Index: sys/geom/mirror/g_mirror.h
===================================================================
--- sys/geom/mirror/g_mirror.h
+++ sys/geom/mirror/g_mirror.h
@@ -148,6 +148,7 @@
 	u_int		 d_genid;	/* Disk's generation ID. */
 	struct g_mirror_disk_sync d_sync;/* Sync information. */
 	LIST_ENTRY(g_mirror_disk) d_next;
+	u_int		 d_init_ndisks;	/* Initial number of mirror components */
 };
 #define	d_name	d_consumer->provider->name
 
Index: sys/geom/mirror/g_mirror.c
===================================================================
--- sys/geom/mirror/g_mirror.c
+++ sys/geom/mirror/g_mirror.c
@@ -59,6 +59,11 @@
 int g_mirror_debug = 0;
 SYSCTL_INT(_kern_geom_mirror, OID_AUTO, debug, CTLFLAG_RWTUN, &g_mirror_debug, 0,
     "Debug level");
+bool g_launch_mirror_before_timeout = true;
+SYSCTL_BOOL(_kern_geom_mirror, OID_AUTO, launch_mirror_before_timeout,
+    CTLFLAG_RWTUN, &g_launch_mirror_before_timeout, 0,
+    "If false, force gmirror to wait out the full kern.geom.mirror.timeout "
+    "before launching gmirrors");
 static u_int g_mirror_timeout = 4;
 SYSCTL_UINT(_kern_geom_mirror, OID_AUTO, timeout, CTLFLAG_RWTUN, &g_mirror_timeout,
     0, "Time to wait on all mirror components");
@@ -110,6 +115,8 @@
 static void g_mirror_update_device(struct g_mirror_softc *sc, bool force);
 static void g_mirror_dumpconf(struct sbuf *sb, const char *indent,
     struct g_geom *gp, struct g_consumer *cp, struct g_provider *pp);
+static int g_mirror_refresh_device(struct g_mirror_softc *sc,
+    const struct g_provider *pp, const struct g_mirror_metadata *md);
 static void g_mirror_sync_reinit(const struct g_mirror_disk *disk,
     struct bio *bp, off_t offset);
 static void g_mirror_sync_stop(struct g_mirror_disk *disk, int type);
@@ -472,6 +479,7 @@
 	disk->d_sync.ds_update_ts = time_uptime;
 	disk->d_genid = md->md_genid;
 	disk->d_sync.ds_syncid = md->md_syncid;
+	disk->d_init_ndisks = md->md_all;
 	if (errorp != NULL)
 		*errorp = 0;
 	return (disk);
@@ -2362,17 +2370,22 @@
 	case G_MIRROR_DEVICE_STATE_STARTING:
 	    {
 		struct g_mirror_disk *pdisk, *tdisk;
-		u_int dirty, ndisks, genid, syncid;
-		bool broken;
+		u_int dirty, ndisks;
+#ifdef INVARIANTS
+		u_int genid, syncid;
+#endif
 
 		KASSERT(sc->sc_provider == NULL,
 		    ("Non-NULL provider in STARTING state (%s).", sc->sc_name));
 		/*
-		 * Are we ready? We are, if all disks are connected or
-		 * if we have any disks and 'force' is true.
+		 * Are we ready? If the timeout (force is true) has expired, and
+		 * any disks are present, then yes. If we're permitted to launch
+		 * before the timeout has expired and the expected number of
+		 * mirror disks have been tasted, then yes.
 		 */
 		ndisks = g_mirror_ndisks(sc, -1);
-		if (sc->sc_ndisks == ndisks || (force && ndisks > 0)) {
+		if ((force && ndisks > 0) ||
+		    (g_launch_mirror_before_timeout && ndisks == sc->sc_ndisks)) {
 			;
 		} else if (ndisks == 0) {
 			/*
@@ -2419,41 +2432,29 @@
 			callout_drain(&sc->sc_callout);
 		}
 
+#ifdef INVARIANTS
 		/*
-		 * Find the biggest genid.
+		 * Confirm we already detected the newest genid.
 		 */
 		genid = 0;
-		LIST_FOREACH(disk, &sc->sc_disks, d_next) {
+		LIST_FOREACH(disk, &sc->sc_disks, d_next)
 			if (disk->d_genid > genid)
 				genid = disk->d_genid;
-		}
-		sc->sc_genid = genid;
-		/*
-		 * Remove all disks without the biggest genid.
-		 */
-		broken = false;
-		LIST_FOREACH_SAFE(disk, &sc->sc_disks, d_next, tdisk) {
-			if (disk->d_genid < genid) {
-				G_MIRROR_DEBUG(0,
-				    "Component %s (device %s) broken, skipping.",
-				    g_mirror_get_diskname(disk), sc->sc_name);
-				g_mirror_destroy_disk(disk);
-				/*
-				 * Bump the syncid in case we discover a healthy
-				 * replacement disk after starting the mirror.
-				 */
-				broken = true;
-			}
-		}
+		KASSERT(sc->sc_genid == genid,
+		    ("%s: found newer genid %u (sc:%p had %u).", __func__,
+		     genid, sc, sc->sc_genid));
 
