Bring in the GEOM Virtualisation class, which allows to create huge GEOM

providers with limited physical storage and add physical storage as
needed.

Submitted by:	Ivan Voras
Sponsored by:	Google Summer of Code 2006
Approved by:	re (kensmith)

6 files changed, 2437 insertions, 0 deletions

diff --git a/sys/geom/virstor/binstream.c b/sys/geom/virstor/binstream.c
new file mode 100644
index 0000000..bbbafdb
--- /dev/null
+++ b/sys/geom/virstor/binstream.c
@@ -0,0 +1,185 @@
+/*-
+ * Copyright (c) 2005 Ivan Voras <ivoras@gmail.com>
+ * 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.
+ */
+
+// $Id: binstream.c,v 1.1 2006/07/05 10:47:54 ivoras Exp $
+
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
+#include <sys/endian.h>
+#include <sys/param.h>
+
+#include <geom/virstor/binstream.h>
+
+
+/* "Open" a binary stream for reading */
+void
+bs_open(bin_stream_t * bs, void *data)
+{
+	bs->data = (char *)data;
+	bs->pos = 0;
+}
+
+
+/* "Reset" position in binary stream to zero */
+void
+bs_reset(bin_stream_t * bs)
+{
+	bs->pos = 0;
+}
+
+/* Write a zero-terminated string; return next position */
+unsigned
+bs_write_str(bin_stream_t * bs, char *data)
+{
+	int		len = 0;
+	do {
+		*(bs->data + bs->pos + len) = *data;
+		len++;
+	} while (*(data++) != '\0');
+	bs->pos += len;
+	return bs->pos;
+}
+
+
+/* Write an arbitrary buffer; return next position */
+unsigned
+bs_write_buf(bin_stream_t * bs, char *data, unsigned data_size)
+{
+	unsigned	i;
+	for (i = 0; i < data_size; i++)
+		*(bs->data + bs->pos + i) = *(data + i);
+	bs->pos += data_size;
+	return bs->pos;
+}
+
+
+/* Write a 8bit uint; return next position. */
+unsigned
+bs_write_u8(bin_stream_t * bs, uint8_t data)
+{
+	*((uint8_t *) (bs->data + bs->pos)) = data;
+	return ++(bs->pos);
+}
+
+
+/* Write a 16bit uint; return next position. */
+unsigned
+bs_write_u16(bin_stream_t * bs, uint16_t data)
+{
+	le16enc(bs->data + bs->pos, data);
+	return (bs->pos += 2);
+}
+
+
+/* Write a 32bit uint; return next position. */
+unsigned
+bs_write_u32(bin_stream_t * bs, uint32_t data)
+{
+	le32enc(bs->data + bs->pos, data);
+	return (bs->pos += 4);
+}
+
+
+/* Write a 64bit uint; return next position. */
+unsigned
+bs_write_u64(bin_stream_t * bs, uint64_t data)
+{
+	le64enc(bs->data + bs->pos, data);
+	return (bs->pos += 8);
+}
+
+
+/* Read a 8bit uint & return it */
+uint8_t
+bs_read_u8(bin_stream_t * bs)
+{
+	uint8_t		data = *((uint8_t *) (bs->data + bs->pos));
+	bs->pos++;
+	return data;
+}
+
+
+/*
+ * Read a null-terminated string from stream into a buffer; buf_size is size
+ * of the buffer, including the final \0. Returns buf pointer or NULL if
+ * garbage input.
+ */
+char*
+bs_read_str(bin_stream_t * bs, char *buf, unsigned buf_size)
+{
+	unsigned	len = 0;
+	char           *work_buf = buf;
+	if (buf == NULL || buf_size < 1)
+		return NULL;
+	do {
+		*work_buf = *(bs->data + bs->pos + len);
+	} while (len++ < buf_size - 1 && *(work_buf++) != '\0');
+	*(buf + buf_size - 1) = '\0';
+	bs->pos += len;
+	return buf;
+}
+
+
+/* Read an arbitrary buffer. */
+void
+bs_read_buf(bin_stream_t * bs, char *buf, unsigned buf_size)
+{
+	unsigned	i;
+	for (i = 0; i < buf_size; i++)
+		*(buf + i) = *(bs->data + bs->pos + i);
+	bs->pos += buf_size;
+}
+
+
+/* Read a 16bit uint & return it */
+uint16_t
+bs_read_u16(bin_stream_t * bs)
+{
+	uint16_t	data = le16dec(bs->data + bs->pos);
+	bs->pos += 2;
+	return data;
+}
+
+
+/* Read a 32bit uint & return it */
+uint32_t
+bs_read_u32(bin_stream_t * bs)
+{
+	uint32_t	data = le32dec(bs->data + bs->pos);
+	bs->pos += 4;
+	return data;
+}
+
+
+/* Read a 64bit uint & return it */
+uint64_t
+bs_read_u64(bin_stream_t * bs)
+{
+	uint64_t	data = le64dec(bs->data + bs->pos);
+	bs->pos += 8;
+	return data;
+}
diff --git a/sys/geom/virstor/binstream.h b/sys/geom/virstor/binstream.h
new file mode 100644
index 0000000..4ff4e6b
--- /dev/null
+++ b/sys/geom/virstor/binstream.h
@@ -0,0 +1,93 @@
+/*-
+ * Copyright (c) 2005 Ivan Voras <ivoras@gmail.com>
+ * 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.
+ *
+ * $FreeBSD$
+ */
+
+// $Id: binstream.h,v 1.1 2006/07/05 10:47:54 ivoras Exp $
+
+
+#ifndef _BIN_STREAM_
+#define _BIN_STREAM_
+
+#ifndef uint8_t
+#define uint8_t unsigned char
+#endif
+
+typedef struct {
+	unsigned char  *data;
+	int		pos;
+}	bin_stream_t;
+
+
+/* "Open" a binary stream for reading */
+void		bs_open   (bin_stream_t * bs, void *data);
+
+/* "Reset" position in binary stream to zero */
+void		bs_reset  (bin_stream_t * bs);
+
+
+/* Write a zero-terminated string; return next position */
+unsigned	bs_write_str(bin_stream_t * bs, char *data);
+
+/* Write an arbitrary buffer; return next position */
+unsigned	bs_write_buf(bin_stream_t * bs, char *data, unsigned data_size);
+
+/* Write a 8bit uint; return next position. */
+unsigned	bs_write_u8(bin_stream_t * bs, uint8_t data);
+
+/* Write a 16bit uint; return next position. */
+unsigned	bs_write_u16(bin_stream_t * bs, uint16_t data);
+
+/* Write a 32bit uint; return next position. */
+unsigned	bs_write_u32(bin_stream_t * bs, uint32_t data);
+
+/* Write a 64bit uint; return next position. */
+unsigned	bs_write_u64(bin_stream_t * bs, uint64_t data);
+
+
+/*
+ * Read a null-terminated string from stream into a buffer; buf_size is size
+ * of the buffer, including the final \0. Returns buf pointer or NULL if
+ * garbage input.
+ */
+char           *bs_read_str(bin_stream_t * bs, char *buf, unsigned buf_size);
+
+/* Read an arbitrary buffer. */
+void		bs_read_buf(bin_stream_t * bs, char *buf, unsigned buf_size);
+
+/* Read a 8bit uint * return it */
+uint8_t		bs_read_u8(bin_stream_t * bs);
+
+/* Read a 16bit uint * return it */
+uint16_t	bs_read_u16(bin_stream_t * bs);
+
+/* Read a 8bit uint * return it */
+uint32_t	bs_read_u32(bin_stream_t * bs);
+
+/* Read a 8bit uint * return it */
+uint64_t	bs_read_u64(bin_stream_t * bs);
+
+#endif
diff --git a/sys/geom/virstor/g_virstor.c b/sys/geom/virstor/g_virstor.c
new file mode 100644
index 0000000..bcb2ba7
--- /dev/null
+++ b/sys/geom/virstor/g_virstor.c
@@ -0,0 +1,1864 @@
+/*-
+ * Copyright (c) 2006-2007 Ivan Voras <ivoras@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.
