First cut at GEOM based multipath. This is an active/passive{/passive...}

arrangement that has no intrinsic internal knowledge of whether devices
it is given are truly multipath devices. As such, this is a simplistic
approach, but still a useful one.

The basic approach is to (at present- this will change soon) use camcontrol
to find likely identical devices and and label the trailing sector of the
first one. This label contains both a full UUID and a name. The name is
what is presented in /dev/multipath, but the UUID is used as a true
distinguishor at g_taste time, thus making sure we don't have chaos
on a shared SAN where everyone names their data multipath as "Fred".

The first of N identical devices (and N *may* be 1!) becomes the active
path until a BIO request is failed with EIO or ENXIO. When this occurs,
the active disk is ripped away and the next in a list is picked to
(retry and) continue with.

During g_taste events new disks that meet the match criteria for existing
multipath geoms get added to the tail end of the list.

Thus, this active/passive setup actually does work for devices which
go away and come back, as do (now) mpt(4) and isp(4) SAN based disks.

There is still a lot to do to improve this- like about 5 of the 12
recommendations I've received about it,  but it's been functional enough
for a while that it deserves a broader test base.

Reviewed by: pjd
Sponsored by: IronPort Systems
MFC: 2 months

10 files changed, 1121 insertions, 0 deletions

diff --git a/etc/mtree/BSD.include.dist b/etc/mtree/BSD.include.dist
index d1b2558..0ec9f8b 100644
--- a/etc/mtree/BSD.include.dist
+++ b/etc/mtree/BSD.include.dist
@@ -114,6 +114,8 @@
         ..
         mirror
         ..
+        multipath
+        ..
         nop
         ..
         raid3
diff --git a/sbin/geom/class/Makefile b/sbin/geom/class/Makefile
index 7adfe5e..f52695b 100644
--- a/sbin/geom/class/Makefile
+++ b/sbin/geom/class/Makefile
@@ -10,6 +10,7 @@ SUBDIR+=eli
 SUBDIR+=journal
 SUBDIR+=label
 SUBDIR+=mirror
+SUBDIR+=multipath
 SUBDIR+=nop
 SUBDIR+=raid3
 SUBDIR+=shsec
diff --git a/sbin/geom/class/multipath/Makefile b/sbin/geom/class/multipath/Makefile
new file mode 100644
index 0000000..7b418e1
--- /dev/null
+++ b/sbin/geom/class/multipath/Makefile
@@ -0,0 +1,10 @@
+# $FreeBSD$
+
+.PATH: ${.CURDIR}/../../misc
+CLASS=	multipath
+NO_MAN=true
+
+
+.include <bsd.lib.mk>
+
+CFLAGS+=  -I${.CURDIR}/../../../../sys
diff --git a/sbin/geom/class/multipath/geom_multipath.c b/sbin/geom/class/multipath/geom_multipath.c
new file mode 100644
index 0000000..d729af7
--- /dev/null
+++ b/sbin/geom/class/multipath/geom_multipath.c
@@ -0,0 +1,230 @@
+/*-
+ * Copyright (c) 2006 Mathew Jacob <mjacob@FreeBSD.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHORS AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+#include <sys/param.h>
+#include <errno.h>
+#include <paths.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <string.h>
+#include <strings.h>
+#include <assert.h>
+#include <libgeom.h>
+#include <uuid.h>
+#include <geom/multipath/g_multipath.h>
+
+#include "core/geom.h"
+#include "misc/subr.h"
+
+uint32_t lib_version = G_LIB_VERSION;
+uint32_t version = G_MULTIPATH_VERSION;
+
+static void mp_main(struct gctl_req *, unsigned int);
+static void mp_label(struct gctl_req *);
+static void mp_clear(struct gctl_req *);
+
+struct g_command class_commands[] = {
+	{
+		"label", G_FLAG_VERBOSE | G_FLAG_LOADKLD, mp_main, G_NULL_OPTS,
+		"[-v] name prov ..."
+	},
+	{
+		"clear", G_FLAG_VERBOSE, mp_main, G_NULL_OPTS,
+		"[-v] prov ..."
+	},
+	G_CMD_SENTINEL
+};
+
+static void
+mp_main(struct gctl_req *req, unsigned int flags __unused)
+{
+	const char *name;
+
+	name = gctl_get_ascii(req, "verb");
+	if (name == NULL) {
+		gctl_error(req, "No '%s' argument.", "verb");
+		return;
+	}
+	if (strcmp(name, "label") == 0) {
+		mp_label(req);
+	} else if (strcmp(name, "clear") == 0) {
+		mp_clear(req);
+	} else {
+		gctl_error(req, "Unknown command: %s.", name);
+	}
+}
+
+static void
+mp_label(struct gctl_req *req)
+{
+	struct g_multipath_metadata md;
+	off_t disksiz = 0, msize;
+	uint8_t *sector;
+	char *ptr;
+	uuid_t uuid;
+	uint32_t secsize = 0, ssize, status;
+	const char *name;
+	int error, i, nargs;
+
+	nargs = gctl_get_int(req, "nargs");
+	if (nargs < 2) {
+		gctl_error(req, "wrong number of arguments.");
+		return;
+	}
+
+	/*
+	 * First, check each provider to make sure it's the same size.
