diff -rudpN src/sbin/tunefs.orig/tunefs.8 src/sbin/tunefs/tunefs.8
--- src/sbin/tunefs.orig/tunefs.8	2015-08-31 16:12:07.264970000 +0300
+++ src/sbin/tunefs/tunefs.8	2016-05-27 14:15:54.658161000 +0300
@@ -40,6 +40,7 @@
 .Op Fl a Cm enable | disable
 .Op Fl e Ar maxbpg
 .Op Fl f Ar avgfilesize
+.Op Fl G
 .Op Fl j Cm enable | disable
 .Op Fl J Cm enable | disable
 .Op Fl k Ar held-for-metadata-blocks
@@ -93,6 +94,8 @@ For file systems with exclusively large 
 this parameter should be set higher.
 .It Fl f Ar avgfilesize
 Specify the expected average file size.
+.It Fl G
+Generating a new unique filesystem id (/dev/ufsid/...).
 .It Fl j Cm enable | disable
 Turn on/off soft updates journaling.
 .It Fl J Cm enable | disable
diff -rudpN src/sbin/tunefs.orig/tunefs.c src/sbin/tunefs/tunefs.c
--- src/sbin/tunefs.orig/tunefs.c	2015-08-31 16:12:07.263782000 +0300
+++ src/sbin/tunefs/tunefs.c	2016-05-27 14:15:54.658163000 +0300
@@ -91,21 +91,24 @@ main(int argc, char *argv[])
 	int active;
 	int Aflag, aflag, eflag, evalue, fflag, fvalue, jflag, Jflag, kflag;
 	int kvalue, Lflag, lflag, mflag, mvalue, Nflag, nflag, oflag, ovalue;
-	int pflag, sflag, svalue, Svalue, tflag;
+	int pflag, sflag, svalue, Svalue, tflag, Gflag;
 	int ch, found_arg, i;
 	const char *chg[2];
 	struct ufs_args args;
 	struct statfs stfs;
+	int32_t old_fs_id[2];
+	time_t utime;
 
 	if (argc < 3)
 		usage();
 	Aflag = aflag = eflag = fflag = jflag = Jflag = kflag = Lflag = 0;
 	lflag = mflag = Nflag = nflag = oflag = pflag = sflag = tflag = 0;
+	Gflag = 0;
 	avalue = jvalue = Jvalue = Lvalue = lvalue = Nvalue = nvalue = NULL;
 	evalue = fvalue = mvalue = ovalue = svalue = Svalue = 0;
 	active = 0;
 	found_arg = 0;		/* At least one arg is required. */
-	while ((ch = getopt(argc, argv, "Aa:e:f:j:J:k:L:l:m:N:n:o:ps:S:t:"))
+	while ((ch = getopt(argc, argv, "Aa:e:f:j:J:k:L:l:m:N:n:o:ps:S:t:G"))
 	    != -1)
 		switch (ch) {
 
@@ -292,6 +295,11 @@ main(int argc, char *argv[])
 			tflag = 1;
 			break;
 
+		case 'G':
+			found_arg = 1;
+			Gflag++;
+			break;
+
 		default:
 			usage();
 		}
@@ -551,6 +559,15 @@ main(int argc, char *argv[])
 			}
  		}
 	}
+	if (Gflag) {
+		name = "ufsid";
+		time(&utime);
+		old_fs_id[0] = sblock.fs_id[0];
+		old_fs_id[1] = sblock.fs_id[1];
+		sblock.fs_id[0] = (long)utime;
+		sblock.fs_id[1] = arc4random();
+		warnx("%s changed from %08x%08x to new value %08x%08x", name, old_fs_id[0], old_fs_id[1], sblock.fs_id[0], sblock.fs_id[1]);
+	}
 
 	if (sbwrite(&disk, Aflag) == -1)
 		goto err;
@@ -1086,7 +1103,7 @@ static void
 usage(void)
 {
 	fprintf(stderr, "%s\n%s\n%s\n%s\n%s\n%s\n",
-"usage: tunefs [-A] [-a enable | disable] [-e maxbpg] [-f avgfilesize]",
+"usage: tunefs [-A] [-a enable | disable] [-e maxbpg] [-f avgfilesize] [-G]",
 "              [-J enable | disable] [-j enable | disable] [-k metaspace]",
 "              [-L volname] [-l enable | disable] [-m minfree]",
 "              [-N enable | disable] [-n enable | disable]",
@@ -1132,4 +1149,6 @@ printfs(void)
 		warnx(OPTWARN, "space", "<", MINFREE);
 	warnx("volume label: (-L)                                 %s",
 		sblock.fs_volname);
+	warnx("unique filesystem id: (-G)                         %08x%08x",
+		sblock.fs_id[0],sblock.fs_id[1]);
 }