFS_STACK_SIZE=stacksize"
 Limit the default stack size to \fBstacksize\fR bytes for the purpose of
 detecting and debugging kernel stack overflows.  This value defaults to
@@ -160,6 +166,7 @@ required for a NULL procedure in user space.
 
 By default the stack size is limited to 256K.
 .SH "SEE ALSO"
+.BR "spl-module-parameters (5)" ","
 .BR "zpool (1)" ","
 .BR "zfs (1)" ","
 .BR "zdb (1)" ","

man/man5/zfs-module-parameters.5

@@ -1410,6 +1410,79 @@ otherwise would under the old \fBzfs_mg_alloc_failures\fR facility.
 Default value: \fB0\fR.
 .RE
 
+.sp
+.ne 2
+.na
+\fBzfs_multihost_history\fR (int)
+.ad
+.RS 12n
+Historical statistics for the last N multihost updates will be available in
+\fB/proc/spl/kstat/zfs/<pool>/multihost\fR
+.sp
+Default value: \fB0\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_multihost_interval\fR (ulong)
+.ad
+.RS 12n
+Used to control the frequency of multihost writes which are performed when the
+\fBmultihost\fR pool property is on.  This is one factor used to determine
+the length of the activity check during import.
+.sp
+The multihost write period is \fBzfs_multihost_interval / leaf-vdevs\fR milliseconds.
+This means that on average a multihost write will be issued for each leaf vdev every
+\fBzfs_multihost_interval\fR 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.
+.sp
+On import the activity check waits a minimum amount of time determined by
+\fBzfs_multihost_interval * zfs_multihost_import_intervals\fR.  The activity
+check time may be further extended if the value of mmp delay found in the best
+uberblock indicates actual multihost updates happened at longer intervals than
+\fBzfs_multihost_interval\fR.  A minimum value of \fB100ms\fR is enforced.
+.sp
+Default value: \fB1000\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_multihost_import_intervals\fR (uint)
+.ad
+.RS 12n
+Used to control the duration of the activity test on import.  Smaller values of
+\fBzfs_multihost_import_intervals\fR 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
+.sp
+Default value: \fB10\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_multihost_fail_intervals\fR (uint)
+.ad
+.RS 12n
+Controls the behavior of the pool when multihost write failures are detected.
+.sp
+When \fBzfs_multihost_fail_intervals = 0\fR then multihost 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 offlining a device.
+.sp
+When \fBzfs_multihost_fail_intervals > 0\fR then sequential multihost write failures
+will cause the pool to be suspended.  This occurs when
+\fBzfs_multihost_fail_intervals * zfs_multihost_interval\fR milliseconds have
+passed since the last successful multihost write.  This guarantees the activity test
+will see multihost writes if the pool is imported.
+.sp
+Default value: \fB5\fR.
+.RE
+
 .sp
 .ne 2
 .na
@@ -1529,8 +1602,8 @@ Default value: \fB1,048,576\fR.
 \fBzfs_read_history\fR (int)
 .ad
 .RS 12n
-Historic statistics for the last N reads will be available in
+Historical statistics for the last N reads will be available in
-\fR/proc/spl/kstat/zfs/POOLNAME/reads\fB
+\fB/proc/spl/kstat/zfs/<pool>/reads\fR
 .sp
 Default value: \fB0\fR (no data is kept).
 .RE
@@ -1684,8 +1757,8 @@ Default value: \fB32\fR.
 \fBzfs_txg_history\fR (int)
 .ad
 .RS 12n
-Historic statistics for the last N txgs will be available in
+Historical statistics for the last N txgs will be available in
-\fR/proc/spl/kstat/zfs/POOLNAME/txgs\fB
+\fB/proc/spl/kstat/zfs/<pool>/txgs\fR
 .sp
 Default value: \fB0\fR.
 .RE

man/man8/zpool.8

@@ -717,6 +717,25 @@ The default value is
 .Sy off .
 This property can also be referred to by its shortened name,
 .Sy listsnaps .
+.It Sy multihost Ns = Ns Sy on Ns | Ns Sy off
+Controls whether a pool activity check should be performed during
+.Nm zpool Cm import .
+When a pool is determined to be active it cannot be imported, even with the
+.Fl f
+option.  This property is intended to be used in failover configurations
+where multiple hosts have access to a pool on shared storage.  When this
+property is on, periodic writes to storage occur to show the pool is in use.
+See
+.Sy zfs_multihost_interval
+in the
+.Xr zfs-module-parameters 5
+man page.  In order to enable this property each host must set a unique hostid.
+See
+.Xr genhostid 1
+and
+.Xr spl-module-paramters 5
+for additional details.  The default value is
+.Sy off .
 .It Sy version Ns = Ns Ar version
 The current on-disk version of the pool.
 This can be increased, but never decreased.
@@ -2335,7 +2354,7 @@ is not set, it is assumed that the user is allowed to run
 .Sh INTERFACE STABILITY
 .Sy Evolving
 .Sh SEE ALSO
-.Xr zed 8
+.Xr zed 8 ,
 .Xr zfs 8 ,
 .Xr zfs-events 5 ,
 .Xr zfs-module-parameters 5 ,

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",

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

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();

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

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.
 	 */
@@ -2323,6 +2337,197 @@ 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;
 
 	/*
@@ -2440,6 +2646,33 @@ spa_load_impl(spa_t *spa, uint64_t pool_guid, nvlist_t *config,
 		return (spa_vdev_err(rvd, VDEV_AUX_CORRUPT_DATA, ENXIO));
 	}
 