+	 * This also gets us our size and sectorsize for the metadata.
+	 */
+	for (i = 1; i < nargs; i++) {
+		name = gctl_get_ascii(req, "arg%d", i);
+		msize = g_get_mediasize(name);
+		ssize = g_get_sectorsize(name);
+		if (msize == 0 || ssize == 0) {
+			gctl_error(req, "cannot get information about %s: %s.",
+			    name, strerror(errno));
+			return;
+		}
+		if (i == 1) {
+			secsize = ssize;
+			disksiz	= msize;
+		} else {
+			if (secsize != ssize) {
+				gctl_error(req, "%s sector size %u different.",
+				    name, ssize);
+				return;
+			}
+			if (disksiz != msize) {
+				gctl_error(req, "%s media size %ju different.",
+				    name, (intmax_t)msize);
+				return;
+			}
+		}
+		
+	}
+
+	/*
+	 * Allocate a sector to write as metadata.
+	 */
+	sector = malloc(secsize);
+	if (sector == NULL) {
+		gctl_error(req, "unable to allocate metadata buffer");
+		return;
+	}
+	memset(sector, 0, secsize);
+
+	/*
+	 * Generate metadata.
+	 */
+	strlcpy(md.md_magic, G_MULTIPATH_MAGIC, sizeof(md.md_magic));
+	md.md_version = G_MULTIPATH_VERSION;
+	name = gctl_get_ascii(req, "arg0");
+	strlcpy(md.md_name, name, sizeof(md.md_name));
+	md.md_size = disksiz;
+	md.md_sectorsize = secsize;
+	uuid_create(&uuid, &status);
+	if (status != uuid_s_ok) {
+		gctl_error(req, "cannot create a UUID.");
+		return;
+	}
+	uuid_to_string(&uuid, &ptr, &status);
+	if (status != uuid_s_ok) {
+		gctl_error(req, "cannot stringify a UUID.");
+		return;
+	}
+	strlcpy(md.md_uuid, ptr, sizeof (md.md_uuid));
+	free(ptr);
+
+	/*
+	 * Clear last sector first for each provider to spoil anything extant
+	 */
+	for (i = 1; i < nargs; i++) {
+		name = gctl_get_ascii(req, "arg%d", i);
+		error = g_metadata_clear(name, NULL);
+		if (error != 0) {
+			gctl_error(req, "cannot clear metadata on %s: %s.",
+			    name, strerror(error));
+			return;
+		}
+	}
+
+	multipath_metadata_encode(&md, sector);
+
+	/*
+	 * Ok, store metadata.
+	 */
+	for (i = 1; i < nargs; i++) {
+		name = gctl_get_ascii(req, "arg%d", i);
+		error = g_metadata_store(name, sector, secsize);
+		if (error != 0) {
+			fprintf(stderr, "Can't store metadata on %s: %s.\n",
+			    name, strerror(error));
+			goto fail;
+		}
+	}
+	return;
+
+fail:
+	/*
+	 * Clear last sector first for each provider to spoil anything extant
+	 */
+	for (i = 1; i < nargs; i++) {
+		name = gctl_get_ascii(req, "arg%d", i);
+		error = g_metadata_clear(name, NULL);
+		if (error != 0) {
+			gctl_error(req, "cannot clear metadata on %s: %s.",
+			    name, strerror(error));
+			continue;
+		}
+	}
+}
+
+static void
+mp_clear(struct gctl_req *req)
+{
+	const char *name;
+	int error, i, nargs;
+
+	nargs = gctl_get_int(req, "nargs");
+	if (nargs < 1) {
+		gctl_error(req, "Too few arguments.");
+		return;
+	}
+
+        for (i = 0; i < nargs; i++) {
+		name = gctl_get_ascii(req, "arg%d", i);
+                error = g_metadata_clear(name, G_MULTIPATH_MAGIC);
+		if (error != 0) {
+			fprintf(stderr, "Can't clear metadata on %s: %s.\n",
+			    name, strerror(error));
+			gctl_error(req, "Not fully done.");
+			continue;
+                }
+        }
+}
diff --git a/sys/conf/NOTES b/sys/conf/NOTES
index 3174ad4..e309655 100644
--- a/sys/conf/NOTES
+++ b/sys/conf/NOTES
@@ -147,6 +147,7 @@ options 	GEOM_JOURNAL		# Journaling.
 options 	GEOM_LABEL		# Providers labelization.
 options 	GEOM_MBR		# DOS/MBR partitioning
 options 	GEOM_MIRROR		# Disk mirroring.
