diff --git a/sys/geom/geom_io.c b/sys/geom/geom_io.c
--- a/sys/geom/geom_io.c
+++ b/sys/geom/geom_io.c
@@ -872,17 +872,24 @@
 	struct bio *bp;
 	void *ptr;
 	int errorc;
+	off_t start_align;
+	off_t end_align;
 
-	KASSERT(length > 0 && length >= cp->provider->sectorsize &&
+	KASSERT(length > 0 &&
 	    length <= maxphys, ("g_read_data(): invalid length %jd",
 	    (intmax_t)length));
 
+	/* allow unaligned reads */
+	start_align = (offset % cp->provider->sectorsize);
+	end_align = (cp->provider->sectorsize - ((offset + length) %
+	    cp->provider->sectorsize)) % cp->provider->sectorsize;
+
 	bp = g_alloc_bio();
 	bp->bio_cmd = BIO_READ;
 	bp->bio_done = NULL;
-	bp->bio_offset = offset;
-	bp->bio_length = length;
-	ptr = g_malloc(length, M_WAITOK);
+	bp->bio_offset = offset - start_align;
+	bp->bio_length = length + start_align + end_align;
+	ptr = g_malloc(bp->bio_length, M_WAITOK);
 	bp->bio_data = ptr;
 	g_io_request(bp, cp);
 	errorc = biowait(bp, "gread");
@@ -894,6 +901,10 @@
 	if (errorc) {
 		g_free(ptr);
 		ptr = NULL;
+	} else {
+		/* check if buffer data should be shifted down */
+		if (start_align != 0)
+			memmove(ptr, (uint8_t *)ptr + start_align, length);
 	}
 	return (ptr);
 }
@@ -928,23 +939,45 @@
 g_write_data(struct g_consumer *cp, off_t offset, void *ptr, off_t length)
 {
 	struct bio *bp;
+	void *fixup;
 	int error;
+	off_t start_align;
+	off_t end_align;
 
-	KASSERT(length > 0 && length >= cp->provider->sectorsize &&
+	KASSERT(length > 0 &&
 	    length <= maxphys, ("g_write_data(): invalid length %jd",
 	    (intmax_t)length));
 
+	/* allow unaligned writes */
+	start_align = (offset % cp->provider->sectorsize);
+	end_align = (cp->provider->sectorsize - ((offset + length) %
+	    cp->provider->sectorsize)) % cp->provider->sectorsize;
+
+	/* check if we need to do a fixup request */
+	if (start_align != 0 || end_align != 0) {
+		fixup = g_read_data(cp, offset - start_align,
+		    length + start_align + end_align, &error);
+		if (fixup == NULL)
+			return (error);
+		memcpy((uint8_t *)fixup + start_align, ptr, length);
+	} else {
+		fixup = ptr;
+	}
+
+	/* setup BIO request */
 	bp = g_alloc_bio();
 	bp->bio_cmd = BIO_WRITE;
 	bp->bio_done = NULL;
-	bp->bio_offset = offset;
-	bp->bio_length = length;
-	bp->bio_data = ptr;
+	bp->bio_offset = offset - start_align;
+	bp->bio_length = length + start_align + end_align;
+	bp->bio_data = fixup;
 	g_io_request(bp, cp);
 	error = biowait(bp, "gwrite");
 	if (error == 0 && bp->bio_completed != length)
 		error = EIO;
 	g_destroy_bio(bp);
+	if (fixup != ptr)
+		g_free(fixup);
 	return (error);
 }
 
diff --git a/sys/geom/part/g_part_bsd.c b/sys/geom/part/g_part_bsd.c
--- a/sys/geom/part/g_part_bsd.c
+++ b/sys/geom/part/g_part_bsd.c
@@ -53,12 +53,24 @@
 #define	BOOT2_OFF	(BOOT1_SIZE + LABEL_SIZE)
 #define	BOOT2_SIZE	(BBSIZE - BOOT2_OFF)
 