+	/*
+	 * 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.
 	 */
@@ -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 = spa_get_hostid();
-			myhostid = zone_get_hostid(NULL);
+			if (hostid && myhostid && hostid != myhostid) {
-#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) {
 				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));
 			}
 		}
@@ -2850,12 +3068,25 @@ 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);
 
 		spa->spa_autoreplace = (autoreplace != 0);
 	}
 
+	/*
+	 * 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
@@ -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;

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 = spa_get_hostid();
-	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 */
 	if (hostid != 0)
 		fnvlist_add_uint64(config, ZPOOL_CONFIG_HOSTID, hostid);
 	fnvlist_add_string(config, ZPOOL_CONFIG_HOSTNAME, utsname()->nodename);

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);

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.
@@ -37,6 +38,11 @@ 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

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);
 }

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 (int c = 0; c < vd->vdev_children; c++)
-
-	for (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 (int l = 0; l < VDEV_LABELS; l++) {
-			for (n = 0; n < VDEV_UBERBLOCK_COUNT(vd); n++) {
+			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;
+	for (int c = 0; c < vd->vdev_children; c++)
-	int c, l, n;
-
-	for (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 &&
+	if (ub->ub_txg < txg) {
-	    uberblock_update(ub, spa->spa_root_vdev, txg) == B_FALSE &&
+		boolean_t changed = uberblock_update(ub, spa->spa_root_vdev,
-	    list_is_empty(&spa->spa_config_dirty_list))
+		    txg, spa->spa_mmp.mmp_delay);
-		return (0);
+
+		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

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,

scripts/zfs-tests.sh

@@ -43,7 +43,8 @@ SINGLETEST=()
 SINGLETESTUSER="root"
 ZFS_DBGMSG="$STF_SUITE/callbacks/zfs_dbgmsg.ksh"
 ZFS_DMESG="$STF_SUITE/callbacks/zfs_dmesg.ksh"
-TESTFAIL_CALLBACKS=${TESTFAIL_CALLBACKS:-"$ZFS_DBGMSG:$ZFS_DMESG"}
+ZFS_MMP="$STF_SUITE/callbacks/zfs_mmp.ksh"
+TESTFAIL_CALLBACKS=${TESTFAIL_CALLBACKS:-"$ZFS_DBGMSG:$ZFS_DMESG:$ZFS_MMP"}
 
 #
 # Attempt to remove loopback devices and files which where created earlier

tests/runfiles/linux.run

@@ -410,6 +410,10 @@ tests = ['migration_001_pos', 'migration_002_pos', 'migration_003_pos',
 [tests/functional/mmap]
 tests = ['mmap_write_001_pos', 'mmap_read_001_pos']
 
+[tests/functional/mmp]
+tests = ['mmp_on_thread', 'mmp_on_uberblocks', 'mmp_on_off', 'mmp_interval',
+    'mmp_active_import', 'mmp_inactive_import', 'mmp_exported_import']
+
 [tests/functional/mount]
 tests = ['umount_001', 'umountall_001']
 

tests/zfs-tests/callbacks/Makefile.am

@@ -1,4 +1,5 @@
 pkgdatadir = $(datadir)/@PACKAGE@/zfs-tests/callbacks
 dist_pkgdata_SCRIPTS = \
 	zfs_dbgmsg.ksh \
-	zfs_dmesg.ksh
+	zfs_dmesg.ksh \
+	zfs_mmp.ksh

tests/zfs-tests/callbacks/zfs_mmp.ksh

@@ -0,0 +1,37 @@
+#!/bin/ksh -p
+
+#
+# This file and its contents are supplied under the terms of the
+# Common Development and Distribution License ("CDDL"), version 1.0.
+# You may only use this file in accordance with the terms of version
+# 1.0 of the CDDL.
+#
+# A full copy of the text of the CDDL should have accompanied this
+# source.  A copy of the CDDL is also available via the Internet at
+# http://www.illumos.org/license/CDDL.
+#
+
+#
+# Copyright (c) 2017 by Lawrence Livermore National Security.
+# All rights reserved.
+#
+
+# $1: number of lines to output (default: 40)
+typeset lines=${1:-40}
+typeset history=$(cat /sys/module/zfs/parameters/zfs_multihost_history)
+
+if [ $history -eq 0 ]; then
+	exit
+fi
+
+for f in /proc/spl/kstat/zfs/*/multihost; do
+	echo "================================================================="
+	echo " Last $lines lines of $f"
+	echo "================================================================="
+
+	sudo tail -n $lines $f
+done
+
+echo "================================================================="
+echo " End of zfs multihost log"
+echo "================================================================="

tests/zfs-tests/include/commands.cfg

@@ -52,6 +52,7 @@ export SYSTEM_FILES='arp
     gunzip
     gzip
     head
+    hostid
     hostname
     id
     iostat