+options 	GEOM_MULTIPATH		# Disk multipath
 options 	GEOM_NOP		# Test class.
 options 	GEOM_PART_APM		# Apple partitioning
 options 	GEOM_PART_GPT		# GPT partitioning
diff --git a/sys/conf/files b/sys/conf/files
index 9881214..d8d6c80 100644
--- a/sys/conf/files
+++ b/sys/conf/files
@@ -1210,6 +1210,7 @@ geom/label/g_label_reiserfs.c	optional geom_label
 geom/label/g_label_ufs.c	optional geom_label
 geom/mirror/g_mirror.c		optional geom_mirror
 geom/mirror/g_mirror_ctl.c	optional geom_mirror
+geom/multipath/g_multipath.c	optional geom_multipath
 geom/nop/g_nop.c		optional geom_nop
 geom/part/g_part.c		standard
 geom/part/g_part_if.m		standard
diff --git a/sys/geom/multipath/g_multipath.c b/sys/geom/multipath/g_multipath.c
new file mode 100644
index 0000000..0398628
--- /dev/null
+++ b/sys/geom/multipath/g_multipath.c
@@ -0,0 +1,768 @@
+/*-
+ * Copyright (c) 2006-2007 Matthew Jacob <mjacob@FreeBSD.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHORS AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+/*
+ * Based upon work by Pawel Jakub Dawidek <pjd@FreeBSD.org> for all of the
+ * fine geom examples, and by Poul Henning Kamp <phk@FreeBSD.org> for GEOM
+ * itself, all of which is most gratefully acknowledged.
+ */
+
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+#include <sys/param.h>
+#include <sys/systm.h>
+#include <sys/kernel.h>
+#include <sys/module.h>
+#include <sys/lock.h>
+#include <sys/mutex.h>
+#include <sys/bio.h>
+#include <sys/sysctl.h>
+#include <sys/kthread.h>
+#include <sys/malloc.h>
+#include <geom/geom.h>
+#include <geom/multipath/g_multipath.h>
+
+
+SYSCTL_DECL(_kern_geom);
+SYSCTL_NODE(_kern_geom, OID_AUTO, multipath, CTLFLAG_RW, 0,
+    "GEOM_MULTIPATH tunables");
+static u_int g_multipath_debug = 0;
+SYSCTL_UINT(_kern_geom_multipath, OID_AUTO, debug, CTLFLAG_RW,
+    &g_multipath_debug, 0, "Debug level");
+
+static enum {
+	GKT_NIL,
+	GKT_RUN,
+	GKT_DIE
+} g_multipath_kt_state;
+static struct bio_queue_head gmtbq;
+static struct mtx gmtbq_mtx;
+
+static void g_multipath_orphan(struct g_consumer *);
+static void g_multipath_start(struct bio *);
+static void g_multipath_done(struct bio *);
+static void g_multipath_done_error(struct bio *);
+static void g_multipath_kt(void *);
+
+static int g_multipath_destroy(struct g_geom *);
+static int
+g_multipath_destroy_geom(struct gctl_req *, struct g_class *, struct g_geom *);
+
+static g_taste_t g_multipath_taste;
+static g_ctl_req_t g_multipath_config;
+static g_init_t g_multipath_init;
+static g_fini_t g_multipath_fini;
+
+struct g_class g_multipath_class = {
+	.name		= G_MULTIPATH_CLASS_NAME,
+	.version	= G_VERSION,
+	.ctlreq		= g_multipath_config,
+	.taste		= g_multipath_taste,
+	.destroy_geom	= g_multipath_destroy_geom,
+	.init		= g_multipath_init,
+	.fini		= g_multipath_fini
+};
+
+#define	MP_BAD		0x1
+#define	MP_POSTED	0x2
+
+static void
+g_mpd(void *arg, int flags __unused)
+{
+	struct g_consumer *cp;
+
+	g_topology_assert();
+	cp = arg;
+	if (cp->acr > 0 || cp->acw > 0 || cp->ace > 0) {
+		g_access(cp, -cp->acr, -cp->acw, -cp->ace);
+	}
+	if (cp->provider) {
+		printf("GEOM_MULTIPATH: %s removed from %s\n",
+		    cp->provider->name, cp->geom->name);
+		g_detach(cp);
+	}
+	g_destroy_consumer(cp);
+}
+
+static void
+g_multipath_orphan(struct g_consumer *cp)
+{
+	if ((cp->index & MP_POSTED) == 0) {
+		cp->index |= MP_POSTED;
+		printf("GEOM_MULTIPATH: %s orphaned in %s\n",
+		    cp->provider->name, cp->geom->name);
+		g_mpd(cp, 0);
+	}
+}
+
+static void
+g_multipath_start(struct bio *bp)
+{
+	struct g_multipath_softc *sc;
+	struct g_geom *gp;
+	struct g_consumer *cp;
+	struct bio *cbp;
+
+	gp = bp->bio_to->geom;
+	sc = gp->softc;
+	KASSERT(sc != NULL, ("NULL sc"));
+	cp = sc->cp_active;
+	if (cp == NULL) {
+		g_io_deliver(bp, ENXIO);
+		return;
+	}
+	cbp = g_clone_bio(bp);
+	if (cbp == NULL) {
+		g_io_deliver(bp, ENOMEM);
+		return;
+	}
+	cbp->bio_done = g_multipath_done;
+	g_io_request(cbp, cp);
+}
+
+static void
+g_multipath_done(struct bio *bp)
+{
+	if (bp->bio_error == ENXIO || bp->bio_error == EIO) {
+		mtx_lock(&gmtbq_mtx);
+		bioq_insert_tail(&gmtbq, bp);
+		wakeup(&g_multipath_kt_state);
+		mtx_unlock(&gmtbq_mtx);
+	} else {
+		g_std_done(bp);
+	}
+}
+
+static void
+g_multipath_done_error(struct bio *bp)
+{
+	struct bio *pbp;
+	struct g_geom *gp;
+	struct g_multipath_softc *sc;
+	struct g_consumer *cp;
+	struct g_provider *pp;
+
+	/*
+	 * If we had a failure, we have to check first to see
+	 * whether the consumer it failed on was the currently
+	 * active consumer (i.e., this is the first in perhaps
+	 * a number of failures). If so, we then switch consumers
+	 * to the next available consumer.
