Index: sbin/geom/class/label/geom_label.c
===================================================================
--- sbin/geom/class/label/geom_label.c	(revision 211037)
+++ sbin/geom/class/label/geom_label.c	(working copy)
@@ -71,8 +71,12 @@
 	{ "dump", 0, label_main, G_NULL_OPTS, NULL,
 	    "dev ..."
 	},
-	{ "label", G_FLAG_VERBOSE | G_FLAG_LOADKLD, label_main, G_NULL_OPTS,
-	    NULL, "[-v] name dev"
+	{ "label", G_FLAG_VERBOSE | G_FLAG_LOADKLD, label_main, 
+	    {
+		{ 's', "sectorsize", NULL, G_TYPE_NUMBER },
+		G_OPT_SENTINEL
+	    },
+	    NULL, "[-v] [-s sectorsize] name dev"
 	},
 	{ "stop", G_FLAG_VERBOSE, NULL,
 	    {
@@ -144,7 +148,29 @@
 		    strerror(errno));
 		return;
 	}
+	md.md_sectorsize = gctl_has_param(req, "sectorsize") ? 
+	    gctl_get_intmax(req, "sectorsize") : 0;
+	if (md.md_sectorsize > 0) {
+		unsigned ssize;
 
+		ssize = g_get_sectorsize(name);
+		if (ssize == 0) {
+			gctl_error(req, "Can't get sectorsize of %s: %s",
+			    name, strerror(errno));
+			return;
+		}
+		if (md.md_sectorsize < ssize) {
+			gctl_error(req, "Cannot force sectorsize smaller "
+			    "than original sectorsize on %s", name);
+			return;
+		}
+		if (md.md_sectorsize % ssize != 0) {
+			gctl_error(req, "Forced sectorsize must be a "
+			    "multiple of original sectorsize on %s", name);
+			return;
+		}
+	}
+
 	/*
 	 * Ok, store metadata.
 	 */
@@ -189,9 +215,11 @@
 label_metadata_dump(const struct g_label_metadata *md)
 {
 
-	printf("    Magic string: %s\n", md->md_magic);
-	printf("Metadata version: %u\n", (u_int)md->md_version);
-	printf("           Label: %s\n", md->md_label);
+	printf("      Magic string: %s\n", md->md_magic);
+	printf("  Metadata version: %u\n", (u_int)md->md_version);
+	printf("             Label: %s\n", md->md_label);
+	if (md->md_version > 2 && md->md_sectorsize > 0)
+		printf("Forced sector size: %u\n", md->md_sectorsize);
 }
 
 static void
Index: sbin/geom/class/label/glabel.8
===================================================================
--- sbin/geom/class/label/glabel.8	(revision 211037)
+++ sbin/geom/class/label/glabel.8	(working copy)
@@ -43,6 +43,7 @@
 .Nm
 .Cm label
 .Op Fl v
+.Op Fl s Ar sectorsize
 .Ar name
 .Ar dev
 .Nm
@@ -157,6 +158,11 @@
 The kernel module
 .Pa geom_label.ko
 will be loaded if it is not loaded already.
+In this mode
+.Nm
+can optionally force a sector size on the labeled device, which is intended
+to be used with drives whose internal sector size (e.g. 4k) is different from
+their advertised sector size (e.g. 512 bytes).
 .It Cm stop
 Turn off the given label by its
 .Ar name .
Index: sbin/geom/misc/subr.c
===================================================================
--- sbin/geom/misc/subr.c	(revision 211037)
+++ sbin/geom/misc/subr.c	(working copy)
@@ -436,7 +436,8 @@
 				abort();
 			}
 		} else if ((int)len != argp->len) {
-			fprintf(stderr, "Wrong length %s argument.\n", param);
+			fprintf(stderr, "Wrong length %s argument. "
+			    "Got %d, expecting %d.\n", param, argp->len, len);
 			abort();
 		}
 		return (p);
Index: sys/geom/label/g_label.c
===================================================================
--- sys/geom/label/g_label.c	(revision 211037)
+++ sys/geom/label/g_label.c	(working copy)
@@ -138,13 +138,18 @@
 
 static struct g_geom *
 g_label_create(struct gctl_req *req, struct g_class *mp, struct g_provider *pp,
-    const char *label, const char *dir, off_t mediasize)
+    const char *label, const char *dir, off_t mediasize, unsigned sectorsize)
 {
 	struct g_geom *gp;
 	struct g_provider *pp2;
 	struct g_consumer *cp;
 	char name[64];
 