+ */
+
+/* Implementation notes:
+ * - "Components" are wrappers around providers that make up the
+ *   virtual storage (i.e. a virstor has "physical" components)
+ */
+
+#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/sx.h>
+#include <sys/bio.h>
+#include <sys/sysctl.h>
+#include <sys/malloc.h>
+#include <sys/time.h>
+#include <sys/proc.h>
+#include <sys/kthread.h>
+#include <sys/mutex.h>
+#include <vm/uma.h>
+#include <geom/geom.h>
+
+#include <geom/virstor/g_virstor.h>
+#include <geom/virstor/g_virstor_md.h>
+
+/* Declare malloc(9) label */
+static MALLOC_DEFINE(M_GVIRSTOR, "gvirstor", "GEOM_VIRSTOR Data");
+
+/* GEOM class methods */
+static g_init_t g_virstor_init;
+static g_fini_t g_virstor_fini;
+static g_taste_t g_virstor_taste;
+static g_ctl_req_t g_virstor_config;
+static g_ctl_destroy_geom_t g_virstor_destroy_geom;
+
+/* Declare & initialize class structure ("geom class") */
+struct g_class g_virstor_class = {
+	.name =		G_VIRSTOR_CLASS_NAME,
+	.version =	G_VERSION,
+	.init =		g_virstor_init,
+	.fini =		g_virstor_fini,
+	.taste =	g_virstor_taste,
+	.ctlreq =	g_virstor_config,
+	.destroy_geom = g_virstor_destroy_geom
+	/* The .dumpconf and the rest are only usable for a geom instance, so
+	 * they will be set when such instance is created. */
+};
+
+/* Declare sysctl's and loader tunables */
+SYSCTL_DECL(_kern_geom);
+SYSCTL_NODE(_kern_geom, OID_AUTO, virstor, CTLFLAG_RW, 0, "GEOM_GVIRSTOR information");
+
+static u_int g_virstor_debug = 2; /* XXX: lower to 2 when released to public */
+TUNABLE_INT("kern.geom.virstor.debug", &g_virstor_debug);
+SYSCTL_UINT(_kern_geom_virstor, OID_AUTO, debug, CTLFLAG_RW, &g_virstor_debug,
+    0, "Debug level (2=production, 5=normal, 15=excessive)");
+
+static u_int g_virstor_chunk_watermark = 100;
+TUNABLE_INT("kern.geom.virstor.chunk_watermark", &g_virstor_chunk_watermark);
+SYSCTL_UINT(_kern_geom_virstor, OID_AUTO, chunk_watermark, CTLFLAG_RW,
+    &g_virstor_chunk_watermark, 0,
+    "Minimum number of free chunks before issuing administrative warning");
+
+static u_int g_virstor_component_watermark = 1;
+TUNABLE_INT("kern.geom.virstor.component_watermark",
+    &g_virstor_component_watermark);
+SYSCTL_UINT(_kern_geom_virstor, OID_AUTO, component_watermark, CTLFLAG_RW,
+    &g_virstor_component_watermark, 0,
+    "Minimum number of free components before issuing administrative warning");
+
+static int read_metadata(struct g_consumer *, struct g_virstor_metadata *);
+static int write_metadata(struct g_consumer *, struct g_virstor_metadata *);
+static int clear_metadata(struct g_virstor_component *);
+static int add_provider_to_geom(struct g_virstor_softc *, struct g_provider *,
+    struct g_virstor_metadata *);
+static struct g_geom *create_virstor_geom(struct g_class *,
+    struct g_virstor_metadata *);
+static void virstor_check_and_run(struct g_virstor_softc *);
+static u_int virstor_valid_components(struct g_virstor_softc *);
+static int virstor_geom_destroy(struct g_virstor_softc *, boolean_t,
+    boolean_t);
+static void remove_component(struct g_virstor_softc *,
+    struct g_virstor_component *, boolean_t);
+static void bioq_dismantle(struct bio_queue_head *);
+static int allocate_chunk(struct g_virstor_softc *,
+    struct g_virstor_component **, u_int *, u_int *);
+static void delay_destroy_consumer(void *, int);
+static void dump_component(struct g_virstor_component *comp);
+#if 0
+static void dump_me(struct virstor_map_entry *me, unsigned int nr);
+#endif
+
+static void virstor_ctl_stop(struct gctl_req *, struct g_class *);
+static void virstor_ctl_add(struct gctl_req *, struct g_class *);
+static void virstor_ctl_remove(struct gctl_req *, struct g_class *);
+static struct g_virstor_softc * virstor_find_geom(const struct g_class *,
+    const char *);
+static void update_metadata(struct g_virstor_softc *);
+static void fill_metadata(struct g_virstor_softc *, struct g_virstor_metadata *,
+    u_int, u_int);
+
+static void g_virstor_orphan(struct g_consumer *);
+static int g_virstor_access(struct g_provider *, int, int, int);
+static void g_virstor_start(struct bio *);
+static void g_virstor_dumpconf(struct sbuf *, const char *, struct g_geom *,
+    struct g_consumer *, struct g_provider *);
+static void g_virstor_done(struct bio *);
+
+static void invalid_call(void);
+/*
+ * Initialise GEOM class (per-class callback)
+ */
+static void
+g_virstor_init(struct g_class *mp __unused)
+{
+
+	/* Catch map struct size mismatch at compile time; Map entries must
+	 * fit into MAXPHYS exactly, with no wasted space. */
+	CTASSERT(VIRSTOR_MAP_BLOCK_ENTRIES*VIRSTOR_MAP_ENTRY_SIZE == MAXPHYS);
+
+	/* Init UMA zones, TAILQ's, other global vars */
+}
+
+/*
+ * Finalise GEOM class (per-class callback)
+ */
+static void
+g_virstor_fini(struct g_class *mp __unused)
+{
+
+	/* Deinit UMA zones & global vars */
+}
+
+/*
+ * Config (per-class callback)
+ */
+static void
+g_virstor_config(struct gctl_req *req, struct g_class *cp, char const *verb)
+{
+	uint32_t *version;
+
+	g_topology_assert();
+
+	version = gctl_get_paraml(req, "version", sizeof(*version));
+	if (version == NULL) {
+		gctl_error(req, "Failed to get 'version' argument");
+		return;
+	}
+	if (*version != G_VIRSTOR_VERSION) {
+		gctl_error(req, "Userland and kernel versions out of sync");
+		return;
+	}
+
+	g_topology_unlock();
+	if (strcmp(verb, "add") == 0)
+		virstor_ctl_add(req, cp);
+	else if (strcmp(verb, "stop") == 0 || strcmp(verb, "destroy") == 0)
+		virstor_ctl_stop(req, cp);
+	else if (strcmp(verb, "remove") == 0)
+		virstor_ctl_remove(req, cp);
+	else
+		gctl_error(req, "unknown verb: '%s'", verb);
+	g_topology_lock();
+}
+
+/*
+ * "stop" verb from userland
+ */
+static void
+virstor_ctl_stop(struct gctl_req *req, struct g_class *cp)
+{
+	int *force, *nargs;
+	int i;
+
+	nargs = gctl_get_paraml(req, "nargs", sizeof *nargs);
+	if (nargs == NULL) {
+		gctl_error(req, "Error fetching argument '%s'", "nargs");
+		return;
+	}
+	if (*nargs < 1) {
+		gctl_error(req, "Invalid number of arguments");
+		return;
+	}
+	force = gctl_get_paraml(req, "force", sizeof *force);
+	if (force == NULL) {
+		gctl_error(req, "Error fetching argument '%s'", "force");
+		return;
+	}
+
+	g_topology_lock();
+	for (i = 0; i < *nargs; i++) {
+		char param[8];
+		const char *name;
+		struct g_virstor_softc *sc;
+		int error;
+
+		sprintf(param, "arg%d", i);
+		name = gctl_get_asciiparam(req, param);
+		sc = virstor_find_geom(cp, name);
+		LOG_MSG(LVL_INFO, "Stopping %s by the userland command",
+		    sc->geom->name);
+		update_metadata(sc);
+		if ((error = virstor_geom_destroy(sc, TRUE, TRUE)) != 0) {
+			LOG_MSG(LVL_ERROR, "Cannot destroy %s: %d",
+			    sc->geom->name, error);
+		}
+	}
+	g_topology_unlock();
+}
+
+/*
+ * "add" verb from userland - add new component(s) to the structure.
+ * This will be done all at once in here, without going through the
+ * .taste function for new components.
+ */
+static void
+virstor_ctl_add(struct gctl_req *req, struct g_class *cp)
+{
+	/* Note: while this is going on, I/O is being done on
+	 * the g_up and g_down threads. The idea is to make changes
+	 * to softc members in a way that can atomically activate
+	 * them all at once. */
+	struct g_virstor_softc *sc;
+	int *hardcode, *nargs;
+	const char *geom_name;	/* geom to add a component to */
+	struct g_consumer *fcp;
+	struct g_virstor_bio_q *bq;
+	u_int added;
+	int error;
+	int i;
+
+	nargs = gctl_get_paraml(req, "nargs", sizeof(*nargs));
+	if (nargs == NULL) {
+		gctl_error(req, "Error fetching argument '%s'", "nargs");
+		return;
+	}
+	if (*nargs < 2) {
+		gctl_error(req, "Invalid number of arguments");
+		return;
+	}
+	hardcode = gctl_get_paraml(req, "hardcode", sizeof(*hardcode));
+	if (hardcode == NULL) {
+		gctl_error(req, "Error fetching argument '%s'", "hardcode");
+		return;
+	}
+
+	/* Find "our" geom */
+	geom_name = gctl_get_asciiparam(req, "arg0");
+	if (geom_name == NULL) {
+		gctl_error(req, "Error fetching argument '%s'", "geom_name (arg0)");
+		return;
+	}
+	sc = virstor_find_geom(cp, geom_name);
+	if (sc == NULL) {
+		gctl_error(req, "Don't know anything about '%s'", geom_name);
+		return;
+	}
+
+	if (virstor_valid_components(sc) != sc->n_components) {
+		LOG_MSG(LVL_ERROR, "Cannot add components to incomplete "
+		    "virstor %s", sc->geom->name);
+		gctl_error(req, "Virstor %s is incomplete", sc->geom->name);
+		return;
+	}
+
+	fcp = sc->components[0].gcons;
+	added = 0;
+	g_topology_lock();
+	for (i = 1; i < *nargs; i++) {
+		struct g_virstor_metadata md;
+		char aname[8];
+		const char *prov_name;
+		struct g_provider *pp;
+		struct g_consumer *cp;
+		u_int nc;
+		u_int j;
+
+		snprintf(aname, sizeof aname, "arg%d", i);
+		prov_name = gctl_get_asciiparam(req, aname);
+		if (strncmp(prov_name, _PATH_DEV, strlen(_PATH_DEV)) == 0)
+			prov_name += strlen(_PATH_DEV);
+
+		pp = g_provider_by_name(prov_name);
+		if (pp == NULL) {
+			/* This is the most common error so be verbose about it */
+			if (added != 0) {
+				gctl_error(req, "Invalid provider: '%s' (added"
+				    " %u components)", prov_name, added);
+				update_metadata(sc);
+			} else {
+				gctl_error(req, "Invalid provider: '%s'",
+				    prov_name);
+			}
+			g_topology_unlock();
+			return;
+		}
+		cp = g_new_consumer(sc->geom);
+		if (cp == NULL) {
+			gctl_error(req, "Cannot create consumer");
+			g_topology_unlock();
+			return;
+		}
+		error = g_attach(cp, pp);
+		if (error != 0) {
+			gctl_error(req, "Cannot attach a consumer to %s",
+			    pp->name);
+			g_destroy_consumer(cp);
+			g_topology_unlock();
+			return;
+		}
+		if (fcp->acr != 0 || fcp->acw != 0 || fcp->ace != 0) {
+			error = g_access(cp, fcp->acr, fcp->acw, fcp->ace);
+			if (error != 0) {
+				gctl_error(req, "Access request failed for %s",
+				    pp->name);
+				g_destroy_consumer(cp);
+				g_topology_unlock();
+				return;
+			}
+		}
+		if (fcp->provider->sectorsize != pp->sectorsize) {
+			gctl_error(req, "Sector size doesn't fit for %s",
+			    pp->name);
+			g_destroy_consumer(cp);
+			g_topology_unlock();
+			return;
+		}
+		for (j = 0; j < sc->n_components; j++) {
+			if (strcmp(sc->components[j].gcons->provider->name,
+			    pp->name) == 0) {
+				gctl_error(req, "Component %s already in %s",
+				    pp->name, sc->geom->name);
+				g_destroy_consumer(cp);
+				g_topology_unlock();
+				return;
+			}
+		}
+		sc->components = realloc(sc->components,
+		    sizeof(*sc->components) * (sc->n_components + 1),
+		    M_GVIRSTOR, M_WAITOK);
+
+		nc = sc->n_components;
+		sc->components[nc].gcons = cp;
+		sc->components[nc].sc = sc;
+		sc->components[nc].index = nc;
+		sc->components[nc].chunk_count = cp->provider->mediasize /
+		    sc->chunk_size;
+		sc->components[nc].chunk_next = 0;
+		sc->components[nc].chunk_reserved = 0;
+
+		if (sc->components[nc].chunk_count < 4) {
+			gctl_error(req, "Provider too small: %s",
+			    cp->provider->name);
+			g_destroy_consumer(cp);
+			g_topology_unlock();
+			return;
+		}
+		fill_metadata(sc, &md, nc, *hardcode);
+		write_metadata(cp, &md);
+		/* The new component becomes visible when n_components is
+		 * incremented */
+		sc->n_components++;
+		added++;
+
+	}
+	/* This call to update_metadata() is critical. In case there's a
+	 * power failure in the middle of it and some components are updated
+	 * while others are not, there will be trouble on next .taste() iff
+	 * a non-updated component is detected first */
+	update_metadata(sc);
+	g_topology_unlock();
+	LOG_MSG(LVL_INFO, "Added %d component(s) to %s", added,
+	    sc->geom->name);
+	/* Fire off BIOs previously queued because there wasn't any
+	 * physical space left. If the BIOs still can't be satisfied
+	 * they will again be added to the end of the queue (during
+	 * which the mutex will be recursed) */
+	bq = malloc(sizeof(*bq), M_GVIRSTOR, M_WAITOK);
+	bq->bio = NULL;
+	mtx_lock(&sc->delayed_bio_q_mtx);
+	/* First, insert a sentinel to the queue end, so we don't
+	 * end up in an infinite loop if there's still no free
+	 * space available. */
+	STAILQ_INSERT_TAIL(&sc->delayed_bio_q, bq, linkage);
+	while (!STAILQ_EMPTY(&sc->delayed_bio_q)) {
+		bq = STAILQ_FIRST(&sc->delayed_bio_q);
+		if (bq->bio != NULL) {
+			g_virstor_start(bq->bio);
+			STAILQ_REMOVE_HEAD(&sc->delayed_bio_q, linkage);
+			free(bq, M_GVIRSTOR);
+		} else {
+			STAILQ_REMOVE_HEAD(&sc->delayed_bio_q, linkage);
+			free(bq, M_GVIRSTOR);
+			break;
+		}
+	}
+	mtx_unlock(&sc->delayed_bio_q_mtx);
+
+}
+
+/*
+ * Find a geom handled by the class
+ */
+static struct g_virstor_softc *
+virstor_find_geom(const struct g_class *cp, const char *name)
+{
+	struct g_geom *gp;
+
+	LIST_FOREACH(gp, &cp->geom, geom) {
+		if (strcmp(name, gp->name) == 0)
+			return (gp->softc);
+	}
+	return (NULL);
+}
+
+/*
+ * Update metadata on all components to reflect the current state
+ * of these fields:
+ *    - chunk_next
+ *    - flags
+ *    - md_count
+ * Expects things to be set up so write_metadata() can work, i.e.