+	 */
+
+	g_topology_lock();
+	pbp = bp->bio_parent;
+	gp = pbp->bio_to->geom;
+	sc = gp->softc;
+	cp = bp->bio_from;
+	pp = cp->provider;
+
+	cp->index |= MP_BAD;
+	if (cp->nend == cp->nstart && pp->nend == pp->nstart) {
+		cp->index |= MP_POSTED;
+		g_post_event(g_mpd, cp, M_NOWAIT, NULL);
+	}
+	if (cp == sc->cp_active) {
+		struct g_consumer *lcp;
+		printf("GEOM_MULTIPATH: %s failed in %s\n",
+		    pp->name, sc->sc_name);
+		sc->cp_active = NULL;
+		LIST_FOREACH(lcp, &gp->consumer, consumer) {
+			if ((lcp->index & MP_BAD) == 0) {
+				sc->cp_active = lcp;
+				break;
+			}
+		}
+		if (sc->cp_active == NULL) {
+			printf("GEOM_MULTIPATH: out of providers for %s\n",
+			    sc->sc_name);
+			return;
+		} else {
+			printf("GEOM_MULTIPATH: %s now active path in %s\n",
+			    sc->cp_active->provider->name, sc->sc_name);
+		}
+	}
+	g_topology_unlock();
+
+	/*
+	 * If we can fruitfully restart the I/O, do so.
+	 */
+	if (sc->cp_active) {
+		g_destroy_bio(bp);
+		pbp->bio_children--;
+		g_multipath_start(pbp);
+	} else {
+		g_std_done(bp);
+	}
+}
+
+static void
+g_multipath_kt(void *arg)
+{
+	g_multipath_kt_state = GKT_RUN;
+	mtx_lock(&gmtbq_mtx);
+	while (g_multipath_kt_state == GKT_RUN) {
+		for (;;) {
+			struct bio *bp;
+			bp = bioq_takefirst(&gmtbq);
+			if (bp == NULL) {
+				break;
+			}
+			mtx_unlock(&gmtbq_mtx);
+			g_multipath_done_error(bp);
+			mtx_lock(&gmtbq_mtx);
+		}
+		msleep(&g_multipath_kt_state, &gmtbq_mtx, PRIBIO,
+		    "gkt:wait", hz / 10);
+	}
+	mtx_unlock(&gmtbq_mtx);
+	wakeup(&g_multipath_kt_state);
+	kthread_exit(0);
+}
+
+
+static int
+g_multipath_access(struct g_provider *pp, int dr, int dw, int de)
+{
+	struct g_geom *gp;
+	struct g_consumer *cp, *badcp = NULL;
+	int error;
+
+	gp = pp->geom;
+
+	LIST_FOREACH(cp, &gp->consumer, consumer) {
+		error = g_access(cp, dr, dw, de);
+		if (error) {
+			badcp = cp;
+			goto fail;
+		}
+	}
+	return (0);
+
+fail:
+	LIST_FOREACH(cp, &gp->consumer, consumer) {
+		if (cp == badcp) {
+			break;
+		}
+		(void) g_access(cp, -dr, -dw, -de);
+	}
+	return (error);
+}
+
+static struct g_geom *
+g_multipath_create(struct g_class *mp, struct g_multipath_metadata *md)
+{
+	struct g_multipath_softc *sc;
+	struct g_geom *gp;
+	struct g_provider *pp;
+
+	g_topology_assert();
+
+	LIST_FOREACH(gp, &mp->geom, geom) {
+		if (strcmp(gp->name, md->md_name) == 0) {
+			printf("GEOM_MULTIPATH: name %s already exists\n",
+			    md->md_name);
+			return (NULL);
+		}
+	}
+
+	gp = g_new_geomf(mp, md->md_name);
+	if (gp == NULL) {
+		goto fail;
+	}
+
+	sc = g_malloc(sizeof(*sc), M_WAITOK | M_ZERO);
+	if (sc == NULL) {
+		goto fail;
+	}
+
+	gp->softc = sc;
+	gp->start = g_multipath_start;
+	gp->orphan = g_multipath_orphan;
+	gp->access = g_multipath_access;
+	memcpy(sc->sc_uuid, md->md_uuid, sizeof (sc->sc_uuid));
+	memcpy(sc->sc_name, md->md_name, sizeof (sc->sc_name));
+
+	pp = g_new_providerf(gp, "multipath/%s", md->md_name);
+	if (pp == NULL) {
+		goto fail;
+	}
+	/* limit the provider to not have it stomp on metadata */