+CTASSERT(sizeof(struct disklabel) <= DEV_BSIZE);
+
 FEATURE(geom_part_bsd, "GEOM partitioning class for BSD disklabels");
 
+SYSCTL_DECL(_kern_geom_part);
+static SYSCTL_NODE(_kern_geom_part, OID_AUTO, bsd, CTLFLAG_RW, 0,
+    "GEOM_PART_BSD BSD disklabels");
+
+static u_int enforce_512bbs = 1;
+SYSCTL_UINT(_kern_geom_part_bsd, OID_AUTO, enforce_512bbs,
+    CTLFLAG_RWTUN, &enforce_512bbs, 0, "Enforce 512 byte block size");
+
 struct g_part_bsd_table {
 	struct g_part_table	base;
 	u_char			*bbarea;
 	uint32_t		offset;
+	off_t			sectorsize;
+	off_t			sectorfact;
 };
 
 struct g_part_bsd_entry {
@@ -204,7 +216,7 @@
 
 	pp = gpp->gpp_provider;
 
-	if (pp->sectorsize < sizeof(struct disklabel))
+	if (pp->sectorsize < DEV_BSIZE)
 		return (ENOSPC);
 	if (BBSIZE % pp->sectorsize)
 		return (ENOTBLK);
@@ -215,15 +227,24 @@
 
 	table = (struct g_part_bsd_table *)basetable;
 	table->bbarea = g_malloc(BBSIZE, M_WAITOK | M_ZERO);
-	ptr = table->bbarea + pp->sectorsize;
+
+	if (enforce_512bbs) {
+		table->sectorfact = pp->sectorsize / 512;
+		table->sectorsize = 512;
+	} else {
+		table->sectorfact = 1;
+		table->sectorsize = pp->sectorsize;
+	}
+
+	ptr = table->bbarea + table->sectorsize;
 
 	le32enc(ptr + 0, DISKMAGIC);			/* d_magic */
-	le32enc(ptr + 40, pp->sectorsize);		/* d_secsize */
+	le32enc(ptr + 40, table->sectorsize);		/* d_secsize */
 	le32enc(ptr + 44, basetable->gpt_sectors);	/* d_nsectors */
 	le32enc(ptr + 48, basetable->gpt_heads);	/* d_ntracks */
 	le32enc(ptr + 52, ncyls);			/* d_ncylinders */
 	le32enc(ptr + 56, secpercyl);			/* d_secpercyl */
-	le32enc(ptr + 60, msize);			/* d_secperunit */
+	le32enc(ptr + 60, msize * table->sectorfact);	/* d_secperunit */
 	le16enc(ptr + 72, 3600);			/* d_rpm */
 	le32enc(ptr + 132, DISKMAGIC);			/* d_magic2 */
 	le16enc(ptr + 138, basetable->gpt_entries);	/* d_npartitions */
@@ -313,7 +334,7 @@
 
 	table = (struct g_part_bsd_table *)basetable;
 	msize = MIN(pp->mediasize / pp->sectorsize, UINT32_MAX);
-	le32enc(table->bbarea + pp->sectorsize + 60, msize); /* d_secperunit */
+	le32enc(table->bbarea + pp->sectorsize + 60, msize * table->sectorfact); /* d_secperunit */
 	basetable->gpt_last = msize - 1;
 	LIST_FOREACH(baseentry, &basetable->gpt_entry, gpe_entry) {
 		if (baseentry->gpe_index != RAW_PART + 1)
@@ -354,24 +375,33 @@
 }
 
 static int
-g_part_bsd_probe(struct g_part_table *table, struct g_consumer *cp)
+g_part_bsd_probe(struct g_part_table *basetable, struct g_consumer *cp)
 {
+	struct g_part_bsd_table *table;
 	struct g_provider *pp;
 	u_char *buf;
 	uint32_t magic1, magic2;
 	int error;
 
+	table = (struct g_part_bsd_table *)basetable;
 	pp = cp->provider;
 