 		/*
-		 * Find the biggest syncid.
+		 * Confirm we already detected the newest syncid.
 		 */
 		syncid = 0;
-		LIST_FOREACH(disk, &sc->sc_disks, d_next) {
+		LIST_FOREACH(disk, &sc->sc_disks, d_next)
 			if (disk->d_sync.ds_syncid > syncid)
 				syncid = disk->d_sync.ds_syncid;
-		}
+		KASSERT(sc->sc_syncid == syncid,
+		    ("%s: found newer syncid %u (sc:%p had %u).", __func__,
+		     syncid, sc, sc->sc_syncid));
+#endif
 
 		/*
 		 * Here we need to look for dirty disks and if all disks
@@ -2467,8 +2468,29 @@
 		 */
 		dirty = ndisks = 0;
 		pdisk = NULL;
-		LIST_FOREACH(disk, &sc->sc_disks, d_next) {
-			if (disk->d_sync.ds_syncid != syncid)
+		LIST_FOREACH_SAFE(disk, &sc->sc_disks, d_next, tdisk) {
+			if (disk->d_init_ndisks != sc->sc_ndisks) {
+				G_MIRROR_DEBUG(0, "Found a mismatching 'md_all' "
+				    "field on %s (device %s) (found=%u newest=%u).",
+				    g_mirror_get_diskname(disk), sc->sc_name,
+				    disk->d_init_ndisks, sc->sc_ndisks);
+				/*
+				 * XXX Do we want to kill same-genid mirrors
+				 * with different md_all?  Force them dirty?
+				 * Take sc_ndisks = max(, md_all) over current
+				 * generation?
+				 *
+				 * For now, preserve existing behavior and kill
+				 * them.
+				 */
+#if 1
+				g_mirror_destroy_disk(disk);
+				sc->sc_bump_id |= G_MIRROR_BUMP_SYNCID;
+				continue;
+#endif
+			}
+
+			if (disk->d_sync.ds_syncid != sc->sc_syncid)
 				continue;
 			if ((disk->d_flags &
 			    G_MIRROR_DISK_FLAG_SYNCHRONIZING) != 0) {
@@ -2495,7 +2517,7 @@
 			    "master disk for synchronization.",
 			    g_mirror_get_diskname(pdisk), sc->sc_name);
 			LIST_FOREACH(disk, &sc->sc_disks, d_next) {
-				if (disk->d_sync.ds_syncid != syncid)
+				if (disk->d_sync.ds_syncid != sc->sc_syncid)
 					continue;
 				if ((disk->d_flags &
 				    G_MIRROR_DISK_FLAG_SYNCHRONIZING) != 0) {
@@ -2516,7 +2538,7 @@
 			 * We have some non-dirty disks.
 			 */
 			LIST_FOREACH(disk, &sc->sc_disks, d_next) {
-				if (disk->d_sync.ds_syncid != syncid)
+				if (disk->d_sync.ds_syncid != sc->sc_syncid)
 					continue;
 				if ((disk->d_flags &
 				    G_MIRROR_DISK_FLAG_SYNCHRONIZING) != 0) {
@@ -2532,8 +2554,7 @@
 
 		/* Reset hint. */
 		sc->sc_hint = NULL;
-		sc->sc_syncid = syncid;
-		if (force || broken) {
+		if (force) {
 			/* Remember to bump syncid on first write. */
 			sc->sc_bump_id |= G_MIRROR_BUMP_SYNCID;
 		}
@@ -2870,37 +2891,23 @@
     struct g_mirror_metadata *md)
 {
 