tests/zfs-tests/include/libtest.shlib

@@ -3428,3 +3428,83 @@ function swap_cleanup
 
 	return 0
 }
+
+#
+# Set a global system tunable (64-bit value)
+#
+# $1 tunable name
+# $2 tunable values
+#
+function set_tunable64
+{
+	set_tunable_impl "$1" "$2" Z
+}
+
+#
+# Set a global system tunable (32-bit value)
+#
+# $1 tunable name
+# $2 tunable values
+#
+function set_tunable32
+{
+	set_tunable_impl "$1" "$2" W
+}
+
+function set_tunable_impl
+{
+	typeset tunable="$1"
+	typeset value="$2"
+	typeset mdb_cmd="$3"
+	typeset module="${4:-zfs}"
+
+	[[ -z "$tunable" ]] && return 1
+	[[ -z "$value" ]] && return 1
+	[[ -z "$mdb_cmd" ]] && return 1
+
+	case "$(uname)" in
+	Linux)
+		typeset zfs_tunables="/sys/module/$module/parameters"
+		[[ -w "$zfs_tunables/$tunable" ]] || return 1
+		echo -n "$value" > "$zfs_tunables/$tunable"
+		return "$?"
+		;;
+	SunOS)
+		[[ "$module" -eq "zfs" ]] || return 1
+		echo "${tunable}/${mdb_cmd}0t${value}" | mdb -kw
+		return "$?"
+		;;
+	esac
+}
+
+#
+# Get a global system tunable
+#
+# $1 tunable name
+#
+function get_tunable
+{
+	get_tunable_impl "$1"
+}
+
+function get_tunable_impl
+{
+	typeset tunable="$1"
+	typeset module="${2:-zfs}"
+
+	[[ -z "$tunable" ]] && return 1
+
+	case "$(uname)" in
+	Linux)
+		typeset zfs_tunables="/sys/module/$module/parameters"
+		[[ -f "$zfs_tunables/$tunable" ]] || return 1
+		cat $zfs_tunables/$tunable
+		return "$?"
+		;;
+	SunOS)
+		[[ "$module" -eq "zfs" ]] || return 1
+		;;
+	esac
+
+	return 1
+}

tests/zfs-tests/tests/functional/Makefile.am

@@ -29,6 +29,7 @@ SUBDIRS = \
 	link_count \
 	migration \
 	mmap \
+	mmp \
 	mount \
 	mv_files \
 	nestedfs \

tests/zfs-tests/tests/functional/cli_root/zpool_get/zpool_get.cfg

@@ -54,6 +54,7 @@ typeset -a properties=(
     "freeing"
     "fragmentation"
     "leaked"
+    "multihost"
     "feature@async_destroy"
     "feature@empty_bpobj"
     "feature@lz4_compress"

tests/zfs-tests/tests/functional/mmp/Makefile.am

@@ -0,0 +1,13 @@
+pkgdatadir = $(datadir)/@PACKAGE@/zfs-tests/tests/functional/mmp
+dist_pkgdata_SCRIPTS = \
+	mmp_on_thread.ksh \
+	mmp_on_uberblocks.ksh \
+	mmp_on_off.ksh \
+	mmp_interval.ksh \
+	mmp_active_import.ksh \
+	mmp_inactive_import.ksh \
+	mmp_exported_import.ksh \
+	setup.ksh \
+	cleanup.ksh \
+	mmp.kshlib \
+	mmp.cfg

tests/zfs-tests/tests/functional/mmp/cleanup.ksh

@@ -0,0 +1,28 @@
+#!/bin/ksh -p
+#
+# CDDL HEADER START
+#
+# This file and its contents are supplied under the terms of the
+# Common Development and Distribution License ("CDDL"), version 1.0.
+# You may only use this file in accordance with the terms of version
+# 1.0 of the CDDL.
+#
+# A full copy of the text of the CDDL should have accompanied this
+# source.  A copy of the CDDL is also available via the Internet at
+# http://www.illumos.org/license/CDDL.
+#
+# CDDL HEADER END
+#
+
+#
+# Copyright (c) 2017 by Lawrence Livermore National Security, LLC.
+#
+
+. $STF_SUITE/include/libtest.shlib
+. $STF_SUITE/tests/functional/mmp/mmp.cfg
+
+verify_runnable "global"
+
+log_must set_tunable64 zfs_multihost_history 0
+
+log_pass "mmp cleanup passed"

tests/zfs-tests/tests/functional/mmp/mmp.cfg

@@ -0,0 +1,38 @@
+#
+# CDDL HEADER START
+#
+# This file and its contents are supplied under the terms of the
+# Common Development and Distribution License ("CDDL"), version 1.0.
+# You may only use this file in accordance with the terms of version
+# 1.0 of the CDDL.
+#
+# A full copy of the text of the CDDL should have accompanied this
+# source.  A copy of the CDDL is also available via the Internet at
+# http://www.illumos.org/license/CDDL.
+#
+# CDDL HEADER END
+#
+
+#
+# Copyright (c) 2017 by Lawrence Livermore National Security, LLC.
+#
+
+export PREV_UBER="$TEST_BASE_DIR/mmp-uber-prev.txt"
+export CURR_UBER="$TEST_BASE_DIR/mmp-uber-curr.txt"
+export DISK=${DISKS%% *}
+
+export HOSTID_FILE="/etc/hostid"
+export HOSTID1=01234567
+export HOSTID2=89abcdef
+
+export TXG_TIMEOUT_LONG=5000
+export TXG_TIMEOUT_DEFAULT=5
+
+export MMP_POOL=mmppool
+export MMP_DIR=$TEST_BASE_DIR/mmp
+export MMP_HISTORY=100
+
+export MMP_INTERVAL_DEFAULT=1000
+export MMP_INTERVAL_MIN=100
+
+export ZPOOL_IMPORT_DURATION=9