+	pp->mediasize = md->md_size - md->md_sectorsize;
+	pp->sectorsize = md->md_sectorsize;
+	sc->pp = pp;
+	g_error_provider(pp, 0);
+	return (gp);
+fail:
+	if (gp != NULL) {
+		if (gp->softc != NULL) {
+			g_free(gp->softc);
+		}
+		g_destroy_geom(gp);
+	}
+	return (NULL);
+}
+
+static int
+g_multipath_add_disk(struct g_geom *gp, struct g_provider *pp)
+{
+	struct g_multipath_softc *sc;
+	struct g_consumer *cp, *nxtcp;
+	int error;
+
+	g_topology_assert();
+
+	sc = gp->softc;
+	KASSERT(sc, ("no softc"));
+
+	/*
+	 * Make sure that the passed provider isn't already attached
+	 */
+	LIST_FOREACH(cp, &gp->consumer, consumer) {
+		if (cp->provider == pp) {
+			break;
+		}
+	}
+	if (cp) {
+		printf("GEOM_MULTIPATH: provider %s already attached to %s\n",
+		    pp->name, gp->name);
+		return (EEXIST);
+	}
+	nxtcp = LIST_FIRST(&gp->consumer);
+	cp = g_new_consumer(gp);
+	if (cp == NULL) {
+		return (ENOMEM);
+	}
+	error = g_attach(cp, pp);
+	if (error != 0) {
+		printf("GEOM_MULTIPATH: cannot attach %s to %s",
+		    pp->name, sc->sc_name);
+		g_destroy_consumer(cp);
+		return (error);
+	}
+	cp->private = sc;
+	cp->index = 0;
+
+	/*
+	 * Set access permissions on new consumer to match other consumers
+	 */
+	if (nxtcp && (nxtcp->acr + nxtcp->acw +  nxtcp->ace)) {
+		error = g_access(cp, nxtcp->acr, nxtcp->acw, nxtcp->ace);
+		if (error) {
+			printf("GEOM_MULTIPATH: cannot set access in "
+			    "attaching %s to %s/%s (%d)\n",
+			    pp->name, sc->sc_name, sc->sc_uuid, error);
+			g_detach(cp);
+			g_destroy_consumer(cp);
+			return (error);
+		}
+	}
+	printf("GEOM_MULTIPATH: adding %s to %s/%s\n",
+	    pp->name, sc->sc_name, sc->sc_uuid);
+	if (sc->cp_active == NULL) {
+		sc->cp_active = cp;
+		printf("GEOM_MULTIPATH: %s now active path in %s\n",
+		    pp->name, sc->sc_name);
+	}
+	return (0);
+}
+
+static int
+g_multipath_destroy(struct g_geom *gp)
+{
+	struct g_provider *pp;
+
+	g_topology_assert();
+	if (gp->softc == NULL) {
+		return (ENXIO);
+	}
+	pp = LIST_FIRST(&gp->provider);
+	if (pp != NULL && (pp->acr != 0 || pp->acw != 0 || pp->ace != 0)) {
+		return (EBUSY);
+	}
+	printf("GEOM_MULTIPATH: destroying %s\n", gp->name);
+	g_free(gp->softc);
+	gp->softc = NULL;
+	g_wither_geom(gp, ENXIO);
+	return (0);
+}
+
+static int
+g_multipath_destroy_geom(struct gctl_req *req, struct g_class *mp,
+    struct g_geom *gp)
+{
+	return (g_multipath_destroy(gp));
+}
+
+static void
+g_multipath_init(struct g_class *mp)
+{
+	bioq_init(&gmtbq);
+	mtx_init(&gmtbq_mtx, "gmtbq", NULL, MTX_DEF);
+	if (kthread_create(g_multipath_kt, mp, NULL, 0, 0, "g_mp_kt") == 0) {
+		g_multipath_kt_state = GKT_RUN;
+	}
+}
+
+static void
+g_multipath_fini(struct g_class *mp)
+{
+	if (g_multipath_kt_state == GKT_RUN) {
+		mtx_lock(&gmtbq_mtx);
+		g_multipath_kt_state = GKT_DIE;
+		wakeup(&g_multipath_kt_state);
+		msleep(&g_multipath_kt_state, &gmtbq_mtx, PRIBIO,
+		    "gmp:fini", 0);
+		mtx_unlock(&gmtbq_mtx);
+	}
+}
+
+static int
+g_multipath_read_metadata(struct g_consumer *cp,