+ * the topology lock must be held.
+ */
+static void
+update_metadata(struct g_virstor_softc *sc)
+{
+	struct g_virstor_metadata md;
+	int n;
+
+	if (virstor_valid_components(sc) != sc->n_components)
+		return; /* Incomplete device */
+	LOG_MSG(LVL_DEBUG, "Updating metadata on components for %s",
+	    sc->geom->name);
+	/* Update metadata on components */
+	g_trace(G_T_TOPOLOGY, "%s(%s, %s)", __func__,
+	    sc->geom->class->name, sc->geom->name);
+	g_topology_assert();
+	for (n = 0; n < sc->n_components; n++) {
+		read_metadata(sc->components[n].gcons, &md);
+		md.chunk_next = sc->components[n].chunk_next;
+		md.flags = sc->components[n].flags;
+		md.md_count = sc->n_components;
+		write_metadata(sc->components[n].gcons, &md);
+	}
+}
+
+/*
+ * Fills metadata (struct md) from information stored in softc and the nc'th
+ * component of virstor
+ */
+static void
+fill_metadata(struct g_virstor_softc *sc, struct g_virstor_metadata *md,
+    u_int nc, u_int hardcode)
+{
+	struct g_virstor_component *c;
+
+	bzero(md, sizeof *md);
+	c = &sc->components[nc];
+
+	strncpy(md->md_magic, G_VIRSTOR_MAGIC, sizeof md->md_magic);
+	md->md_version = G_VIRSTOR_VERSION;
+	strncpy(md->md_name, sc->geom->name, sizeof md->md_name);
+	md->md_id = sc->id;
+	md->md_virsize = sc->virsize;
+	md->md_chunk_size = sc->chunk_size;
+	md->md_count = sc->n_components;
+
+	if (hardcode) {
+		strncpy(md->provider, c->gcons->provider->name,
+		    sizeof md->provider);
+	}
+	md->no = nc;
+	md->provsize = c->gcons->provider->mediasize;
+	md->chunk_count = c->chunk_count;
+	md->chunk_next = c->chunk_next;
+	md->chunk_reserved = c->chunk_reserved;
+	md->flags = c->flags;
+}
+
+/*
+ * Remove a component from virstor device.
+ * Can only be done if the component is unallocated.
+ */
+static void
+virstor_ctl_remove(struct gctl_req *req, struct g_class *cp)
+{
+	/* As this is executed in parallel to I/O, operations on virstor
+	 * structures must be as atomic as possible. */
+	struct g_virstor_softc *sc;
+	int *nargs;
+	const char *geom_name;
+	u_int removed;
+	int i;
+
+	nargs = gctl_get_paraml(req, "nargs", sizeof(*nargs));
+	if (nargs == NULL) {
+		gctl_error(req, "Error fetching argument '%s'", "nargs");
+		return;
+	}
+	if (*nargs < 2) {
+		gctl_error(req, "Invalid number of arguments");
+		return;
+	}
+	/* Find "our" geom */
+	geom_name = gctl_get_asciiparam(req, "arg0");
+	if (geom_name == NULL) {
+		gctl_error(req, "Error fetching argument '%s'",
+		    "geom_name (arg0)");
+		return;
+	}
+	sc = virstor_find_geom(cp, geom_name);
+	if (sc == NULL) {
+		gctl_error(req, "Don't know anything about '%s'", geom_name);
+		return;
+	}
+
+	if (virstor_valid_components(sc) != sc->n_components) {
+		LOG_MSG(LVL_ERROR, "Cannot remove components from incomplete "
+		    "virstor %s", sc->geom->name);
+		gctl_error(req, "Virstor %s is incomplete", sc->geom->name);
+		return;
+	}
+
+	removed = 0;
+	for (i = 1; i < *nargs; i++) {
+		char param[8];
+		const char *prov_name;
+		int j, found;
+		struct g_virstor_component *newcomp, *compbak;
+
+		sprintf(param, "arg%d", i);
+		prov_name = gctl_get_asciiparam(req, param);
+		if (strncmp(prov_name, _PATH_DEV, strlen(_PATH_DEV)) == 0)
+			prov_name += strlen(_PATH_DEV);
+
+		found = -1;
+		for (j = 0; j < sc->n_components; j++) {
+			if (strcmp(sc->components[j].gcons->provider->name,
+			    prov_name) == 0) {
+				found = j;
+				break;
+			}
+		}
+		if (found == -1) {
+			LOG_MSG(LVL_ERROR, "No %s component in %s",
+			    prov_name, sc->geom->name);
+			continue;
+		}
+
+		compbak = sc->components;
+		newcomp = malloc(sc->n_components * sizeof(*sc->components),
+		    M_GVIRSTOR, M_WAITOK | M_ZERO);
+		bcopy(sc->components, newcomp, found * sizeof(*sc->components));
+		bcopy(&sc->components[found + 1], newcomp + found,
+		    found * sizeof(*sc->components));
+		if ((sc->components[j].flags & VIRSTOR_PROVIDER_ALLOCATED) != 0) {
+			LOG_MSG(LVL_ERROR, "Allocated provider %s cannot be "
+			    "removed from %s",
+			    prov_name, sc->geom->name);
+			free(newcomp, M_GVIRSTOR);
+			/* We'll consider this non-fatal error */
+			continue;
+		}
+		/* Renumerate unallocated components */
+		for (j = 0; j < sc->n_components-1; j++) {
+			if ((sc->components[j].flags &
+			    VIRSTOR_PROVIDER_ALLOCATED) == 0) {
+				sc->components[j].index = j;
+			}
+		}
+		/* This is the critical section. If a component allocation
+		 * event happens while both variables are not yet set,
+		 * there will be trouble. Something will panic on encountering
+		 * NULL sc->components[x].gcomp member.
+		 * Luckily, component allocation happens very rarely and
+		 * removing components is an abnormal action in any case. */
+		sc->components = newcomp;
+		sc->n_components--;
+		/* End critical section */
+
+		g_topology_lock();
+		if (clear_metadata(&compbak[found]) != 0) {
+			LOG_MSG(LVL_WARNING, "Trouble ahead: cannot clear "
+			    "metadata on %s", prov_name);
+		}
+		g_detach(compbak[found].gcons);
+		g_destroy_consumer(compbak[found].gcons);
+		g_topology_unlock();
+
+		free(compbak, M_GVIRSTOR);
+
+		removed++;
+	}
+
+	/* This call to update_metadata() is critical. In case there's a
+	 * power failure in the middle of it and some components are updated
+	 * while others are not, there will be trouble on next .taste() iff
+	 * a non-updated component is detected first */
+	g_topology_lock();
+	update_metadata(sc);
+	g_topology_unlock();
+	LOG_MSG(LVL_INFO, "Removed %d component(s) from %s", removed,
+	    sc->geom->name);
+}
+
+/*
+ * Clear metadata sector on component
+ */
+static int
+clear_metadata(struct g_virstor_component *comp)
+{
+	char *buf;
+	int error;
+
+	LOG_MSG(LVL_INFO, "Clearing metadata on %s",
+	    comp->gcons->provider->name);
+	g_topology_assert();
+	error = g_access(comp->gcons, 0, 1, 0);
+	if (error != 0)
+		return (error);
+	buf = malloc(comp->gcons->provider->sectorsize, M_GVIRSTOR,
+	    M_WAITOK | M_ZERO);
+	error = g_write_data(comp->gcons,
+	    comp->gcons->provider->mediasize -
+	    comp->gcons->provider->sectorsize,
+	    buf,
+	    comp->gcons->provider->sectorsize);
+	free(buf, M_GVIRSTOR);
+	g_access(comp->gcons, 0, -1, 0);
+	return (error);
+}
+
+/*
+ * Destroy geom forcibly.