tests/zfs-tests/tests/functional/mmp/mmp.kshlib

@@ -0,0 +1,186 @@
+#
+# 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.
+# Use is subject to license terms.
+#
+
+. $STF_SUITE/include/libtest.shlib
+. $STF_SUITE/tests/functional/mmp/mmp.cfg
+
+
+function check_pool_import # pool opts token keyword
+{
+	typeset pool=${1:-$MMP_POOL}
+	typeset opts=$2
+	typeset token=$3
+	typeset keyword=$4
+
+	zpool import $opts 2>&1 | \
+	    nawk -v token="$token:" '($1==token) {print $0}' | \
+	    grep -i "$keyword" > /dev/null 2>&1
+
+	return $?
+}
+
+function is_pool_imported # pool opts
+{
+	typeset pool=${1:-$MMP_POOL}
+	typeset opts=$2
+
+	check_pool_import "$pool" "$opts" "status" \
+	    "The pool is currently imported"
+	return $?
+}
+
+function wait_pool_imported # pool opts
+{
+	typeset pool=${1:-$MMP_POOL}
+	typeset opts=$2
+
+	while is_pool_imported "$pool" "$opts"; do
+		log_must sleep 5
+	done
+
+	return 0
+}
+
+function try_pool_import # pool opts message
+{
+	typeset pool=${1:-$MMP_POOL}
+	typeset opts=$2
+	typeset msg=$3
+
+	zpool import $opts $pool 2>&1 | grep -i "$msg"
+
+	return $?
+}
+
+function mmp_set_hostid
+{
+	typeset hostid=$1
+
+	a=${hostid:6:2}
+	b=${hostid:4:2}
+	c=${hostid:2:2}
+	d=${hostid:0:2}
+
+	printf "\\x$a\\x$b\\x$c\\x$d" >$HOSTID_FILE
+
+	if [ $(hostid) != "$hostid" ]; then
+		return 1
+	fi
+
+	return 0
+}
+
+function mmp_clear_hostid
+{
+	rm -f $HOSTID_FILE
+}
+
+function mmp_pool_create # pool dir
+{
+	typeset pool=${1:-$MMP_POOL}
+	typeset dir=${2:-$MMP_DIR}
+	typeset opts="-T120 -M -k0 -f $dir -E -p $pool"
+
+	log_must mkdir -p $dir
+	log_must truncate -s $MINVDEVSIZE $dir/vdev1 $dir/vdev2
+
+	log_must mmp_set_hostid $HOSTID1
+	log_must zpool create -f $pool mirror $dir/vdev1 $dir/vdev2
+	log_must zpool set multihost=on $pool
+	log_must zpool export $pool
+	log_must mmp_set_hostid $HOSTID2
+
+	log_note "Starting ztest in the background as hostid $HOSTID1"
+	log_must eval "ZFS_HOSTID=$HOSTID1 ztest $opts >/dev/null 2>&1 &"
+
+	while ! is_pool_imported "$pool" "-d $dir"; do
+		log_must pgrep ztest
+		log_must sleep 5
+	done
+}
+
+function mmp_pool_destroy # pool dir
+{
+	typeset pool=${1:-$MMP_POOL}
+	typeset dir=${2:-$MMP_DIR}
+
+	ZTESTPID=$(pgrep ztest)
+	if [ -n "$ZTESTPID" ]; then
+		log_must kill $ZTESTPID
+		wait $ZTESTPID
+	fi
+
+	if poolexists $pool; then
+		destroy_pool $pool
+        fi
+
+	rm -Rf $dir
+	mmp_clear_hostid
+}
+
+function mmp_pool_set_hostid # pool hostid
+{
+	typeset pool=$1
+	typeset hostid=$2
+
+	log_must mmp_set_hostid $hostid
+	log_must zpool export $pool
+	log_must zpool import $pool
+
+	return 0
+}
+
+function import_no_activity_check # pool opts
+{
+	typeset pool=$1
+	typeset opts=$2
+
+	SECONDS=0
+	zpool import $opts $pool
+	typeset rc=$?
+
+	if [[ $SECONDS -gt $ZPOOL_IMPORT_DURATION ]]; then
+		log_fail "unexpected activity check (${SECONDS}s)"
+	fi
+
+	return $rc
+}
+
+function import_activity_check # pool opts
+{
+	typeset pool=$1
+	typeset opts=$2
+
+	SECONDS=0
+	zpool import $opts $pool
+	typeset rc=$?
+
+	if [[ $SECONDS -le $ZPOOL_IMPORT_DURATION ]]; then
+		log_fail "expected activity check (${SECONDS}s)"
+	fi
+
+	return $rc
+}