+    struct g_multipath_metadata *md)
+{
+	struct g_provider *pp;
+	u_char *buf;
+	int error;
+
+	g_topology_assert();
+	error = g_access(cp, 1, 0, 0);
+	if (error != 0) {
+		return (error);
+	}
+	pp = cp->provider;
+	g_topology_unlock();
+	buf = g_read_data(cp, pp->mediasize - pp->sectorsize,
+	    pp->sectorsize, &error);
+	g_topology_lock();
+	g_access(cp, -1, 0, 0);
+	if (buf == NULL) {
+		return (error);
+	}
+	multipath_metadata_decode(buf, md);
+	g_free(buf);
+	return (0);
+}
+
+static struct g_geom *
+g_multipath_taste(struct g_class *mp, struct g_provider *pp, int flags __unused)
+{
+	struct g_multipath_metadata md;
+	struct g_multipath_softc *sc;
+	struct g_consumer *cp;
+	struct g_geom *gp, *gp1;
+	int error, isnew;
+
+	g_topology_assert();
+
+	gp = g_new_geomf(mp, "multipath:taste");
+	gp->start = g_multipath_start;
+	gp->access = g_multipath_access;
+	gp->orphan = g_multipath_orphan;
+	cp = g_new_consumer(gp);
+	g_attach(cp, pp);
+	error = g_multipath_read_metadata(cp, &md);
+	g_detach(cp);
+	g_destroy_consumer(cp);
+	g_destroy_geom(gp);
+	if (error != 0) {
+		return (NULL);
+	}
+	gp = NULL;
+
+	if (strcmp(md.md_magic, G_MULTIPATH_MAGIC) != 0) {
+		if (g_multipath_debug) {
+			printf("%s is not MULTIPATH\n", pp->name);
+		}
+		return (NULL);
+	}
+	if (md.md_version != G_MULTIPATH_VERSION) {
+		printf("%s has version %d multipath id- this module is version "
+		    " %d: rejecting\n", pp->name, md.md_version,
+		    G_MULTIPATH_VERSION);
+		return (NULL);
+	}
+	if (g_multipath_debug) {
+		printf("MULTIPATH: %s/%s\n", md.md_name, md.md_uuid);
+	}
+
+	/*
+	 * Let's check if such a device already is present. We check against
+	 * uuid alone first because that's the true distinguishor. If that
+	 * passes, then we check for name conflicts. If there are conflicts, 
+	 * modify the name.
+	 *
+	 * The whole purpose of this is to solve the problem that people don't
+	 * pick good unique names, but good unique names (like uuids) are a
+	 * pain to use. So, we allow people to build GEOMs with friendly names
+	 * and uuids, and modify the names in case there's a collision.
+	 */
+	sc = NULL;
+	LIST_FOREACH(gp, &mp->geom, geom) {
+		sc = gp->softc;
+		if (sc == NULL) {
+			continue;
+		}
+		if (strncmp(md.md_uuid, sc->sc_uuid, sizeof(md.md_uuid)) == 0) {
+			break;
+		}
+	}
+
+	LIST_FOREACH(gp1, &mp->geom, geom) {
+		if (gp1 == gp) {
+			continue;
+		}
+		sc = gp1->softc;
+		if (sc == NULL) {
+			continue;
+		}
+		if (strncmp(md.md_name, sc->sc_name, sizeof(md.md_name)) == 0) {
+			break;
+		}
+	}
+
+	/*
+	 * If gp is NULL, we had no extant MULTIPATH geom with this uuid.
+	 *
+	 * If gp1 is *not* NULL, that means we have a MULTIPATH geom extant
+	 * with the same name (but a different UUID).
+	 *
+	 * If gp is NULL, then modify the name with a random number and
+  	 * complain, but allow the creation of the geom to continue.
+	 *
+	 * If gp is *not* NULL, just use the geom's name as we're attaching
+	 * this disk to the (previously generated) name.