+ */
+static int
+g_virstor_destroy_geom(struct gctl_req *req __unused, struct g_class *mp,
+    struct g_geom *gp)
+{
+	struct g_virstor_softc *sc;
+	int exitval;
+
+	sc = gp->softc;
+	KASSERT(sc != NULL, ("%s: NULL sc", __func__));
+	
+	exitval = 0;
+	LOG_MSG(LVL_DEBUG, "%s called for %s, sc=%p", __func__, gp->name,
+	    gp->softc);
+
+	if (sc != NULL) {
+#ifdef INVARIANTS
+		char *buf;
+		int error;
+		off_t off;
+		int isclean, count;
+		int n;
+
+		LOG_MSG(LVL_INFO, "INVARIANTS detected");
+		LOG_MSG(LVL_INFO, "Verifying allocation "
+		    "table for %s", sc->geom->name);
+		count = 0;
+		for (n = 0; n < sc->chunk_count; n++) {
+			if (sc->map[n].flags || VIRSTOR_MAP_ALLOCATED != 0)
+				count++;
+		}
+		LOG_MSG(LVL_INFO, "Device %s has %d allocated chunks",
+		    sc->geom->name, count);
+		n = off = count = 0;
+		isclean = 1;
+		if (virstor_valid_components(sc) != sc->n_components) {
+			/* This is a incomplete virstor device (not all
+			 * components have been found) */
+			LOG_MSG(LVL_ERROR, "Device %s is incomplete",
+			    sc->geom->name);
+			goto bailout;
+		}
+		error = g_access(sc->components[0].gcons, 1, 0, 0);
+		KASSERT(error == 0, ("%s: g_access failed (%d)", __func__,
+		    error));
+		/* Compare the whole on-disk allocation table with what's
+		 * currently in memory */
+		while (n < sc->chunk_count) {
+			buf = g_read_data(sc->components[0].gcons, off,
+			    sc->sectorsize, &error);
+			KASSERT(buf != NULL, ("g_read_data returned NULL (%d) "
+			    "for read at %jd", error, off));
+			if (bcmp(buf, &sc->map[n], sc->sectorsize) != 0) {
+				LOG_MSG(LVL_ERROR, "ERROR in allocation table, "
+				    "entry %d, offset %jd", n, off);
+				isclean = 0;
+				count++;
+			}
+			n += sc->me_per_sector;
+			off += sc->sectorsize;
+			g_free(buf);
+		}
+		error = g_access(sc->components[0].gcons, -1, 0, 0);
+		KASSERT(error == 0, ("%s: g_access failed (%d) on exit",
+		    __func__, error));
+		if (isclean != 1) {
+			LOG_MSG(LVL_ERROR, "ALLOCATION TABLE CORRUPTED FOR %s "
+			    "(%d sectors don't match, max %d allocations)",
+			    sc->geom->name, count,
+			    count * sc->me_per_sector);
+		} else {
+			LOG_MSG(LVL_INFO, "Allocation table ok for %s",
+			    sc->geom->name);
+		}
+bailout:
+#endif
+		update_metadata(sc);
+		virstor_geom_destroy(sc, FALSE, FALSE);
+		exitval = EAGAIN;
+	} else
+		exitval = 0;
+	return (exitval);
+}
+
+/*
+ * Taste event (per-class callback)
+ * Examines a provider and creates geom instances if needed
+ */
+static struct g_geom *
+g_virstor_taste(struct g_class *mp, struct g_provider *pp, int flags)
+{
+	struct g_virstor_metadata md;
+	struct g_geom *gp;
+	struct g_consumer *cp;
+	struct g_virstor_softc *sc;
+	int error;
+
+	g_trace(G_T_TOPOLOGY, "%s(%s, %s)", __func__, mp->name, pp->name);
+	g_topology_assert();
+	LOG_MSG(LVL_DEBUG, "Tasting %s", pp->name);
+
+	/* We need a dummy geom to attach a consumer to the given provider */
+	gp = g_new_geomf(mp, "virstor:taste.helper");
+	gp->start = (void *)invalid_call;	/* XXX: hacked up so the        */
+	gp->access = (void *)invalid_call;	/* compiler doesn't complain.   */
+	gp->orphan = (void *)invalid_call;	/* I really want these to fail. */
+
+	cp = g_new_consumer(gp);
+	g_attach(cp, pp);
+	error = read_metadata(cp, &md);
+	g_detach(cp);
+	g_destroy_consumer(cp);
+	g_destroy_geom(gp);
+
+	if (error != 0)
+		return (NULL);
+
+	if (strcmp(md.md_magic, G_VIRSTOR_MAGIC) != 0)
+		return (NULL);
+	if (md.md_version != G_VIRSTOR_VERSION) {
+		LOG_MSG(LVL_ERROR, "Kernel module version invalid "
+		    "to handle %s (%s) : %d should be %d",
+		    md.md_name, pp->name, md.md_version, G_VIRSTOR_VERSION);
+		return (NULL);
+	}
+	if (md.provsize != pp->mediasize)
+		return (NULL);
+
+	/* If the provider name is hardcoded, use the offered provider only
+	 * if it's been offered with its proper name (the one used in
+	 * the label command). */
+	if (md.provider[0] != '\0') {
+		if (strcmp(md.provider, pp->name) != 0)
+			return (NULL);
+	}
+
+	/* Iterate all geoms this class already knows about to see if a new
+	 * geom instance of this class needs to be created (in case the provider
+	 * is first from a (possibly) multi-consumer geom) or it just needs
+	 * to be added to an existing instance. */
+	sc = NULL;
+	gp = NULL;
+	LIST_FOREACH(gp, &mp->geom, geom) {
+		sc = gp->softc;
+		if (sc == NULL)
+			continue;
+		if (strcmp(md.md_name, sc->geom->name) != 0)
+			continue;
+		if (md.md_id != sc->id)
+			continue;
+		break;
+	}
+	if (gp != NULL) { /* We found an existing geom instance; add to it */
+		LOG_MSG(LVL_INFO, "Adding %s to %s", pp->name, md.md_name);
+		error = add_provider_to_geom(sc, pp, &md);
+		if (error != 0) {
+			LOG_MSG(LVL_ERROR, "Error adding %s to %s (error %d)",
+			    pp->name, md.md_name, error);
+			return (NULL);
+		}
+	} else { /* New geom instance needs to be created */
+		gp = create_virstor_geom(mp, &md);
+		if (gp == NULL) {
+			LOG_MSG(LVL_ERROR, "Error creating new instance of "
+			    "class %s: %s", mp->name, md.md_name);
+			LOG_MSG(LVL_DEBUG, "Error creating %s at %s",
+			    md.md_name, pp->name);
+			return (NULL);
+		}
+		sc = gp->softc;
+		LOG_MSG(LVL_INFO, "Adding %s to %s (first found)", pp->name,
+		    md.md_name);
+		error = add_provider_to_geom(sc, pp, &md);
+		if (error != 0) {
+			LOG_MSG(LVL_ERROR, "Error adding %s to %s (error %d)",
+			    pp->name, md.md_name, error);
+			virstor_geom_destroy(sc, TRUE, FALSE);
+			return (NULL);
+		}
+	}
+
+	return (gp);
+}
+
+/*
+ * Destroyes consumer passed to it in arguments. Used as a callback
+ * on g_event queue.
+ */
+static void
+delay_destroy_consumer(void *arg, int flags __unused)
+{
+	struct g_consumer *c = arg;
+	KASSERT(c != NULL, ("%s: invalid consumer", __func__));
+	LOG_MSG(LVL_DEBUG, "Consumer %s destroyed with delay",
+	    c->provider->name);
+	g_detach(c);
+	g_destroy_consumer(c);
+}
+
+/*
+ * Remove a component (consumer) from geom instance; If it's the first
+ * component being removed, orphan the provider to announce geom's being
+ * dismantled
+ */
+static void
+remove_component(struct g_virstor_softc *sc, struct g_virstor_component *comp,
+    boolean_t delay)
+{
+	struct g_consumer *c;
+
+	KASSERT(comp->gcons != NULL, ("Component with no consumer in %s",
+	    sc->geom->name));
+	c = comp->gcons;
+
+	comp->gcons = NULL;
+	KASSERT(c->provider != NULL, ("%s: no provider", __func__));
+	LOG_MSG(LVL_DEBUG, "Component %s removed from %s", c->provider->name,
+	    sc->geom->name);
+	if (sc->provider != NULL) {
+		/* Whither, GEOM? */
+		sc->provider->flags |= G_PF_WITHER;
+		g_orphan_provider(sc->provider, ENXIO);
+		sc->provider = NULL;
+		LOG_MSG(LVL_INFO, "Removing provider %s", sc->geom->name);
+	}
+
+	if (c->acr > 0 || c->acw > 0 || c->ace > 0)
+		g_access(c, -c->acr, -c->acw, -c->ace);
+	if (delay) {
+		/* Destroy consumer after it's tasted */
+		g_post_event(delay_destroy_consumer, c, M_WAITOK, NULL);
+	} else {
+		g_detach(c);
+		g_destroy_consumer(c);
+	}
+}
+
+/*
+ * Destroy geom - called internally
+ * See g_virstor_destroy_geom for the other one
+ */
+static int
+virstor_geom_destroy(struct g_virstor_softc *sc, boolean_t force,
+    boolean_t delay)
+{
+	struct g_provider *pp;
+	struct g_geom *gp;
+	int n;
+
+	g_topology_assert();
+
+	if (sc == NULL)
+		return (ENXIO);
+
+	pp = sc->provider;
+	if (pp != NULL && (pp->acr != 0 || pp->acw != 0 || pp->ace != 0)) {
+		LOG_MSG(force ? LVL_WARNING : LVL_ERROR,
+		    "Device %s is still open.", pp->name);
+		if (!force)
+			return (EBUSY);
+	}
+
+	for (n = 0; n < sc->n_components; n++) {
+		if (sc->components[n].gcons != NULL)
+			remove_component(sc, &sc->components[n], delay);
+	}
+
+	gp = sc->geom;
+	gp->softc = NULL;
+
+	KASSERT(sc->provider == NULL, ("Provider still exists for %s",
+	    gp->name));
+
+	/* XXX: This might or might not work, since we're called with
+	 * the topology lock held. Also, it might panic the kernel if
+	 * the error'd BIO is in softupdates code. */
+	mtx_lock(&sc->delayed_bio_q_mtx);
+	while (!STAILQ_EMPTY(&sc->delayed_bio_q)) {
+		struct g_virstor_bio_q *bq;
+		bq = STAILQ_FIRST(&sc->delayed_bio_q);
+		bq->bio->bio_error = ENOSPC;
+		g_io_deliver(bq->bio, EIO);
+		STAILQ_REMOVE_HEAD(&sc->delayed_bio_q, linkage);
+		free(bq, M_GVIRSTOR);
+	}
+	mtx_unlock(&sc->delayed_bio_q_mtx);
+	mtx_destroy(&sc->delayed_bio_q_mtx);
+
+	free(sc->map, M_GVIRSTOR);
+	free(sc->components, M_GVIRSTOR);
+	bzero(sc, sizeof *sc);
+	free(sc, M_GVIRSTOR);
+
+	pp = LIST_FIRST(&gp->provider); /* We only offer one provider */
+	if (pp == NULL || (pp->acr == 0 && pp->acw == 0 && pp->ace == 0))
+		LOG_MSG(LVL_DEBUG, "Device %s destroyed", gp->name);
+
+	g_wither_geom(gp, ENXIO);
+
+	return (0);
+}
+
+/*
+ * Utility function: read metadata & decode. Wants topology lock to be
+ * held.