tests/zfs-tests/tests/functional/mmp/mmp_active_import.ksh

@@ -0,0 +1,104 @@
+#!/bin/ksh -p
+#
+# CDDL HEADER START
+#
+# This file and its contents are supplied under the terms of the
+# Common Development and Distribution License ("CDDL"), version 1.0.
+# You may only use this file in accordance with the terms of version
+# 1.0 of the CDDL.
+#
+# A full copy of the text of the CDDL should have accompanied this
+# source.  A copy of the CDDL is also available via the Internet at
+# http://www.illumos.org/license/CDDL.
+#
+# CDDL HEADER END
+#
+
+#
+# Copyright (c) 2017 by Lawrence Livermore National Security, LLC.
+#
+
+# DESCRIPTION:
+#	Under no circumstances when multihost is active, should an active pool
+#	with one hostid be importable by a host with a different hostid.
+#
+# STRATEGY:
+#	 1. Simulate an active pool on another host with ztest.
+#	 2. Verify 'zpool import' reports an active pool.
+#	 3. Verify 'zpool import [-f] $MMP_POOL' cannot import the pool.
+#	 4. Kill ztest to make pool eligible for import.
+#	 5. Verify 'zpool import' fails with the expected error message.
+#	 6. Verify 'zpool import $MMP_POOL' fails with the expected message.
+#	 7. Verify 'zpool import -f $MMP_POOL' can now import the pool.
+#	 8. Verify pool may be exported/imported without -f argument.
+#
+
+. $STF_SUITE/include/libtest.shlib
+. $STF_SUITE/tests/functional/mmp/mmp.cfg
+. $STF_SUITE/tests/functional/mmp/mmp.kshlib
+
+verify_runnable "both"
+
+function cleanup
+{
+	mmp_pool_destroy $MMP_DIR $MMP_POOL
+	log_must set_tunable64 zfs_multihost_interval $MMP_INTERVAL_DEFAULT
+	log_must mmp_clear_hostid
+}
+
+log_assert "multihost=on|off active pool activity checks"
+log_onexit cleanup
+
+# 1. Simulate an active pool on another host with ztest.
+mmp_pool_destroy $MMP_POOL $MMP_DIR
+mmp_pool_create $MMP_POOL $MMP_DIR
+
+# 2. Verify 'zpool import' reports an active pool.
+log_must mmp_set_hostid $HOSTID2
+log_must set_tunable64 zfs_multihost_interval $MMP_INTERVAL_MIN
+log_must is_pool_imported $MMP_POOL "-d $MMP_DIR"
+
+# 3. Verify 'zpool import [-f] $MMP_POOL' cannot import the pool.
+MMP_IMPORTED_MSG="Cannot import '$MMP_POOL': pool is imported"
+log_must try_pool_import $MMP_POOL "-d $MMP_DIR" $MMP_IMPORTED_MSG
+for i in {1..10}; do
+	log_must pgrep ztest >/dev/null
+	log_must try_pool_import $MMP_POOL "-f -d $MMP_DIR" $MMP_IMPORTED_MSG
+done
+
+# 4. Kill ztest to make pool eligible for import.  Poll with 'zpool status'.
+ZTESTPID=$(pgrep ztest)
+if [ -n "$ZTESTPID" ]; then
+	log_must kill -9 $ZTESTPID
+fi
+log_must wait_pool_imported $MMP_POOL "-d $MMP_DIR"
+
+# 5. Verify 'zpool import' fails with the expected error message, when
+#    - hostid=0:        - configuration error
+#    - hostid=matches   - safe to import the pool
+#    - hostid=different - previously imported on a different system
+#
+log_must mmp_clear_hostid
+MMP_IMPORTED_MSG="Set the system hostid"
+log_must check_pool_import $MMP_POOL "-d $MMP_DIR" "action" $MMP_IMPORTED_MSG
+
+log_must mmp_set_hostid $HOSTID1
+MMP_IMPORTED_MSG="The pool can be imported"
+log_must check_pool_import $MMP_POOL "-d $MMP_DIR" "action" $MMP_IMPORTED_MSG
+
+log_must mmp_set_hostid $HOSTID2
+MMP_IMPORTED_MSG="The pool was last accessed by another system."
+log_must check_pool_import $MMP_POOL "-d $MMP_DIR" "status" $MMP_IMPORTED_MSG
+
+# 6. Verify 'zpool import $MMP_POOL' fails with the expected message.
+MMP_IMPORTED_MSG="pool was previously in use from another system."
+log_must try_pool_import $MMP_POOL "-d $MMP_DIR" $MMP_IMPORTED_MSG
+
+# 7. Verify 'zpool import -f $MMP_POOL' can now import the pool.
+log_must import_activity_check $MMP_POOL "-f -d $MMP_DIR"
+
+# 8 Verify pool may be exported/imported without -f argument.
+log_must zpool export $MMP_POOL
+log_must import_no_activity_check $MMP_POOL "-d $MMP_DIR"
+
+log_pass "multihost=on|off active pool activity checks passed"

tests/zfs-tests/tests/functional/mmp/mmp_exported_import.ksh

@@ -0,0 +1,104 @@
+#!/bin/ksh -p
+#
+# CDDL HEADER START
+#
+# This file and its contents are supplied under the terms of the
+# Common Development and Distribution License ("CDDL"), version 1.0.
+# You may only use this file in accordance with the terms of version
+# 1.0 of the CDDL.
+#
+# A full copy of the text of the CDDL should have accompanied this
+# source.  A copy of the CDDL is also available via the Internet at
+# http://www.illumos.org/license/CDDL.
+#
+# CDDL HEADER END
+#
+
+#
+# Copyright (c) 2017 by Lawrence Livermore National Security, LLC.
+#
+
+# DESCRIPTION:
+#	Verify import behavior for exported pool (no activity check)
+#
+# STRATEGY:
+#	1. Create a zpool
+#	2. Verify multihost=off and hostids match (no activity check)
+#	3. Verify multihost=off and hostids differ (no activity check)
+#	4. Verify multihost=off and hostid zero allowed (no activity check)
+#	5. Verify multihost=on and hostids match (no activity check)
+#	6. Verify multihost=on and hostids differ (no activity check)
+#	7. Verify multihost=on and hostid zero fails (no activity check)
+#
+
+. $STF_SUITE/include/libtest.shlib
+. $STF_SUITE/tests/functional/mmp/mmp.cfg
+. $STF_SUITE/tests/functional/mmp/mmp.kshlib
+
+verify_runnable "both"
+
+function cleanup
+{
+	default_cleanup_noexit
+	log_must mmp_clear_hostid
+}
+
+log_assert "multihost=on|off activity checks exported pool"
+log_onexit cleanup
+
+# 1. Create a zpool
+log_must mmp_set_hostid $HOSTID1
+default_setup_noexit $DISK
+
+# 2. Verify multihost=off and hostids match (no activity check)
+log_must zpool set multihost=off $TESTPOOL
+
+for opt in "" "-f"; do
+	log_must zpool export $TESTPOOL
+	log_must import_no_activity_check $TESTPOOL $opt
+done
+
+# 3. Verify multihost=off and hostids differ (no activity check)
+for opt in "" "-f"; do
+	log_must mmp_pool_set_hostid $TESTPOOL $HOSTID1
+	log_must zpool export $TESTPOOL
+	log_must mmp_set_hostid $HOSTID2
+	log_must import_no_activity_check $TESTPOOL $opt
+done
+
+# 4. Verify multihost=off and hostid zero allowed (no activity check)
+log_must mmp_clear_hostid
+
+for opt in "" "-f"; do
+	log_must zpool export $TESTPOOL
+	log_must import_no_activity_check $TESTPOOL $opt
+done
+
+# 5. Verify multihost=on and hostids match (no activity check)
+log_must mmp_pool_set_hostid $TESTPOOL $HOSTID1
+log_must zpool set multihost=on $TESTPOOL
+
+for opt in "" "-f"; do
+	log_must zpool export $TESTPOOL
+	log_must import_no_activity_check $TESTPOOL $opt
+done
+
+# 6. Verify multihost=on and hostids differ (no activity check)
+for opt in "" "-f"; do
+	log_must mmp_pool_set_hostid $TESTPOOL $HOSTID1
+	log_must zpool export $TESTPOOL
+	log_must mmp_set_hostid $HOSTID2
+	log_must import_no_activity_check $TESTPOOL $opt
+done
+
+# 7. Verify multihost=on and hostid zero fails (no activity check)
+log_must zpool export $TESTPOOL
+log_must mmp_clear_hostid
+
+for opt in "" "-f"; do
+	MMP_IMPORTED_MSG="Set the system hostid"
+	log_must check_pool_import $TESTPOOL "" "action" $MMP_IMPORTED_MSG
+	log_mustnot import_no_activity_check $TESTPOOL $opt
+done
+
+log_pass "multihost=on|off exported pool activity checks passed"