+	 */
+
+	if (gp1) {
+		sc = gp1->softc;
+		if (gp == NULL) {
+			char buf[16];
+			u_long rand = random();
+
+			snprintf(buf, sizeof (buf), "%s-%lu", md.md_name, rand);
+			printf("GEOM_MULTIPATH: geom %s/%s exists already\n",
+			    sc->sc_name, sc->sc_uuid);
+			printf("GEOM_MULTIPATH: %s will be (temporarily) %s\n",
+			    md.md_uuid, buf);
+			strlcpy(md.md_name, buf, sizeof (md.md_name));
+		} else {
+			strlcpy(md.md_name, sc->sc_name, sizeof (md.md_name));
+		}
+	}
+
+	if (gp == NULL) {
+		gp = g_multipath_create(mp, &md);
+		if (gp == NULL) {
+			printf("GEOM_MULTIPATH: cannot create geom %s/%s\n",
+			    md.md_name, md.md_uuid);
+			return (NULL);
+		}
+		isnew = 1;
+	} else {
+		isnew = 0;
+	}
+
+	sc = gp->softc;
+	KASSERT(sc != NULL, ("sc is NULL"));
+	error = g_multipath_add_disk(gp, pp);
+	if (error != 0) {
+		if (isnew) {
+			g_multipath_destroy(gp);
+		}
+		return (NULL);
+	}
+	return (gp);
+}
+
+static void
+g_multipath_ctl_create(struct gctl_req *req, struct g_class *mp)
+{
+	struct g_geom *gp;
+	struct g_provider *pp0, *pp1;
+	struct g_multipath_metadata md;
+	const char *name, *mpname, *uuid;
+	static const char devpf[6] = "/dev/";
+	int *nargs, error;
+
+	g_topology_assert();
+
+	mpname = gctl_get_asciiparam(req, "arg0");
+        if (mpname == NULL) {
+                gctl_error(req, "No 'arg0' argument");
+                return;
+        }
+
+	nargs = gctl_get_paraml(req, "nargs", sizeof(*nargs));
+	if (nargs == NULL) {
+		gctl_error(req, "No 'nargs' argument");
+		return;
+	}
+	if (*nargs != 4) {
+		gctl_error(req, "missing device or uuid arguments");
+		return;
+	}
+
+	name = gctl_get_asciiparam(req, "arg1");
+	if (name == NULL) {
+		gctl_error(req, "No 'arg1' argument");
+		return;
+	}
+	if (strncmp(name, devpf, 5) == 0) {
+		name += 5;
+	}
+	pp0 = g_provider_by_name(name);
+	if (pp0 == NULL) {
+		gctl_error(req, "Provider %s is invalid", name);
+		return;
+	}
+
+	name = gctl_get_asciiparam(req, "arg2");
+	if (name == NULL) {
+		gctl_error(req, "No 'arg2' argument");
+		return;
+	}
+	if (strncmp(name, devpf, 5) == 0) {
+		name += 5;
+	}
+	pp1 = g_provider_by_name(name);
+	if (pp1 == NULL) {
+		gctl_error(req, "Provider %s is invalid", name);
+		return;
+	}
+
+	uuid = gctl_get_asciiparam(req, "arg3");
+	if (uuid == NULL) {
+		gctl_error(req, "No uuid argument");
+		return;
+	}
+	if (strlen(uuid) != 36) {
+		gctl_error(req, "Malformed uuid argument");
+		return;
+	}
+
+	/*
+	 * Check to make sure parameters from the two providers are the same
+	 */
+	if (pp0 == pp1) {
+		gctl_error(req, "providers %s and %s are the same",
+		    pp0->name, pp1->name);
+		return;
+	}
+    	if (pp0->mediasize != pp1->mediasize) {
+		gctl_error(req, "Provider %s is %jd; Provider %s is %jd",
+		    pp0->name, (intmax_t) pp0->mediasize,
+		    pp1->name, (intmax_t) pp1->mediasize);
+		return;
+	}
+    	if (pp0->sectorsize != pp1->sectorsize) {
+		gctl_error(req, "Provider %s has sectorsize %u; Provider %s "
+		    "has sectorsize %u", pp0->name, pp0->sectorsize,
+		    pp1->name, pp1->sectorsize);
+		return;
+	}
+
+	/*
+	 * cons up enough of a metadata structure to use.
+	 */
+	memset(&md, 0, sizeof(md));
+	md.md_size = pp0->mediasize;
+	md.md_sectorsize = pp0->sectorsize;
+	strncpy(md.md_name, mpname, sizeof (md.md_name));
+	strncpy(md.md_uuid, uuid, sizeof (md.md_uuid));
+
+	gp = g_multipath_create(mp, &md);
+	if (gp == NULL) {
+		return;
+	}
+	error = g_multipath_add_disk(gp, pp0);
+	if (error) {
+		g_multipath_destroy(gp);
+		return;
+	}
+	error = g_multipath_add_disk(gp, pp1);
+	if (error) {
+		g_multipath_destroy(gp);
+		return;
+	}
+}
+
+static struct g_geom *
+g_multipath_find_geom(struct g_class *mp, const char *name)
+{
+	struct g_geom *gp;
+
+	LIST_FOREACH(gp, &mp->geom, geom) {
+		if (strcmp(gp->name, name) == 0) {
+			return (gp);
+		}
+	}
+	return (NULL);
+}
+
+static void
+g_multipath_ctl_destroy(struct gctl_req *req, struct g_class *mp)
+{
+	struct g_geom *gp;
+	const char *name;
+	int error;
+
+	g_topology_assert();
+
+	name = gctl_get_asciiparam(req, "arg0");
+        if (name == NULL) {
+                gctl_error(req, "No 'arg0' argument");
+                return;
+        }
+	gp = g_multipath_find_geom(mp, name);
+	if (gp == NULL) {
+		gctl_error(req, "Device %s is invalid", name);
+		return;
+	}
+	error = g_multipath_destroy(gp);
+	if (error != 0) {
+		gctl_error(req, "failed to destroy %s (err=%d)", name, error);
+	}
+}
+
+static void
+g_multipath_config(struct gctl_req *req, struct g_class *mp, const char *verb)
+{
+	uint32_t *version;
+	g_topology_assert();
+	version = gctl_get_paraml(req, "version", sizeof(*version));
+	if (version == NULL) {
+		gctl_error(req, "No 'version' argument");
+	} else if (*version != G_MULTIPATH_VERSION) {
+		gctl_error(req, "Userland and kernel parts are out of sync");
+	} else if (strcmp(verb, "create") == 0) {
+		g_multipath_ctl_create(req, mp);
+	} else if (strcmp(verb, "destroy") == 0) {
+		g_multipath_ctl_destroy(req, mp);
+	} else {
+		gctl_error(req, "Unknown verb %s", verb);
+	}
+}
+DECLARE_GEOM_CLASS(g_multipath_class, g_multipath);
diff --git a/sys/geom/multipath/g_multipath.h b/sys/geom/multipath/g_multipath.h
new file mode 100644
index 0000000..22d6157
--- /dev/null
+++ b/sys/geom/multipath/g_multipath.h
@@ -0,0 +1,99 @@
+/*-
+ * Copyright (c) 2006-2007 Matthew Jacob <mjacob@FreeBSD.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHORS AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ *
+ * $FreeBSD$
+ */
+/*
+ * Based upon work by Pawel Jakub Dawidek <pjd@FreeBSD.org> for all of the
+ * fine geom examples, and by Poul Henning Kamp <phk@FreeBSD.org> for GEOM
+ * itself, all of which is most gratefully acknowledged.