 	/* Sanity-check the provider. */
-	if (pp->sectorsize < sizeof(struct disklabel) ||
-	    pp->mediasize < BBSIZE)
+	if (pp->sectorsize < DEV_BSIZE || pp->mediasize < BBSIZE)
 		return (ENOSPC);
 	if (BBSIZE % pp->sectorsize)
 		return (ENOTBLK);
 
+	if (enforce_512bbs) {
+		table->sectorfact = pp->sectorsize / 512;
+		table->sectorsize = 512;
+	} else {
+		table->sectorfact = 1;
+		table->sectorsize = pp->sectorsize;
+	}
+
 	/* Check that there's a disklabel. */
-	buf = g_read_data(cp, pp->sectorsize, pp->sectorsize, &error);
+	buf = g_read_data(cp, table->sectorsize, table->sectorsize, &error);
 	if (buf == NULL)
 		return (error);
 	magic1 = le32dec(buf + 0);
@@ -402,9 +432,9 @@
 	if (table->bbarea == NULL)
 		return (error);
 
-	buf = table->bbarea + pp->sectorsize;
+	buf = table->bbarea + table->sectorsize;
 
-	if (le32dec(buf + 40) != pp->sectorsize)
+	if (le32dec(buf + 40) != table->sectorsize)
 		goto invalid_label;
 	sectors = le32dec(buf + 44);
 	if (sectors < 1 || sectors > 255)
@@ -423,8 +453,10 @@
 		basetable->gpt_heads = heads;
 
 	chs = le32dec(buf + 60);
-	if (chs < 1)
+	if ((chs % table->sectorfact) != 0 || chs < 1)
 		goto invalid_label;
+	chs /= table->sectorfact;
+
 	/* Fix-up a sysinstall bug. */
 	if (chs > msize) {
 		chs = msize;
@@ -449,10 +481,12 @@
 		part.p_fstype = p[12];
 		part.p_frag = p[13];
 		part.p_cpg = le16dec(p + 14);
-		if (part.p_size == 0)
+		if (part.p_size == 0 || (part.p_size % table->sectorfact) != 0)
 			continue;
-		if (part.p_offset < table->offset)
+		part.p_size /= table->sectorfact;
+		if (part.p_offset < table->offset || (part.p_offset % table->sectorfact) != 0)
 			continue;
+		part.p_offset /= table->sectorfact;
 		if (part.p_offset - table->offset > basetable->gpt_last)
 			goto invalid_label;
 		baseentry = g_part_new_entry(basetable, index + 1,
@@ -510,14 +544,14 @@
 	pp = cp->provider;
 	table = (struct g_part_bsd_table *)basetable;
 	baseentry = LIST_FIRST(&basetable->gpt_entry);
-	label = table->bbarea + pp->sectorsize;
+	label = table->bbarea + table->sectorsize;
 	for (index = 1; index <= basetable->gpt_entries; index++) {
 		p = label + 148 + (index - 1) * 16;
 		entry = (baseentry != NULL && index == baseentry->gpe_index)
 		    ? (struct g_part_bsd_entry *)baseentry : NULL;
 		if (entry != NULL && !baseentry->gpe_deleted) {
-			le32enc(p + 0, entry->part.p_size);
-			le32enc(p + 4, entry->part.p_offset);
+			le32enc(p + 0, entry->part.p_size * table->sectorfact);
+			le32enc(p + 4, entry->part.p_offset * table->sectorfact);
 			le32enc(p + 8, entry->part.p_fsize);
 			p[12] = entry->part.p_fstype;
 			p[13] = entry->part.p_frag;
diff --git a/sys/geom/part/g_part_mbr.c b/sys/geom/part/g_part_mbr.c
--- a/sys/geom/part/g_part_mbr.c
+++ b/sys/geom/part/g_part_mbr.c
@@ -60,10 +60,15 @@
 SYSCTL_UINT(_kern_geom_part_mbr, OID_AUTO, enforce_chs,
     CTLFLAG_RWTUN, &enforce_chs, 0, "Enforce alignment to CHS addressing");
 