tests/zfs-tests/tests/functional/mmp/mmp_inactive_import.ksh

@@ -0,0 +1,95 @@
+#!/bin/ksh -p
+#
+# CDDL HEADER START
+#
+# This file and its contents are supplied under the terms of the
+# Common Development and Distribution License ("CDDL"), version 1.0.
+# You may only use this file in accordance with the terms of version
+# 1.0 of the CDDL.
+#
+# A full copy of the text of the CDDL should have accompanied this
+# source.  A copy of the CDDL is also available via the Internet at
+# http://www.illumos.org/license/CDDL.
+#
+# CDDL HEADER END
+#
+
+#
+# Copyright (c) 2017 by Lawrence Livermore National Security, LLC.
+#
+
+# DESCRIPTION:
+#	Verify import behavior for inactive, but not exported, pools
+#
+# STRATEGY:
+#	1. Create a zpool
+#	2. Verify multihost=off and hostids match (no activity check)
+#	3. Verify multihost=off and hostids differ (no activity check)
+#	4. Verify multihost=off and hostid allowed (no activity check)
+#	5. Verify multihost=on and hostids match (no activity check)
+#	6. Verify multihost=on and hostids differ (activity check)
+#	7. Verify multihost=on and hostid zero fails (activity check)
+#
+
+. $STF_SUITE/include/libtest.shlib
+. $STF_SUITE/tests/functional/mmp/mmp.cfg
+. $STF_SUITE/tests/functional/mmp/mmp.kshlib
+
+verify_runnable "both"
+
+function cleanup
+{
+	default_cleanup_noexit
+	log_must mmp_clear_hostid
+}
+
+log_assert "multihost=on|off inactive pool activity checks"
+log_onexit cleanup
+
+# 1. Create a zpool
+log_must mmp_set_hostid $HOSTID1
+default_setup_noexit $DISK
+
+# 2. Verify multihost=off and hostids match (no activity check)
+log_must zpool set multihost=off $TESTPOOL
+
+for opt in "" "-f"; do
+	log_must zpool export -F $TESTPOOL
+	log_must import_no_activity_check $TESTPOOL $opt
+done
+
+# 3. Verify multihost=off and hostids differ (no activity check)
+log_must zpool export -F $TESTPOOL
+log_must mmp_set_hostid $HOSTID2
+log_mustnot import_no_activity_check $TESTPOOL ""
+log_must import_no_activity_check $TESTPOOL "-f"
+
+# 4. Verify multihost=off and hostid zero allowed (no activity check)
+log_must zpool export -F $TESTPOOL
+log_must mmp_clear_hostid
+log_mustnot import_no_activity_check $TESTPOOL ""
+log_must import_no_activity_check $TESTPOOL "-f"
+
+# 5. Verify multihost=on and hostids match (no activity check)
+log_must mmp_pool_set_hostid $TESTPOOL $HOSTID1
+log_must zpool set multihost=on $TESTPOOL
+
+for opt in "" "-f"; do
+	log_must zpool export -F $TESTPOOL
+	log_must import_no_activity_check $TESTPOOL $opt
+done
+
+# 6. Verify multihost=on and hostids differ (activity check)
+log_must zpool export -F $TESTPOOL
+log_must mmp_set_hostid $HOSTID2
+log_mustnot import_activity_check $TESTPOOL ""
+log_must import_activity_check $TESTPOOL "-f"
+
+# 7. Verify multihost=on and hostid zero fails (activity check)
+log_must zpool export -F $TESTPOOL
+log_must mmp_clear_hostid
+MMP_IMPORTED_MSG="Set the system hostid"
+log_must check_pool_import $TESTPOOL "-f" "action" $MMP_IMPORTED_MSG
+log_mustnot import_activity_check $TESTPOOL "-f"
+
+log_pass "multihost=on|off inactive pool activity checks passed"