+ */
+static int
+read_metadata(struct g_consumer *cp, struct g_virstor_metadata *md)
+{
+	struct g_provider *pp;
+	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);
+
+	virstor_metadata_decode(buf, md);
+	g_free(buf);
+
+	return (0);
+}
+
+/**
+ * Utility function: encode & write metadata. Assumes topology lock is
+ * held.
+ */
+static int
+write_metadata(struct g_consumer *cp, struct g_virstor_metadata *md)
+{
+	struct g_provider *pp;
+	char *buf;
+	int error;
+
+	KASSERT(cp != NULL && md != NULL && cp->provider != NULL,
+	    ("Something's fishy in %s", __func__));
+	LOG_MSG(LVL_DEBUG, "Writing metadata on %s", cp->provider->name);
+	g_topology_assert();
+	error = g_access(cp, 0, 1, 0);
+	if (error != 0)
+		return (error);
+	pp = cp->provider;
+
+	buf = malloc(pp->sectorsize, M_GVIRSTOR, M_WAITOK);
+	virstor_metadata_encode(md, buf);
+	g_topology_unlock();
+	error = g_write_data(cp, pp->mediasize - pp->sectorsize, buf,
+	    pp->sectorsize);
+	g_topology_lock();
+	g_access(cp, 0, -1, 0);
+
+	free(buf, M_GVIRSTOR);
+	return (0);
+}
+
+/*
+ * Creates a new instance of this GEOM class, initialise softc
+ */
+static struct g_geom *
+create_virstor_geom(struct g_class *mp, struct g_virstor_metadata *md)
+{
+	struct g_geom *gp;
+	struct g_virstor_softc *sc;
+
+	LOG_MSG(LVL_DEBUG, "Creating geom instance for %s (id=%u)",
+	    md->md_name, md->md_id);
+
+	if (md->md_count < 1 || md->md_chunk_size < 1 ||
+	    md->md_virsize < md->md_chunk_size) {
+		/* This is bogus configuration, and probably means data is
+		 * somehow corrupted. Panic, maybe? */
+		LOG_MSG(LVL_ERROR, "Nonsensical metadata information for %s",
+		    md->md_name);
+		return (NULL);
+	}
+
+	/* Check if it's already created */
+	LIST_FOREACH(gp, &mp->geom, geom) {
+		sc = gp->softc;
+		if (sc != NULL && strcmp(sc->geom->name, md->md_name) == 0) {
+			LOG_MSG(LVL_WARNING, "Geom %s already exists",
+			    md->md_name);
+			if (sc->id != md->md_id) {
+				LOG_MSG(LVL_ERROR,
+				    "Some stale or invalid components "
+				    "exist for virstor device named %s. "
+				    "You will need to <CLEAR> all stale "
+				    "components and maybe reconfigure "
+				    "the virstor device. Tune "
+				    "kern.geom.virstor.debug sysctl up "
+				    "for more information.",
+				    sc->geom->name);
+			}
+			return (NULL);
+		}
+	}
+	gp = g_new_geomf(mp, "%s", md->md_name);
+	gp->softc = NULL; /* to circumevent races that test softc */
+
+	gp->start = g_virstor_start;
+	gp->spoiled = g_virstor_orphan;
+	gp->orphan = g_virstor_orphan;
+	gp->access = g_virstor_access;
+	gp->dumpconf = g_virstor_dumpconf;
+
+	sc = malloc(sizeof(*sc), M_GVIRSTOR, M_WAITOK | M_ZERO);
+	sc->id = md->md_id;
+	sc->n_components = md->md_count;
+	sc->components = malloc(sizeof(struct g_virstor_component) * md->md_count,
+	    M_GVIRSTOR, M_WAITOK | M_ZERO);
+	sc->chunk_size = md->md_chunk_size;
+	sc->virsize = md->md_virsize;
+	STAILQ_INIT(&sc->delayed_bio_q);
+	mtx_init(&sc->delayed_bio_q_mtx, "gvirstor_delayed_bio_q_mtx",
+	    "gvirstor", MTX_DEF | MTX_RECURSE);
+
+	sc->geom = gp;
+	sc->provider = NULL; /* virstor_check_and_run will create it */
+	gp->softc = sc;
+
+	LOG_MSG(LVL_ANNOUNCE, "Device %s created", sc->geom->name);
+
+	return (gp);
+}
+
+/*
+ * Add provider to a GEOM class instance
+ */
+static int
+add_provider_to_geom(struct g_virstor_softc *sc, struct g_provider *pp,
+    struct g_virstor_metadata *md)
+{
+	struct g_virstor_component *component;
+	struct g_consumer *cp, *fcp;
+	struct g_geom *gp;
+	int error;
+
+	if (md->no >= sc->n_components)
+		return (EINVAL);
+
+	/* "Current" compontent */
+	component = &(sc->components[md->no]);
+	if (component->gcons != NULL)
+		return (EEXIST);
+
+	gp = sc->geom;
+	fcp = LIST_FIRST(&gp->consumer);
+
+	cp = g_new_consumer(gp);
+	error = g_attach(cp, pp);
+
+	if (error != 0) {
+		g_destroy_consumer(cp);
+		return (error);
+	}
+
+	if (fcp != NULL) {
+		if (fcp->provider->sectorsize != pp->sectorsize) {
+			/* TODO: this can be made to work */
+			LOG_MSG(LVL_ERROR, "Provider %s of %s has invalid "
+			    "sector size (%d)", pp->name, sc->geom->name,
+			    pp->sectorsize);
+			return (EINVAL);
+		}
+		if (fcp->acr > 0 || fcp->acw || fcp->ace > 0) {
+			/* Replicate access permissions from first "live" consumer
+			 * to the new one */
+			error = g_access(cp, fcp->acr, fcp->acw, fcp->ace);
+			if (error != 0) {
+				g_detach(cp);
+				g_destroy_consumer(cp);
+				return (error);
+			}
+		}
+	}
+
+	/* Bring up a new component */
+	cp->private = component;
+	component->gcons = cp;
+	component->sc = sc;
+	component->index = md->no;
+	component->chunk_count = md->chunk_count;
+	component->chunk_next = md->chunk_next;
+	component->chunk_reserved = md->chunk_reserved;
+	component->flags = md->flags;
+
+	LOG_MSG(LVL_DEBUG, "%s attached to %s", pp->name, sc->geom->name);
+
+	virstor_check_and_run(sc);
+	return (0);
+}
+
+/*
+ * Check if everything's ready to create the geom provider & device entry,
+ * create and start provider.
+ * Called ultimately by .taste, from g_event thread
+ */
+static void
+virstor_check_and_run(struct g_virstor_softc *sc)
+{
+	off_t off;
+	size_t n, count;
+	int index;
+	int error;
+
+	if (virstor_valid_components(sc) != sc->n_components)
+		return;
+
+	if (virstor_valid_components(sc) == 0) {
+		/* This is actually a candidate for panic() */
+		LOG_MSG(LVL_ERROR, "No valid components for %s?",
+		    sc->provider->name);
+		return;
+	}
+
+	sc->sectorsize = sc->components[0].gcons->provider->sectorsize;
+
+	/* Initialise allocation map from the first consumer */
+	sc->chunk_count = sc->virsize / sc->chunk_size;
+	if (sc->chunk_count * (off_t)sc->chunk_size != sc->virsize) {
+		LOG_MSG(LVL_WARNING, "Device %s truncated to %ju bytes",
+		    sc->provider->name,
+		    sc->chunk_count * (off_t)sc->chunk_size);
+	}
+	sc->map_size = sc->chunk_count * sizeof *(sc->map);
+	/* The following allocation is in order of 4MB - 8MB */
+	sc->map = malloc(sc->map_size, M_GVIRSTOR, M_WAITOK);
+	KASSERT(sc->map != NULL, ("%s: Memory allocation error (%zu bytes) for %s",
+	    __func__, sc->map_size, sc->provider->name));
+	sc->map_sectors = sc->map_size / sc->sectorsize;
+
+	count = 0;
+	for (n = 0; n < sc->n_components; n++)
+		count += sc->components[n].chunk_count;
+	LOG_MSG(LVL_INFO, "Device %s has %zu physical chunks and %zu virtual "
+	    "(%zu KB chunks)",
+	    sc->geom->name, count, sc->chunk_count, sc->chunk_size / 1024);
+
+	error = g_access(sc->components[0].gcons, 1, 0, 0);
+	if (error != 0) {
+		LOG_MSG(LVL_ERROR, "Cannot acquire read access for %s to "
+		    "read allocation map for %s",
+		    sc->components[0].gcons->provider->name,
+		    sc->geom->name);
+		return;
+	}
+	/* Read in the allocation map */
+	LOG_MSG(LVL_DEBUG, "Reading map for %s from %s", sc->geom->name,
+	    sc->components[0].gcons->provider->name);
+	off = count = n = 0;
+	while (count < sc->map_size) {
+		struct g_virstor_map_entry *mapbuf;
+		size_t bs;
+
+		bs = MIN(MAXPHYS, sc->map_size - count);
+		if (bs % sc->sectorsize != 0) {
+			/* Check for alignment errors */
+			bs = (bs / sc->sectorsize) * sc->sectorsize;
+			if (bs == 0)
+				break;
+			LOG_MSG(LVL_ERROR, "Trouble: map is not sector-aligned "
+			    "for %s on %s", sc->geom->name,
+			    sc->components[0].gcons->provider->name);
+		}
+		mapbuf = g_read_data(sc->components[0].gcons, off, bs, &error);
+		if (mapbuf == NULL) {
+			free(sc->map, M_GVIRSTOR);
+			LOG_MSG(LVL_ERROR, "Error reading allocation map "
+			    "for %s from %s (offset %ju) (error %d)",
+			    sc->geom->name,
+			    sc->components[0].gcons->provider->name,
+			    off, error);
+			return;
+		}
+
+		bcopy(mapbuf, &sc->map[n], bs);
+		off += bs;
+		count += bs;
+		n += bs / sizeof *(sc->map);
+		g_free(mapbuf);
+	}
+	g_access(sc->components[0].gcons, -1, 0, 0);
+	LOG_MSG(LVL_DEBUG, "Read map for %s", sc->geom->name);
+
+	/* find first component with allocatable chunks */
+	index = -1;
+	for (n = 0; n < sc->n_components; n++) {
+		if (sc->components[n].chunk_next <
+		    sc->components[n].chunk_count) {
+			index = n;
+			break;
+		}
+	}
+	if (index == -1)
+		/* not found? set it to the last component and handle it
+		 * later */
+		index = sc->n_components - 1;
+
+	if (index >= sc->n_components - g_virstor_component_watermark - 1) {
+		LOG_MSG(LVL_WARNING, "Device %s running out of components "
+		    "(%d/%u: %s)", sc->geom->name,
+		    index+1,
+		    sc->n_components,