+	KASSERT(sectorsize > 0, ("Passed 0 sectorsize to g_label_create"));
+	KASSERT(sectorsize >= pp->sectorsize, ("Forced sectorsize smaller than"
+	    " physical sectorsize on %s", pp->name));
+	KASSERT(sectorsize % pp->sectorsize == 0, ("Forced sectorsize not a"
+	    " multiple of physical sectorsize on %s", pp->name));
 	g_topology_assert();
 
 	if (!g_label_is_name_ok(label)) {
@@ -181,8 +186,11 @@
 	gp->spoiled = g_label_spoiled;
 	g_access(cp, -1, 0, 0);
 	g_slice_config(gp, 0, G_SLICE_CONFIG_SET, (off_t)0, mediasize,
-	    pp->sectorsize, name);
+	    sectorsize, name);
 	G_LABEL_DEBUG(1, "Label for provider %s is %s.", pp->name, name);
+	if (sectorsize != pp->sectorsize)
+		G_LABEL_DEBUG(1, "Forcing sector size %u on %s",
+		    sectorsize, pp->name);
 	return (gp);
 }
 
@@ -284,6 +292,8 @@
 	if (g_access(cp, 1, 0, 0) != 0)
 		goto end;
 	do {
+		unsigned forcedssize;
+
 		if (g_label_read_metadata(cp, &md) != 0)
 			break;
 		if (strcmp(md.md_magic, G_LABEL_MAGIC) != 0)
@@ -307,8 +317,13 @@
 		if (md.md_provsize != pp->mediasize)
 			break;
 
+		if (md.md_version > 2 && md.md_sectorsize > 0)
+			forcedssize = md.md_sectorsize;
+		else
+			forcedssize = pp->sectorsize;
+
 		g_label_create(NULL, mp, pp, md.md_label, G_LABEL_DIR,
-		    pp->mediasize - pp->sectorsize);
+		    pp->mediasize - forcedssize, forcedssize);
 	} while (0);
 	for (i = 0; g_labels[i] != NULL; i++) {
 		char label[64];
@@ -321,7 +336,7 @@
 		if (label[0] == '\0')
 			continue;
 		g_label_create(NULL, mp, pp, label, g_labels[i]->ld_dir,
-		    pp->mediasize);
+		    pp->mediasize, pp->sectorsize);
 	}
 	g_access(cp, -1, 0, 0);
 end:
@@ -373,7 +388,8 @@
 		gctl_error(req, "No 'arg%d' argument", 0);
 		return;
 	}
-	g_label_create(req, mp, pp, name, G_LABEL_DIR, pp->mediasize);
+	g_label_create(req, mp, pp, name, G_LABEL_DIR, pp->mediasize,
+	    pp->sectorsize);
 }
 
 static const char *
Index: sys/geom/label/g_label.h
===================================================================
--- sys/geom/label/g_label.h	(revision 211037)
+++ sys/geom/label/g_label.h	(working copy)
@@ -41,8 +41,9 @@
  * Version history:
  * 1 - Initial version number.
  * 2 - Added md_provsize field to metadata.
+ * 3 - Added forced sector size field to metadata.
  */
-#define	G_LABEL_VERSION		2
+#define	G_LABEL_VERSION		3
 #define	G_LABEL_DIR		"label"
 
 #ifdef _KERNEL
@@ -94,6 +95,7 @@
 	uint32_t	md_version;	/* Version number. */
 	char		md_label[16];	/* Label. */
 	uint64_t	md_provsize;	/* Provider's size. */
+	uint32_t	md_sectorsize;	/* Forced sector size (0=disabled). */
 };
 static __inline void
 label_metadata_encode(const struct g_label_metadata *md, u_char *data)
@@ -103,6 +105,7 @@
 	le32enc(data + 16, md->md_version);
 	bcopy(md->md_label, data + 20, sizeof(md->md_label));
 	le64enc(data + 36, md->md_provsize);
+	le32enc(data + 44, md->md_sectorsize);
 }
 static __inline void
 label_metadata_decode(const u_char *data, struct g_label_metadata *md)
@@ -112,5 +115,6 @@
 	md->md_version = le32dec(data + 16);
 	bcopy(data + 20, md->md_label, sizeof(md->md_label));
 	md->md_provsize = le64dec(data + 36);
+	md->md_sectorsize = le32dec(data + 44);
 }
 #endif	/* _G_LABEL_H_ */