tests/zfs-tests/tests/functional/mmp/mmp_interval.ksh

@@ -0,0 +1,47 @@
+#!/bin/ksh -p
+#
+# CDDL HEADER START
+#
+# This file and its contents are supplied under the terms of the
+# Common Development and Distribution License ("CDDL"), version 1.0.
+# You may only use this file in accordance with the terms of version
+# 1.0 of the CDDL.
+#
+# A full copy of the text of the CDDL should have accompanied this
+# source.  A copy of the CDDL is also available via the Internet at
+# http://www.illumos.org/license/CDDL.
+#
+# CDDL HEADER END
+#
+
+#
+# Copyright (c) 2017 by Lawrence Livermore National Security, LLC.
+#
+
+# DESCRIPTION:
+#	zfs_multihost_interval should only accept valid values.
+#
+# STRATEGY:
+#	1. Set zfs_multihost_interval to invalid values (negative).
+#	2. Set zfs_multihost_interval to valid values.
+#
+
+. $STF_SUITE/include/libtest.shlib
+. $STF_SUITE/tests/functional/mmp/mmp.cfg
+. $STF_SUITE/tests/functional/mmp/mmp.kshlib
+
+verify_runnable "both"
+
+function cleanup
+{
+	log_must set_tunable64 zfs_multihost_interval $MMP_INTERVAL_DEFAULT
+}
+
+log_assert "zfs_multihost_interval cannot be set to an invalid value"
+log_onexit cleanup
+
+log_mustnot set_tunable64 zfs_multihost_interval -1
+log_must set_tunable64 zfs_multihost_interval $MMP_INTERVAL_MIN
+log_must set_tunable64 zfs_multihost_interval $MMP_INTERVAL_DEFAULT
+
+log_pass "zfs_multihost_interval cannot be set to an invalid value"

tests/zfs-tests/tests/functional/mmp/mmp_on_off.ksh

@@ -0,0 +1,79 @@
+#!/bin/ksh -p
+#
+# CDDL HEADER START
+#
+# This file and its contents are supplied under the terms of the
+# Common Development and Distribution License ("CDDL"), version 1.0.
+# You may only use this file in accordance with the terms of version
+# 1.0 of the CDDL.
+#
+# A full copy of the text of the CDDL should have accompanied this
+# source.  A copy of the CDDL is also available via the Internet at
+# http://www.illumos.org/license/CDDL.
+#
+# CDDL HEADER END
+#
+
+#
+# Copyright (c) 2017 by Lawrence Livermore National Security, LLC.
+#
+
+# DESCRIPTION:
+#	When multihost=off ensure that leaf vdev uberblocks are not updated.
+#
+# STRATEGY:
+#	1. Set multihost=off (disables mmp)
+#	2. Set zfs_txg_timeout to large value
+#	3. Create a zpool
+#	4. Find the current "best" uberblock
+#	5. Sleep for enough time for uberblocks to change
+#	6. Find the current "best" uberblock
+#	7. If the uberblock changed, fail
+#	8. Set multihost=on
+#	9. Sleep for enough time for uberblocks to change
+#	10. Find the current "best" uberblock
+#	11. If uberblocks didn't change, fail
+#
+
+. $STF_SUITE/include/libtest.shlib
+. $STF_SUITE/tests/functional/mmp/mmp.cfg
+. $STF_SUITE/tests/functional/mmp/mmp.kshlib
+
+verify_runnable "both"
+
+function cleanup
+{
+	default_cleanup_noexit
+	log_must set_tunable64 zfs_txg_timeout $TXG_TIMEOUT_DEFAULT
+	log_must set_tunable64 zfs_multihost_interval $MMP_INTERVAL_DEFAULT
+	log_must rm -f $PREV_UBER $CURR_UBER
+	log_must mmp_clear_hostid
+}
+
+log_assert "mmp thread won't write uberblocks with multihost=off"
+log_onexit cleanup
+
+log_must set_tunable64 zfs_multihost_interval $MMP_INTERVAL_MIN
+log_must set_tunable64 zfs_txg_timeout $TXG_TIMEOUT_LONG
+log_must mmp_set_hostid $HOSTID1
+
+default_setup_noexit $DISK
+log_must zpool set multihost=off $TESTPOOL
+
+log_must zdb -u $TESTPOOL > $PREV_UBER
+log_must sleep 5
+log_must zdb -u $TESTPOOL > $CURR_UBER
+
+if ! diff "$CURR_UBER" "$PREV_UBER"; then
+	log_fail "mmp thread has updated an uberblock"
+fi
+
+log_must zpool set multihost=on $TESTPOOL
+log_must sleep 5
+log_must zdb -u $TESTPOOL > $CURR_UBER
+
+if diff "$CURR_UBER" "$PREV_UBER"; then
+	log_fail "mmp failed to update uberblocks"
+fi
+
+log_pass "mmp thread won't write uberblocks with multihost=off passed"