+		    sc->components[index].gcons->provider->name);
+	}
+	sc->curr_component = index;
+
+	if (sc->components[index].chunk_next >=
+	    sc->components[index].chunk_count - g_virstor_chunk_watermark) {
+		LOG_MSG(LVL_WARNING,
+		    "Component %s of %s is running out of free space "
+		    "(%u chunks left)",
+		    sc->components[index].gcons->provider->name,
+		    sc->geom->name, sc->components[index].chunk_count -
+		    sc->components[index].chunk_next);
+	}
+
+	sc->me_per_sector = sc->sectorsize / sizeof *(sc->map);
+	if (sc->sectorsize % sizeof *(sc->map) != 0) {
+		LOG_MSG(LVL_ERROR,
+		    "%s: Map entries don't fit exactly in a sector (%s)",
+		    __func__, sc->geom->name);
+		return;
+	}
+
+	/* Recalculate allocated chunks in components & at the same time
+	 * verify map data is sane. We could trust metadata on this, but
+	 * we want to make sure. */
+	for (n = 0; n < sc->n_components; n++)
+		sc->components[n].chunk_next = sc->components[n].chunk_reserved;
+
+	for (n = 0; n < sc->chunk_count; n++) {
+		if (sc->map[n].provider_no >= sc->n_components ||
+			sc->map[n].provider_chunk >=
+			sc->components[sc->map[n].provider_no].chunk_count) {
+			LOG_MSG(LVL_ERROR, "%s: Invalid entry %u in map for %s",
+			    __func__, (u_int)n, sc->geom->name);
+			LOG_MSG(LVL_ERROR, "%s: provider_no: %u, n_components: %u"
+			    " provider_chunk: %u, chunk_count: %u", __func__,
+			    sc->map[n].provider_no, sc->n_components,
+			    sc->map[n].provider_chunk,
+			    sc->components[sc->map[n].provider_no].chunk_count);
+			return;
+		}
+		if (sc->map[n].flags & VIRSTOR_MAP_ALLOCATED)
+			sc->components[sc->map[n].provider_no].chunk_next++;
+	}
+
+	sc->provider = g_new_providerf(sc->geom, "virstor/%s",
+	    sc->geom->name);
+
+	sc->provider->sectorsize = sc->sectorsize;
+	sc->provider->mediasize = sc->virsize;
+	g_error_provider(sc->provider, 0);
+
+	LOG_MSG(LVL_INFO, "%s activated", sc->provider->name);
+	LOG_MSG(LVL_DEBUG, "%s starting with current component %u, starting "
+	    "chunk %u", sc->provider->name, sc->curr_component,
+	    sc->components[sc->curr_component].chunk_next);
+}
+
+/*
+ * Returns count of active providers in this geom instance
+ */
+static u_int
+virstor_valid_components(struct g_virstor_softc *sc)
+{
+	unsigned int nc, i;
+
+	nc = 0;
+	KASSERT(sc != NULL, ("%s: softc is NULL", __func__));
+	KASSERT(sc->components != NULL, ("%s: sc->components is NULL", __func__));
+	for (i = 0; i < sc->n_components; i++)
+		if (sc->components[i].gcons != NULL)
+			nc++;
+	return (nc);
+}
+
+/*
+ * Called when the consumer gets orphaned (?)
+ */
+static void
+g_virstor_orphan(struct g_consumer *cp)
+{
+	struct g_virstor_softc *sc;
+	struct g_virstor_component *comp;
+	struct g_geom *gp;
+
+	g_topology_assert();
+	gp = cp->geom;
+	sc = gp->softc;
+	if (sc == NULL)
+		return;
+
+	comp = cp->private;
+	KASSERT(comp != NULL, ("%s: No component in private part of consumer",
+	    __func__));
+	remove_component(sc, comp, FALSE);
+	if (virstor_valid_components(sc) == 0)
+		virstor_geom_destroy(sc, TRUE, FALSE);
+}
+
+/*
+ * Called to notify geom when it's been opened, and for what intent
+ */
+static int
+g_virstor_access(struct g_provider *pp, int dr, int dw, int de)
+{
+	struct g_consumer *c;
+	struct g_virstor_softc *sc;
+	struct g_geom *gp;
+	int error;
+
+	KASSERT(pp != NULL, ("%s: NULL provider", __func__));
+	gp = pp->geom;
+	KASSERT(gp != NULL, ("%s: NULL geom", __func__));
+	sc = gp->softc;
+
+	if (sc == NULL) {
+		/* It seems that .access can be called with negative dr,dw,dx
+		 * in this case but I want to check for myself */
+		LOG_MSG(LVL_WARNING, "access(%d, %d, %d) for %s",
+		    dr, dw, de, pp->name);
+		/* This should only happen when geom is withered so
+		 * allow only negative requests */
+		KASSERT(dr <= 0 && dw <= 0 && de <= 0,
+		    ("%s: Positive access for %s", __func__, pp->name));
+		if (pp->acr + dr == 0 && pp->acw + dw == 0 && pp->ace + de == 0)
+			LOG_MSG(LVL_DEBUG, "Device %s definitely destroyed",
+			    pp->name);
+		return (0);
+	}
+
+	/* Grab an exclusive bit to propagate on our consumers on first open */
+	if (pp->acr == 0 && pp->acw == 0 && pp->ace == 0)
+		de++;
+	/* ... drop it on close */
+	if (pp->acr + dr == 0 && pp->acw + dw == 0 && pp->ace + de == 0) {
+		de--;
+		update_metadata(sc);	/* Writes statistical information */
+	}
+
+	error = ENXIO;
+	LIST_FOREACH(c, &gp->consumer, consumer) {
+		KASSERT(c != NULL, ("%s: consumer is NULL", __func__));
+		error = g_access(c, dr, dw, de);
+		if (error != 0) {
+			struct g_consumer *c2;
+
+			/* Backout earlier changes */
+			LIST_FOREACH(c2, &gp->consumer, consumer) {
+				if (c2 == c) /* all eariler components fixed */
+					return (error);
+				g_access(c2, -dr, -dw, -de);
+			}
+		}
+	}
+
+	return (error);
+}
+
+/*
+ * Generate XML dump of current state
+ */
+static void
+g_virstor_dumpconf(struct sbuf *sb, const char *indent, struct g_geom *gp,
+    struct g_consumer *cp, struct g_provider *pp)
+{
+	struct g_virstor_softc *sc;
+
+	g_topology_assert();
+	sc = gp->softc;
+
+	if (sc == NULL || pp != NULL)
+		return;
+
+	if (cp != NULL) {
+		/* For each component */
+		struct g_virstor_component *comp;
+
+		comp = cp->private;
+		if (comp == NULL)
+			return;
+		sbuf_printf(sb, "%s<ComponentIndex>%u</ComponentIndex>\n",
+		    indent, comp->index);
+		sbuf_printf(sb, "%s<ChunkCount>%u</ChunkCount>\n",
+		    indent, comp->chunk_count);
+		sbuf_printf(sb, "%s<ChunksUsed>%u</ChunksUsed>\n",
+		    indent, comp->chunk_next);
+		sbuf_printf(sb, "%s<ChunksReserved>%u</ChunksReserved>\n",
+		    indent, comp->chunk_reserved);
+		sbuf_printf(sb, "%s<StorageFree>%u%%</StorageFree>\n",
+		    indent,
+		    comp->chunk_next > 0 ? 100 -
+		    ((comp->chunk_next + comp->chunk_reserved) * 100) /
+		    comp->chunk_count : 100);
+	} else {
+		/* For the whole thing */
+		u_int count, used, i;
+		off_t size;
+
+		count = used = size = 0;
+		for (i = 0; i < sc->n_components; i++) {
+			if (sc->components[i].gcons != NULL) {
+				count += sc->components[i].chunk_count;
+				used += sc->components[i].chunk_next +
+				    sc->components[i].chunk_reserved;
+				size += sc->components[i].gcons->
+				    provider->mediasize;
+			}
+		}
+
+		sbuf_printf(sb, "%s<Status>"
+		    "Components=%u, Online=%u</Status>\n", indent,
+		    sc->n_components, virstor_valid_components(sc));
+		sbuf_printf(sb, "%s<State>%u%% physical free</State>\n",
+		    indent, 100-(used * 100) / count);
+		sbuf_printf(sb, "%s<ChunkSize>%zu</ChunkSize>\n", indent,
+		    sc->chunk_size);
+		sbuf_printf(sb, "%s<PhysicalFree>%u%%</PhysicalFree>\n",
+		    indent, used > 0 ? 100 - (used * 100) / count : 100);
+		sbuf_printf(sb, "%s<ChunkPhysicalCount>%u</ChunkPhysicalCount>\n",
+		    indent, count);
+		sbuf_printf(sb, "%s<ChunkVirtualCount>%zu</ChunkVirtualCount>\n",
+		    indent, sc->chunk_count);
+		sbuf_printf(sb, "%s<PhysicalBacking>%zu%%</PhysicalBacking>\n",
+		    indent,
+		    (count * 100) / sc->chunk_count);
+		sbuf_printf(sb, "%s<PhysicalBackingSize>%jd</PhysicalBackingSize>\n",
+		    indent, size);
+		sbuf_printf(sb, "%s<VirtualSize>%jd</VirtualSize>\n", indent,
+		    sc->virsize);
+	}
+}
+
+/*
+ * GEOM .done handler
+ * Can't use standard handler because one requested IO may
+ * fork into additional data IOs
+ */
+static void
+g_virstor_done(struct bio *b)
+{
+	struct g_virstor_softc *sc;
+	struct bio *parent_b;
+
+	parent_b = b->bio_parent;
+	sc = parent_b->bio_to->geom->softc;
+
+	if (b->bio_error != 0) {
+		LOG_MSG(LVL_ERROR, "Error %d for offset=%ju, length=%ju, %s",
+		    b->bio_error, b->bio_offset, b->bio_length,
+		    b->bio_to->name);
+		if (parent_b->bio_error == 0)
+			parent_b->bio_error = b->bio_error;
+	}
+
+	parent_b->bio_inbed++;
+	parent_b->bio_completed += b->bio_completed;
+
+	if (parent_b->bio_children == parent_b->bio_inbed) {
+		parent_b->bio_completed = parent_b->bio_length;
+		g_io_deliver(parent_b, parent_b->bio_error);
+	}
+	g_destroy_bio(b);
+}
+
+/*
+ * I/O starts here
+ * Called in g_down thread
+ */
+static void
+g_virstor_start(struct bio *b)
+{
+	struct g_virstor_softc *sc;
+	struct g_virstor_component *comp;
+	struct bio *cb;
+	struct g_provider *pp;
+	char *addr;
+	off_t offset, length;
+	struct bio_queue_head bq;
+	size_t chunk_size;	/* cached for convenience */
+	u_int count;
+
+	pp = b->bio_to;
+	sc = pp->geom->softc;
+	KASSERT(sc != NULL, ("%s: no softc (error=%d, device=%s)", __func__,
+	    b->bio_to->error, b->bio_to->name));