+static u_int enforce_512bbs = 1;
+SYSCTL_UINT(_kern_geom_part_mbr, OID_AUTO, enforce_512bbs,
+    CTLFLAG_RWTUN, &enforce_512bbs, 0, "Enforce 512 byte block size");
+
 #define	MBRSIZE		512
 
 struct g_part_mbr_table {
 	struct g_part_table	base;
+	off_t		sectorfact;
 	u_char		mbr[MBRSIZE];
 };
 
@@ -296,6 +301,11 @@
 
 	table = (struct g_part_mbr_table *)basetable;
 	le16enc(table->mbr + DOSMAGICOFFSET, DOSMAGIC);
+
+	if (enforce_512bbs)
+		table->sectorfact = pp->sectorsize / MBRSIZE;
+	else
+		table->sectorfact = 1;
 	return (0);
 }
 
@@ -403,14 +413,16 @@
 }
 
 static int
-g_part_mbr_probe(struct g_part_table *table, struct g_consumer *cp)
+g_part_mbr_probe(struct g_part_table *basetable, struct g_consumer *cp)
 {
+	struct g_part_mbr_table *table;
 	char psn[8];
 	struct g_provider *pp;
 	u_char *buf, *p;
 	int error, index, res, sum;
 	uint16_t magic;
 
+	table = (struct g_part_mbr_table *)basetable;
 	pp = cp->provider;
 
 	/* Sanity-check the provider. */
@@ -419,13 +431,18 @@
 	if (pp->sectorsize > 4096)
 		return (ENXIO);
 
+	if (enforce_512bbs)
+		table->sectorfact = pp->sectorsize / MBRSIZE;
+	else
+		table->sectorfact = 1;
+
 	/* We don't nest under an MBR (see EBR instead). */
 	error = g_getattr("PART::scheme", cp, &psn);
 	if (error == 0 && strcmp(psn, g_part_mbr_scheme.name) == 0)
 		return (ELOOP);
 
 	/* Check that there's a MBR. */
-	buf = g_read_data(cp, 0L, pp->sectorsize, &error);
+	buf = g_read_data(cp, 0L, MBRSIZE, &error);
 	if (buf == NULL)
 		return (error);
 
@@ -478,7 +495,7 @@
 	first = basetable->gpt_sectors;
 	msize = MIN(pp->mediasize / pp->sectorsize, UINT32_MAX);
 
-	buf = g_read_data(cp, 0L, pp->sectorsize, &error);
+	buf = g_read_data(cp, 0L, MBRSIZE, &error);
 	if (buf == NULL)
 		return (error);
 
@@ -495,6 +512,12 @@
 		ent.dp_ecyl = p[7];
 		ent.dp_start = le32dec(p + 8);
 		ent.dp_size = le32dec(p + 12);
+		if (ent.dp_start % table->sectorfact)
+			continue;
+		ent.dp_start /= table->sectorfact;
+		if (ent.dp_size % table->sectorfact)
+			continue;
+		ent.dp_size /= table->sectorfact;
 		if (ent.dp_typ == 0 || ent.dp_typ == DOSPTYP_PMBR)
 			continue;
 		if (ent.dp_start == 0 || ent.dp_size == 0)
@@ -605,8 +628,8 @@
 			p[5] = entry->ent.dp_ehd;
 			p[6] = entry->ent.dp_esect;
 			p[7] = entry->ent.dp_ecyl;
-			le32enc(p + 8, entry->ent.dp_start);
-			le32enc(p + 12, entry->ent.dp_size);
+			le32enc(p + 8, entry->ent.dp_start * table->sectorfact);
+			le32enc(p + 12, entry->ent.dp_size * table->sectorfact);
 		} else
 			bzero(p, DOSPARTSIZE);
 
@@ -614,6 +637,6 @@
 			baseentry = LIST_NEXT(baseentry, gpe_entry);
 	}
 
-	error = g_write_data(cp, 0, table->mbr, cp->provider->sectorsize);
+	error = g_write_data(cp, 0, table->mbr, MBRSIZE);
 	return (error);
 }