+	G_MIRROR_DEBUG(2, "%s: md_did 0x%u disk %s device %s md_all 0x%x "
+	    "sc_ndisks 0x%x md_slice 0x%x sc_slice 0x%x md_balance 0x%x "
+	    "sc_balance 0x%x sc_mediasize 0x%lx pp_mediasize 0x%lx "
+	    "md_sectorsize 0x%x sc_sectorsize 0x%x md_mflags 0x%lx "
+	    "md_dflags 0x%lx md_syncid 0x%x md_genid 0x%x md_priority 0x%x "
+	    "sc_state 0x%x.",
+	    __func__, md->md_did, pp->name, sc->sc_name, md->md_all,
+	    sc->sc_ndisks, md->md_slice, sc->sc_slice, md->md_balance,
+	    sc->sc_balance, sc->sc_mediasize, pp->mediasize, md->md_sectorsize,
+	    sc->sc_sectorsize, md->md_mflags, md->md_dflags, md->md_syncid,
+	    md->md_genid, md->md_priority, sc->sc_state);
+
 	if (g_mirror_id2disk(sc, md->md_did) != NULL) {
 		G_MIRROR_DEBUG(1, "Disk %s (id=%u) already exists, skipping.",
 		    pp->name, md->md_did);
 		return (EEXIST);
 	}
-	if (md->md_all != sc->sc_ndisks) {
-		G_MIRROR_DEBUG(1,
-		    "Invalid '%s' field on disk %s (device %s), skipping.",
-		    "md_all", pp->name, sc->sc_name);
-		return (EINVAL);
-	}
-	if (md->md_slice != sc->sc_slice) {
-		G_MIRROR_DEBUG(1,
-		    "Invalid '%s' field on disk %s (device %s), skipping.",
-		    "md_slice", pp->name, sc->sc_name);
-		return (EINVAL);
-	}
-	if (md->md_balance != sc->sc_balance) {
-		G_MIRROR_DEBUG(1,
-		    "Invalid '%s' field on disk %s (device %s), skipping.",
-		    "md_balance", pp->name, sc->sc_name);
-		return (EINVAL);
-	}
-#if 0
-	if (md->md_mediasize != sc->sc_mediasize) {
-		G_MIRROR_DEBUG(1,
-		    "Invalid '%s' field on disk %s (device %s), skipping.",
-		    "md_mediasize", pp->name, sc->sc_name);
-		return (EINVAL);
-	}
-#endif
 	if (sc->sc_mediasize > pp->mediasize) {
 		G_MIRROR_DEBUG(1,
 		    "Invalid size of disk %s (device %s), skipping.", pp->name,
@@ -2938,7 +2945,7 @@
 g_mirror_add_disk(struct g_mirror_softc *sc, struct g_provider *pp,
     struct g_mirror_metadata *md)
 {
-	struct g_mirror_disk *disk;
+	struct g_mirror_disk *disk, *tdisk;
 	int error;
 
 	g_topology_assert_not();
@@ -2947,6 +2954,15 @@
 	error = g_mirror_check_metadata(sc, pp, md);
 	if (error != 0)
 		return (error);
+
+	/*
+	 * If the component disk we're tasting has newer metadata than the
+	 * STARTING gmirror device, refresh the device from the component.
+	 */
+	error = g_mirror_refresh_device(sc, pp, md);
+	if (error != 0)
+		return (error);
+
 	if (sc->sc_state == G_MIRROR_DEVICE_STATE_RUNNING &&
 	    md->md_genid < sc->sc_genid) {
 		G_MIRROR_DEBUG(0, "Component %s (device %s) broken, skipping.",
@@ -2965,6 +2981,24 @@
 		    pp->name, md->md_version, G_MIRROR_VERSION);
 		g_mirror_update_metadata(disk);
 	}
+
+	/*
+	 * If we're still STARTING, kick out any previously tasted stale
+	 * components.
+	 */
+	if (sc->sc_state != G_MIRROR_DEVICE_STATE_STARTING)
+		return (0);
+
+	LIST_FOREACH_SAFE(disk, &sc->sc_disks, d_next, tdisk) {
+		if (disk->d_genid < sc->sc_genid) {
+			G_MIRROR_DEBUG(0, "Stale 'genid' field on %s "
+			    "(device %s) (component=%u latest=%u), skipping.",
+			    g_mirror_get_diskname(disk), sc->sc_name,
+			    disk->d_genid, sc->sc_genid);
+			g_mirror_destroy_disk(disk);
+			sc->sc_bump_id |= G_MIRROR_BUMP_SYNCID;
+		}
+	}
 	return (0);
 }
 