+
+	LOG_REQ(LVL_MOREDEBUG, b, "%s", __func__);
+
+	switch (b->bio_cmd) {
+	case BIO_READ:
+	case BIO_WRITE:
+	case BIO_DELETE:
+		break;
+	default:
+		g_io_deliver(b, EOPNOTSUPP);
+		return;
+	}
+
+	LOG_MSG(LVL_DEBUG2, "BIO arrived, size=%ju", b->bio_length);
+	bioq_init(&bq);
+
+	chunk_size = sc->chunk_size;
+	addr = b->bio_data;
+	offset = b->bio_offset;	/* virtual offset and length */
+	length = b->bio_length;
+
+	while (length > 0) {
+		size_t chunk_index, in_chunk_offset, in_chunk_length;
+		struct virstor_map_entry *me;
+
+		chunk_index = offset / chunk_size; /* round downwards */
+		in_chunk_offset = offset % chunk_size;
+		in_chunk_length = min(length, chunk_size - in_chunk_offset);
+		LOG_MSG(LVL_DEBUG, "Mapped %s(%ju, %ju) to (%zu,%zu,%zu)",
+		    b->bio_cmd == BIO_READ ? "R" : "W",
+		    offset, length,
+		    chunk_index, in_chunk_offset, in_chunk_length);
+		me = &sc->map[chunk_index];
+
+		if (b->bio_cmd == BIO_READ || b->bio_cmd == BIO_DELETE) {
+			if ((me->flags & VIRSTOR_MAP_ALLOCATED) == 0) {
+				/* Reads from unallocated chunks return zeroed
+				 * buffers */
+				if (b->bio_cmd == BIO_READ)
+					bzero(addr, in_chunk_length);
+			} else {
+				comp = &sc->components[me->provider_no];
+
+				cb = g_clone_bio(b);
+				if (cb == NULL) {
+					bioq_dismantle(&bq);
+					if (b->bio_error == 0)
+						b->bio_error = ENOMEM;
+					g_io_deliver(b, b->bio_error);
+					return;
+				}
+				cb->bio_to = comp->gcons->provider;
+				cb->bio_done = g_virstor_done;
+				cb->bio_offset =
+				    (off_t)me->provider_chunk * (off_t)chunk_size
+				    + in_chunk_offset;
+				cb->bio_length = in_chunk_length;
+				cb->bio_data = addr;
+				cb->bio_caller1 = comp;
+				bioq_disksort(&bq, cb);
+			}
+		} else { /* handle BIO_WRITE */
+			KASSERT(b->bio_cmd == BIO_WRITE,
+			    ("%s: Unknown command %d", __func__,
+			    b->bio_cmd));
+
+			if ((me->flags & VIRSTOR_MAP_ALLOCATED) == 0) {
+				/* We have a virtual chunk, represented by
+				 * the "me" entry, but it's not yet allocated
+				 * (tied to) a physical chunk. So do it now. */
+				struct virstor_map_entry *data_me;
+				u_int phys_chunk, comp_no;
+				off_t s_offset;
+				int error;
+
+				error = allocate_chunk(sc, &comp, &comp_no,
+				    &phys_chunk);
+				if (error != 0) {
+					/* We cannot allocate a physical chunk
+					 * to satisfy this request, so we'll
+					 * delay it to when we can...
+					 * XXX: this will prevent the fs from
+					 * being umounted! */
+					struct g_virstor_bio_q *biq;
+					biq = malloc(sizeof *biq, M_GVIRSTOR,
+					    M_NOWAIT);
+					if (biq == NULL) {
+						bioq_dismantle(&bq);
+						if (b->bio_error == 0)
+							b->bio_error = ENOMEM;
+						g_io_deliver(b, b->bio_error);
+						return;
+					}
+					biq->bio = b;
+					mtx_lock(&sc->delayed_bio_q_mtx);
+					STAILQ_INSERT_TAIL(&sc->delayed_bio_q,
+					    biq, linkage);
+					mtx_unlock(&sc->delayed_bio_q_mtx);
+					LOG_MSG(LVL_WARNING, "Delaying BIO "
+					    "(size=%ju) until free physical "
+					    "space can be found on %s",
+					    b->bio_length,
+					    sc->provider->name);
+					return;
+				}
+				LOG_MSG(LVL_DEBUG, "Allocated chunk %u on %s "
+				    "for %s",
+				    phys_chunk,
+				    comp->gcons->provider->name,
+				    sc->provider->name);
+
+				me->provider_no = comp_no;
+				me->provider_chunk = phys_chunk;
+				me->flags |= VIRSTOR_MAP_ALLOCATED;
+
+				cb = g_clone_bio(b);
+				if (cb == NULL) {
+					me->flags &= ~VIRSTOR_MAP_ALLOCATED;
+					me->provider_no = 0;
+					me->provider_chunk = 0;
+					bioq_dismantle(&bq);
+					if (b->bio_error == 0)
+						b->bio_error = ENOMEM;
+					g_io_deliver(b, b->bio_error);
+					return;
+				}
+
+				/* The allocation table is stored continuously
+				 * at the start of the drive. We need to
+				 * calculate the offset of the sector that holds
+				 * this map entry both on the drive and in the
+				 * map array.
+				 * sc_offset will end up pointing to the drive
+				 * sector. */
+				s_offset = chunk_index * sizeof *me;
+				s_offset = (s_offset / sc->sectorsize) *
+				    sc->sectorsize;
+
+				/* data_me points to map entry sector
+				 * in memory (analoguos to offset) */
+				data_me = &sc->map[(chunk_index /
+				    sc->me_per_sector) * sc->me_per_sector];
+
+				/* Commit sector with map entry to storage */
+				cb->bio_to = sc->components[0].gcons->provider;
+				cb->bio_done = g_virstor_done;
+				cb->bio_offset = s_offset;
+				cb->bio_data = (char *)data_me;
+				cb->bio_length = sc->sectorsize;
+				cb->bio_caller1 = &sc->components[0];
+				bioq_disksort(&bq, cb);
+			}
+
+			comp = &sc->components[me->provider_no];
+			cb = g_clone_bio(b);
+			if (cb == NULL) {
+				bioq_dismantle(&bq);
+				if (b->bio_error == 0)
+					b->bio_error = ENOMEM;
+				g_io_deliver(b, b->bio_error);
+				return;
+			}
+			/* Finally, handle the data */
+			cb->bio_to = comp->gcons->provider;
+			cb->bio_done = g_virstor_done;
+			cb->bio_offset = (off_t)me->provider_chunk*(off_t)chunk_size +
+			    in_chunk_offset;
+			cb->bio_length = in_chunk_length;
+			cb->bio_data = addr;
+			cb->bio_caller1 = comp;
+			bioq_disksort(&bq, cb);
+		}
+		addr += in_chunk_length;
+		length -= in_chunk_length;
+		offset += in_chunk_length;
+	}
+
+	/* Fire off bio's here */
+	count = 0;
+	for (cb = bioq_first(&bq); cb != NULL; cb = bioq_first(&bq)) {
+		bioq_remove(&bq, cb);
+		LOG_REQ(LVL_MOREDEBUG, cb, "Firing request");
+		comp = cb->bio_caller1;
+		cb->bio_caller1 = NULL;
+		LOG_MSG(LVL_DEBUG, " firing bio, offset=%ju, length=%ju",
+		    cb->bio_offset, cb->bio_length);
+		g_io_request(cb, comp->gcons);
+		count++;
+	}
+	if (count == 0) { /* We handled everything locally */
+		b->bio_completed = b->bio_length;
+		g_io_deliver(b, 0);
+	}
+
+}
+
+/*
+ * Allocate a chunk from a physical provider. Returns physical component,
+ * chunk index relative to the component and the component's index.
+ */
+static int
+allocate_chunk(struct g_virstor_softc *sc, struct g_virstor_component **comp,
+    u_int *comp_no_p, u_int *chunk)
+{
+	u_int comp_no;
+
+	KASSERT(sc->curr_component < sc->n_components,
+	    ("%s: Invalid curr_component: %u",  __func__, sc->curr_component));
+
+	comp_no = sc->curr_component;
+	*comp = &sc->components[comp_no];
+	dump_component(*comp);
+	if ((*comp)->chunk_next >= (*comp)->chunk_count) {
+		/* This component is full. Allocate next component */
+		if (comp_no >= sc->n_components-1) {
+			LOG_MSG(LVL_ERROR, "All physical space allocated for %s",
+			    sc->geom->name);
+			return (-1);
+		}
+		(*comp)->flags &= ~VIRSTOR_PROVIDER_CURRENT;
+		sc->curr_component = ++comp_no;
+
+		*comp = &sc->components[comp_no];
+		if (comp_no >= sc->n_components - g_virstor_component_watermark-1)
+			LOG_MSG(LVL_WARNING, "Device %s running out of components "
+			    "(switching to %u/%u: %s)", sc->geom->name,
+			    comp_no+1, sc->n_components,
+			    (*comp)->gcons->provider->name);
+		/* Take care not to overwrite reserved chunks */
+		if ( (*comp)->chunk_reserved > 0 &&
+		    (*comp)->chunk_next < (*comp)->chunk_reserved)
+			(*comp)->chunk_next = (*comp)->chunk_reserved;
+
+		(*comp)->flags |=
+		    VIRSTOR_PROVIDER_ALLOCATED | VIRSTOR_PROVIDER_CURRENT;
+		dump_component(*comp);
+		*comp_no_p = comp_no;
+		*chunk = (*comp)->chunk_next++;
+	} else {
+		*comp_no_p = comp_no;
+		*chunk = (*comp)->chunk_next++;
+	}
+	return (0);
+}
+
+/* Dump a component */
+static void
+dump_component(struct g_virstor_component *comp)
+{
+
+	if (g_virstor_debug < LVL_DEBUG2)
+		return;
+	printf("Component %d: %s\n", comp->index, comp->gcons->provider->name);
+	printf("  chunk_count: %u\n", comp->chunk_count);
+	printf("   chunk_next: %u\n", comp->chunk_next);
+	printf("        flags: %u\n", comp->flags);
+}
+
+#if 0
+/* Dump a map entry */
+static void
+dump_me(struct virstor_map_entry *me, unsigned int nr)
+{
+	if (g_virstor_debug < LVL_DEBUG)
+		return;
+	printf("VIRT. CHUNK #%d: ", nr);
+	if ((me->flags & VIRSTOR_MAP_ALLOCATED) == 0)
+		printf("(unallocated)\n");
+	else
+		printf("allocated at provider %u, provider_chunk %u\n",
+		    me->provider_no, me->provider_chunk);
+}
+#endif
+
+/*
+ * Dismantle bio_queue and destroy its components
+ */
+static void
+bioq_dismantle(struct bio_queue_head *bq)
+{
+	struct bio *b;
+
+	for (b = bioq_first(bq); b != NULL; b = bioq_first(bq)) {
+		bioq_remove(bq, b);
+		g_destroy_bio(b);
+	}
+}
+
+/*
+ * The function that shouldn't be called.
