Index: kern/vfs_mount.c
===================================================================
--- kern/vfs_mount.c
+++ kern/vfs_mount.c
@@ -110,6 +110,7 @@
 	"rw",
 	"nosuid",
 	"noexec",
+	"fsid",
 	NULL
 };
 
@@ -838,6 +839,9 @@
 	struct mount *mp;
 	struct vnode *newdp;
 	int error;
+	uint64_t fsidval;
+	char *fsidstr, *fsidend;
+	int fsidlen;
 
 	ASSERT_VOP_ELOCKED(vp, __func__);
 	KASSERT((fsflags & MNT_UPDATE) == 0, ("MNT_UPDATE shouldn't be here"));
@@ -890,6 +894,28 @@
 	 * get.  No freeing of cn_pnbuf.
 	 */
 	error = VFS_MOUNT(mp);
+
+	/*
+	 * Set the fsid if the mount option was specified.
+	 */
+	if (error == 0) {
+		fsidlen = 0;
+		error = vfs_getopt(mp->mnt_optnew, "fsid", (void **)&fsidstr,
+		    &fsidlen);
+		if (error == 0) {
+			if (fsidlen == 0 || fsidstr[fsidlen - 1] != '\0')
+				error = EINVAL;
+			if (error == 0) {
+				fsidval = strtouq(fsidstr, &fsidend, 0);
+				if (*fsidend != '\0')
+					error = EINVAL;
+			}
+			if (error == 0)
+				error = vfs_setfsid(mp, fsidval);
+		} else
+			error = 0;
+	}
+
 	if (error != 0) {
 		vfs_unbusy(mp);
 		mp->mnt_vnodecovered = NULL;
Index: kern/vfs_subr.c
===================================================================
--- kern/vfs_subr.c
+++ kern/vfs_subr.c
@@ -769,6 +769,37 @@
 }
 
 /*
+ * Set the f_fsid to the argument, if possible.
+ */
+int
+vfs_setfsid(struct mount *mp, uint64_t val)
+{
+	struct mount *nmp;
+	fsid_t tfsid;
+
+	CTR2(KTR_VFS, "%s: mp %p", __func__, mp);
+	/*
+	 * Fill in the two 32bit fields of the fsid to "val".
+	 */
+	tfsid.val[0] = val;
+	tfsid.val[1] = val >> 32;
+	mtx_lock(&mntid_mtx);
+	if ((nmp = vfs_getvfs(&tfsid)) != NULL) {
+		vfs_rel(nmp);
+		mtx_unlock(&mntid_mtx);
+		if (nmp != mp)
+			return (EINVAL);
+		return (0);
+	}
+	MNT_ILOCK(mp);
+	mp->mnt_stat.f_fsid.val[0] = tfsid.val[0];
+	mp->mnt_stat.f_fsid.val[1] = tfsid.val[1];
+	MNT_IUNLOCK(mp);
+	mtx_unlock(&mntid_mtx);
+	return (0);
+}
+
+/*
  * Knob to control the precision of file timestamps:
  *
  *   0 = seconds only; nanoseconds zeroed.
Index: sys/mount.h
===================================================================
--- sys/mount.h
+++ sys/mount.h
@@ -918,6 +918,7 @@
 int	vfs_donmount(struct thread *td, uint64_t fsflags,
 	    struct uio *fsoptions);
 void	vfs_getnewfsid(struct mount *);
+int	vfs_setfsid(struct mount *, uint64_t);
 struct cdev *vfs_getrootfsid(struct mount *);
 struct	mount *vfs_getvfs(fsid_t *);      /* return vfs given fsid */
 struct	mount *vfs_busyfs(fsid_t *);