tests/zfs-tests/tests/functional/mmp/mmp_on_thread.ksh

@@ -0,0 +1,64 @@
+#!/bin/ksh -p
+#
+# CDDL HEADER START
+#
+# This file and its contents are supplied under the terms of the
+# Common Development and Distribution License ("CDDL"), version 1.0.
+# You may only use this file in accordance with the terms of version
+# 1.0 of the CDDL.
+#
+# A full copy of the text of the CDDL should have accompanied this
+# source.  A copy of the CDDL is also available via the Internet at
+# http://www.illumos.org/license/CDDL.
+#
+# CDDL HEADER END
+#
+
+#
+# Copyright (c) 2017 by Lawrence Livermore National Security, LLC.
+#
+
+# DESCRIPTION:
+#	Ensure that the MMP thread is writing uberblocks.
+#
+# STRATEGY:
+#	1. Set zfs_txg_timeout to large value
+#	2. Create a zpool
+#	3. Find the current "best" uberblock
+#	4. Sleep for enough time for a potential uberblock update
+#	5. Find the current "best" uberblock
+#	6. If the uberblock never changed, fail
+#
+
+. $STF_SUITE/include/libtest.shlib
+. $STF_SUITE/tests/functional/mmp/mmp.cfg
+. $STF_SUITE/tests/functional/mmp/mmp.kshlib
+
+verify_runnable "both"
+
+function cleanup
+{
+	default_cleanup_noexit
+	log_must set_tunable64 zfs_txg_timeout $TXG_TIMEOUT_DEFAULT
+	log_must rm -f $PREV_UBER $CURR_UBER
+	log_must mmp_clear_hostid
+}
+
+log_assert "mmp thread writes uberblocks (MMP)"
+log_onexit cleanup
+
+log_must set_tunable64 zfs_txg_timeout $TXG_TIMEOUT_LONG
+log_must mmp_set_hostid $HOSTID1
+
+default_setup_noexit $DISK
+log_must zpool set multihost=on $TESTPOOL
+
+log_must zdb -u $TESTPOOL > $PREV_UBER
+log_must sleep 5
+log_must zdb -u $TESTPOOL > $CURR_UBER
+
+if diff -u "$CURR_UBER" "$PREV_UBER"; then
+	log_fail "mmp failed to update uberblocks"
+fi
+
+log_pass "mmp thread writes uberblocks (MMP) passed"

tests/zfs-tests/tests/functional/mmp/mmp_on_uberblocks.ksh

@@ -0,0 +1,79 @@
+#!/bin/ksh -p
+#
+# CDDL HEADER START
+#
+# This file and its contents are supplied under the terms of the
+# Common Development and Distribution License ("CDDL"), version 1.0.
+# You may only use this file in accordance with the terms of version
+# 1.0 of the CDDL.
+#
+# A full copy of the text of the CDDL should have accompanied this
+# source.  A copy of the CDDL is also available via the Internet at
+# http://www.illumos.org/license/CDDL.
+#
+# CDDL HEADER END
+#
+
+#
+# Copyright (c) 2017 by Lawrence Livermore National Security, LLC.
+#
+
+# DESCRIPTION:
+#	Ensure that MMP updates uberblocks at the expected intervals.
+#
+# STRATEGY:
+#	1. Set zfs_txg_timeout to large value
+#	2. Create a zpool
+#	3. Find the current "best" uberblock
+#	4. Loop for 10 seconds, increment counter for each change in UB
+#	5. If number of changes seen is less than min threshold, then fail
+#	6. If number of changes seen is more than max threshold, then fail
+#
+
+. $STF_SUITE/include/libtest.shlib
+. $STF_SUITE/tests/functional/mmp/mmp.cfg
+. $STF_SUITE/tests/functional/mmp/mmp.kshlib
+
+verify_runnable "both"
+
+UBER_CHANGES=0
+
+function cleanup
+{
+	default_cleanup_noexit
+	set_tunable64 zfs_txg_timeout $TXG_TIMEOUT_DEFAULT
+	log_must rm -f $PREV_UBER $CURR_UBER
+	log_must mmp_clear_hostid
+}
+
+log_assert "Ensure MMP uberblocks update at the correct interval"
+log_onexit cleanup
+
+log_must set_tunable64 zfs_txg_timeout $TXG_TIMEOUT_LONG
+log_must mmp_set_hostid $HOSTID1
+
+default_setup_noexit $DISK
+log_must zpool set multihost=on $TESTPOOL
+
+log_must zdb -u $TESTPOOL > $PREV_UBER
+
+SECONDS=0
+while [[ $SECONDS -le 10 ]]; do
+	log_must zdb -u $TESTPOOL > $CURR_UBER
+	if ! diff -u "$CURR_UBER" "$PREV_UBER"; then
+		(( UBER_CHANGES = UBER_CHANGES + 1 ))
+		log_must mv "$CURR_UBER" "$PREV_UBER"
+	fi
+done
+
+log_note "Uberblock changed $UBER_CHANGES times"
+
+if [[ $UBER_CHANGES -lt 8 ]]; then
+	log_fail "Fewer uberblock writes occured than expected (10)"
+fi
+
+if [[ $UBER_CHANGES -gt 12 ]]; then
+	log_fail "More uberblock writes occured than expected (10)"
+fi
+
+log_pass "Ensure MMP uberblocks update at the correct interval passed"

tests/zfs-tests/tests/functional/mmp/setup.ksh

@@ -0,0 +1,32 @@
+#!/bin/ksh -p
+#
+# CDDL HEADER START
+#
+# This file and its contents are supplied under the terms of the
+# Common Development and Distribution License ("CDDL"), version 1.0.
+# You may only use this file in accordance with the terms of version
+# 1.0 of the CDDL.
+#
+# A full copy of the text of the CDDL should have accompanied this
+# source.  A copy of the CDDL is also available via the Internet at
+# http://www.illumos.org/license/CDDL.
+#
+# CDDL HEADER END
+#
+
+#
+# Copyright (c) 2017 by Lawrence Livermore National Security, LLC.
+#
+
+. $STF_SUITE/include/libtest.shlib
+. $STF_SUITE/tests/functional/mmp/mmp.cfg
+
+verify_runnable "global"
+
+if [ -e $HOSTID_FILE ]; then
+	log_unsupported "System has existing $HOSTID_FILE file"
+fi
+
+log_must set_tunable64 zfs_multihost_history $MMP_HISTORY
+
+log_pass "mmp setup pass"