+ * When this is called, the stack is already garbled because of
+ * argument mismatch. There's nothing to do now but panic, which is
+ * accidentally the whole purpose of this function.
+ * Motivation: to guard from accidentally calling geom methods when
+ * they shouldn't be called. (see g_..._taste)
+ */
+static void
+invalid_call(void)
+{
+	panic("invalid_call() has just been called. Something's fishy here.");
+}
+
+DECLARE_GEOM_CLASS(g_virstor_class, g_virstor); /* Let there be light */
diff --git a/sys/geom/virstor/g_virstor.h b/sys/geom/virstor/g_virstor.h
new file mode 100644
index 0000000..062ca94
--- /dev/null
+++ b/sys/geom/virstor/g_virstor.h
@@ -0,0 +1,135 @@
+/*-
+ * Copyright (c) 2006-2007 Ivan Voras <ivoras@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.
+ *
+ * $FreeBSD$
+ */
+
+#ifndef _G_VIRSTOR_H_
+#define _G_VIRSTOR_H_
+
+#define	G_VIRSTOR_CLASS_NAME "VIRSTOR"
+
+
+#define VIRSTOR_MAP_ALLOCATED 1
+struct virstor_map_entry {
+	uint16_t	flags;
+	uint16_t	provider_no;
+	uint32_t	provider_chunk;
+};
+
+#define	VIRSTOR_MAP_ENTRY_SIZE (sizeof(struct virstor_map_entry))
+#define	VIRSTOR_MAP_BLOCK_ENTRIES (MAXPHYS / VIRSTOR_MAP_ENTRY_SIZE)
+/* Struct size is guarded by CTASSERT in main source */
+
+#ifdef _KERNEL
+
+#define	LOG_MSG(lvl, ...)       do {					\
+        if (g_virstor_debug >= (lvl)) {					\
+                printf("GEOM_" G_VIRSTOR_CLASS_NAME);			\
+                if (lvl > 0)						\
+                        printf("[%u]", lvl);				\
+                printf(": ");						\
+                printf(__VA_ARGS__);					\
+                printf("\n");						\
+        }								\
+} while (0)
+#define	LOG_MESSAGE LOG_MSG
+
+#define	LOG_REQ(lvl, bp, ...)  do {					\
+        if (g_virstor_debug >= (lvl)) {					\
+                printf("GEOM_" G_VIRSTOR_CLASS_NAME);			\
+                if (lvl > 0)						\
+                        printf("[%u]", lvl);				\
+                printf(": ");						\
+                printf(__VA_ARGS__);					\
+                printf(" ");						\
+                g_print_bio(bp);					\
+                printf("\n");						\
+        }								\
+} while (0)
+#define	LOG_REQUEST LOG_REQ
+
+/* "critical" system announcements (e.g. "geom is up") */
+#define	LVL_ANNOUNCE	0
+/* errors */
+#define	LVL_ERROR	1
+/* warnings */
+#define	LVL_WARNING	2
+/* info, noncritical for system operation (user doesn't have to see it */
+#define	LVL_INFO	5
+/* debug info */
+#define	LVL_DEBUG	10
+/* more debug info */
+#define	LVL_DEBUG2	12
+/* superfluous debug info (large volumes of data) */
+#define	LVL_MOREDEBUG	15
+
+
+/* Component data */
+struct g_virstor_component {
+	struct g_consumer	*gcons;
+	struct g_virstor_softc	*sc;
+	unsigned int		 index;		/* Component index in array */
+	unsigned int		 chunk_count;
+	unsigned int		 chunk_next;
+	unsigned int		 chunk_reserved;
+	unsigned int		 flags;
+};
+
+
+/* Internal geom instance data */
+struct g_virstor_softc {
+	struct g_geom		*geom;
+	struct g_provider	*provider;
+	struct g_virstor_component *components;
+	u_int			 n_components;
+	u_int			 curr_component; /* Component currently used */
+	uint32_t		 id;		/* Unique ID of this geom */
+	off_t			 virsize;	/* Total size of virstor */
+	off_t			 sectorsize;
+	size_t			 chunk_size;
+	size_t			 chunk_count;	/* governs map_size */
+	struct virstor_map_entry *map;
+	size_t			 map_size;	/* (in bytes) */
+	size_t			 map_sectors;	/* Size of map in sectors */
+	size_t			 me_per_sector;	/* # map entries in a sector */
+	STAILQ_HEAD(, g_virstor_bio_q)	 delayed_bio_q;	/* Queue of delayed BIOs */
+	struct mtx		 delayed_bio_q_mtx;
+};
+
+/* "delayed BIOs" Queue element */
+struct g_virstor_bio_q {
+	struct bio		*bio;
+	STAILQ_ENTRY(g_virstor_bio_q) linkage;
+};
+
+
+#endif	/* _KERNEL */
+
+#ifndef _PATH_DEV
+#define _PATH_DEV "/dev/"
+#endif
+
+#endif	/* !_G_VIRSTOR_H_ */
diff --git a/sys/geom/virstor/g_virstor_md.c b/sys/geom/virstor/g_virstor_md.c
new file mode 100644
index 0000000..59e8b98
--- /dev/null
+++ b/sys/geom/virstor/g_virstor_md.c
@@ -0,0 +1,91 @@
+/*-
+ * Copyright (c) 2005 Ivan Voras <ivoras@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 <sys/endian.h>
+
+#include <geom/virstor/g_virstor_md.h>
+#include <geom/virstor/binstream.h>
+
+/*
+ * Encode data from g_virstor_metadata structure into a endian-independant
+ * byte stream.
+ */
+void
+virstor_metadata_encode(struct g_virstor_metadata *md, unsigned char *data)
+{
+	bin_stream_t bs;
+
+	bs_open(&bs, data);
+
+	bs_write_buf(&bs, md->md_magic, sizeof(md->md_magic));
+	bs_write_u32(&bs, md->md_version);
+	bs_write_buf(&bs, md->md_name, sizeof(md->md_name));
+	bs_write_u64(&bs, md->md_virsize);
+	bs_write_u32(&bs, md->md_chunk_size);
+	bs_write_u32(&bs, md->md_id);
+	bs_write_u16(&bs, md->md_count);
+
+	bs_write_buf(&bs, md->provider, sizeof(md->provider));
+	bs_write_u16(&bs, md->no);
+	bs_write_u64(&bs, md->provsize);
+	bs_write_u32(&bs, md->chunk_count);
+	bs_write_u32(&bs, md->chunk_next);
+	bs_write_u16(&bs, md->chunk_reserved);
+	bs_write_u16(&bs, md->flags);
+}
+
+
+/*
+ * Decode data from endian-independant byte stream into g_virstor_metadata
+ * structure.
+ */
+void
+virstor_metadata_decode(unsigned char *data, struct g_virstor_metadata *md)
+{
+	bin_stream_t bs;
+
+	bs_open(&bs, (char *)(data));
+
+	bs_read_buf(&bs, md->md_magic, sizeof(md->md_magic));
+	md->md_version = bs_read_u32(&bs);
+	bs_read_buf(&bs, md->md_name, sizeof(md->md_name));
+	md->md_virsize = bs_read_u64(&bs);
+	md->md_chunk_size = bs_read_u32(&bs);
+	md->md_id = bs_read_u32(&bs);
+	md->md_count = bs_read_u16(&bs);
+
+	bs_read_buf(&bs, md->provider, sizeof(md->provider));
+	md->no = bs_read_u16(&bs);
+	md->provsize = bs_read_u64(&bs);
+	md->chunk_count = bs_read_u32(&bs);
+	md->chunk_next = bs_read_u32(&bs);
+	md->chunk_reserved = bs_read_u16(&bs);
+	md->flags = bs_read_u16(&bs);
+}
diff --git a/sys/geom/virstor/g_virstor_md.h b/sys/geom/virstor/g_virstor_md.h
new file mode 100644
index 0000000..4537478
--- /dev/null
+++ b/sys/geom/virstor/g_virstor_md.h
@@ -0,0 +1,69 @@
+/*-
+ * Copyright (c) 2005 Ivan Voras <ivoras@gmail.com>
+ * 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.
+ *
+ * $FreeBSD$
+ */
+
+
+#ifndef _G_VIRSTOR_MD_H_
+#define _G_VIRSTOR_MD_H_
+
+/*
+ * Metadata declaration
+ */
+
+#define	G_VIRSTOR_MAGIC		"GEOM::VIRSTOR"
+#define	G_VIRSTOR_VERSION	1
+
+/* flag: provider is allocated */
+#define	VIRSTOR_PROVIDER_ALLOCATED	1
+/* flag: provider is currently being filled (usually it's the last
+ * provider with VIRSTOR_PROVIDER_ALLOCATED flag */
+#define VIRSTOR_PROVIDER_CURRENT	2
+
+struct g_virstor_metadata {
+	/* Data global to the virstor device */
+	char		md_magic[16];		/* Magic value. */
+	uint32_t	md_version;		/* Version number. */
+	char		md_name[16];		/* Device name (e.g. "mydata") */
+	uint32_t	md_id;			/* Unique ID. */
+	uint64_t	md_virsize;		/* Virtual device's size */
+	uint32_t	md_chunk_size;		/* Chunk size in bytes */
+	uint16_t	md_count;		/* Total number of providers */
+
+	/* Data local to this provider */
+	char		provider[16];		/* Hardcoded provider name */
+	uint16_t	no;			/* Provider number/index */
+	uint64_t	provsize;		/* Provider's size */
+	uint32_t	chunk_count;		/* Number of chunks in this pr. */
+	uint32_t	chunk_next;		/* Next chunk to allocate */
+	uint16_t	chunk_reserved;		/* Count of "reserved" chunks */
+	uint16_t	flags;			/* Provider's flags */
+};
+
+void virstor_metadata_encode(struct g_virstor_metadata *md, unsigned char *data);
+void virstor_metadata_decode(unsigned char *data, struct g_virstor_metadata *md);
+
+#endif	/* !_G_VIRSTOR_H_ */