@@ -3029,6 +3063,21 @@
 	return (error);
 }
 
+static void
+g_mirror_reinit_from_metadata(struct g_mirror_softc *sc,
+    const struct g_mirror_metadata *md)
+{
+
+	sc->sc_genid = md->md_genid;
+	sc->sc_syncid = md->md_syncid;
+
+	sc->sc_slice = md->md_slice;
+	sc->sc_balance = md->md_balance;
+	sc->sc_mediasize = md->md_mediasize;
+	sc->sc_ndisks = md->md_all;
+	sc->sc_flags = md->md_mflags;
+}
+
 struct g_geom *
 g_mirror_create(struct g_class *mp, const struct g_mirror_metadata *md,
     u_int type)
@@ -3056,12 +3105,8 @@
 
 	sc->sc_type = type;
 	sc->sc_id = md->md_mid;
-	sc->sc_slice = md->md_slice;
-	sc->sc_balance = md->md_balance;
-	sc->sc_mediasize = md->md_mediasize;
+	g_mirror_reinit_from_metadata(sc, md);
 	sc->sc_sectorsize = md->md_sectorsize;
-	sc->sc_ndisks = md->md_all;
-	sc->sc_flags = md->md_mflags;
 	sc->sc_bump_id = 0;
 	sc->sc_idle = 1;
 	sc->sc_last_write = time_uptime;
@@ -3484,5 +3529,49 @@
 		EVENTHANDLER_DEREGISTER(shutdown_post_sync, g_mirror_post_sync);
 }
 
+/*
+ * Refresh the mirror device's metadata when gmirror encounters a newer
+ * generation as the individual components are being added to the mirror set.
+ */
+static int
+g_mirror_refresh_device(struct g_mirror_softc *sc, const struct g_provider *pp,
+    const struct g_mirror_metadata *md)
+{
+
+	g_topology_assert_not();
+	sx_assert(&sc->sc_lock, SX_XLOCKED);
+
+	if ((sc->sc_genid < md->md_genid) || (sc->sc_genid == md->md_genid &&
+	    sc->sc_syncid < md->md_syncid)) {
+		G_MIRROR_DEBUG(0, "Found newer version for device %s (genid: "
+		    "curr=%u new=%u; syncid: curr=%u new=%u; ndisks: curr=%u "
+		    "new=%u; provider=%s).", sc->sc_name, sc->sc_genid,
+		    md->md_genid, sc->sc_syncid, md->md_syncid, sc->sc_ndisks,
+		    md->md_all, pp->name);
+
+	} else {
+		return (0);
+	}
+
+	if (sc->sc_state != G_MIRROR_DEVICE_STATE_STARTING) {
+		/* Probable data corruption detected */
+		G_MIRROR_DEBUG(0, "Cannot refresh metadata in %s state "
+		    "(device=%s genid=%u). A stale mirror device was launched.",
+		    g_mirror_device_state2str(sc->sc_state), sc->sc_name,
+		    sc->sc_genid);
+		return (EINVAL);
+	}
+
+	/* Update softc */
+	g_mirror_reinit_from_metadata(sc, md);
+
+	G_MIRROR_DEBUG(1, "Refresh device %s (id=%u, state=%s) from disk %s "
+	    "(genid=%u syncid=%u md_all=%u).", sc->sc_name, md->md_mid,
+	    g_mirror_device_state2str(sc->sc_state), pp->name, md->md_genid,
+	    md->md_syncid, (unsigned)md->md_all);
+
+	return (0);
+}
+
 DECLARE_GEOM_CLASS(g_mirror_class, g_mirror);
 MODULE_VERSION(geom_mirror, 0);
Index: sys/geom/mirror/g_mirror_ctl.c
===================================================================
--- sys/geom/mirror/g_mirror_ctl.c
+++ sys/geom/mirror/g_mirror_ctl.c
@@ -982,8 +982,16 @@
 			continue;
 		}
 		sc->sc_ndisks = g_mirror_ndisks(sc, -1);
-		LIST_FOREACH(disk, &sc->sc_disks, d_next) {
-			g_mirror_update_metadata(disk);
+		if (sc->sc_state == G_MIRROR_DEVICE_STATE_RUNNING &&
+		    g_mirror_ndisks(sc, G_MIRROR_DISK_STATE_ACTIVE) > 0) {
+			/* Bump genid after forgetting disconnected disks */
+			sc->sc_bump_id |= G_MIRROR_BUMP_GENID;
+			g_mirror_event_send(sc, 0,
+			    G_MIRROR_EVENT_WAIT | G_MIRROR_EVENT_DEVICE);
+		} else {
+			LIST_FOREACH(disk, &sc->sc_disks, d_next) {
+				g_mirror_update_metadata(disk);
+			}
 		}
 		sx_xunlock(&sc->sc_lock);
 	}
Index: sys/sys/param.h
===================================================================
--- sys/sys/param.h
+++ sys/sys/param.h
@@ -60,7 +60,7 @@
  *		in the range 5 to 9.
  */
 #undef __FreeBSD_version
-#define __FreeBSD_version 1300004	/* Master, propagated to newvers */
+#define __FreeBSD_version 1300005	/* Master, propagated to newvers */
 
 /*
  * __FreeBSD_kernel__ indicates that this system uses the kernel of FreeBSD,