+ */
+
+#ifndef	_G_MULTIPATH_H_
+#define	_G_MULTIPATH_H_
+
+#define	G_MULTIPATH_CLASS_NAME	"MULTIPATH"
+#define	G_MULTIPATH_VERSION	1
+#define	G_MULTIPATH_MAGIC	"GEOM::MULTIPATH"
+
+#include <sys/endian.h>
+
+#ifdef	_KERNEL
+
+struct g_multipath_softc {
+	struct g_provider *	pp;
+	struct g_consumer *	cp_active;
+	char			sc_name[16];
+	char			sc_uuid[40];
+};
+#endif	/* _KERNEL */
+
+struct g_multipath_metadata {
+	char		md_magic[16];	/* Magic Value */
+	char 		md_uuid[40];	/* more magic */
+	char		md_name[16];	/* a friendly name */
+	uint32_t	md_version;	/* version */
+	uint32_t	md_sectorsize;	/* sectorsize of provider */
+	uint64_t	md_size;	/* absolute size of provider */
+};
+
+static __inline void
+multipath_metadata_encode(const struct g_multipath_metadata *, u_char *);
+
+static __inline void
+multipath_metadata_decode(u_char *, struct g_multipath_metadata *);
+
+static __inline void
+multipath_metadata_encode(const struct g_multipath_metadata *md, u_char *data)
+{
+	bcopy(md->md_magic, data, sizeof(md->md_magic));
+	data += sizeof(md->md_magic);
+	bcopy(md->md_uuid, data, sizeof(md->md_uuid));
+	data += sizeof(md->md_uuid);
+	bcopy(md->md_name, data, sizeof(md->md_name));
+	data += sizeof(md->md_name);
+	le32enc(data, md->md_version);
+	data += sizeof(md->md_version);
+	le32enc(data, md->md_sectorsize);
+	data += sizeof(md->md_sectorsize);
+	le64enc(data, md->md_size);
+}
+
+static __inline void
+multipath_metadata_decode(u_char *data, struct g_multipath_metadata *md)
+{
+	bcopy(data, md->md_magic, sizeof(md->md_magic));
+	data += sizeof(md->md_magic);
+	bcopy(data, md->md_uuid, sizeof(md->md_uuid));
+	data += sizeof(md->md_uuid);
+	bcopy(data, md->md_name, sizeof(md->md_name));
+	data += sizeof(md->md_name);
+	md->md_version = le32dec(data);
+	data += sizeof(md->md_version);
+	md->md_sectorsize = le32dec(data);
+	data += sizeof(md->md_sectorsize);
+	md->md_size = le64dec(data);
+}
+#endif	/* _G_MULTIPATH_H_ */
diff --git a/sys/modules/geom/Makefile b/sys/modules/geom/Makefile
index 011ff4c..26e2209 100644
--- a/sys/modules/geom/Makefile
+++ b/sys/modules/geom/Makefile
@@ -12,6 +12,7 @@ SUBDIR=	geom_bde \
 	geom_label \
 	geom_mbr \
 	geom_mirror \
+	geom_multipath \
 	geom_nop \
 	geom_pc98 \
 	geom_raid3 \
diff --git a/sys/modules/geom/geom_multipath/Makefile b/sys/modules/geom/geom_multipath/Makefile
new file mode 100644
index 0000000..d036fe0
--- /dev/null
+++ b/sys/modules/geom/geom_multipath/Makefile
@@ -0,0 +1,8 @@
+# $FreeBSD$
+
+.PATH: ${.CURDIR}/../../../geom/multipath
+
+KMOD=	geom_multipath
+SRCS=	g_multipath.c
+
+.include <bsd.kmod.mk>