diff --git a/UPDATING b/UPDATING
index 231cdf25fd7..7d109a5de10 100644
--- a/UPDATING
+++ b/UPDATING
@@ -51,6 +51,13 @@ NOTE TO PEOPLE WHO THINK THAT FreeBSD 12.x IS SLOW:
 
 ****************************** SPECIAL WARNING: ******************************
 
+20170912:
+	The default serial number format for CTL LUNs has changed.  This will
+	affect users who use /dev/diskid/* device nodes, or whose FibreChannel
+	or iSCSI clients care about their LUNs' serial numbers.  Users who
+	require serial number stability should hardcode serial numbers in
+	/etc/ctl.conf .
+
 20170912:
 	For 32-bit arm compiled for hard-float support, soft-floating point
 	binaries now always get their shared libraries from
diff --git a/sys/cam/ctl/ctl_backend_block.c b/sys/cam/ctl/ctl_backend_block.c
index ab71f8cec13..4038add6bf8 100644
--- a/sys/cam/ctl/ctl_backend_block.c
+++ b/sys/cam/ctl/ctl_backend_block.c
@@ -2324,7 +2324,7 @@ ctl_be_block_create(struct ctl_be_block_softc *softc, struct ctl_lun_req *req)
 	cbe_lun->be = &ctl_be_block_driver;
 
 	if ((params->flags & CTL_LUN_FLAG_SERIAL_NUM) == 0) {
-		snprintf(tmpstr, sizeof(tmpstr), "MYSERIAL%4d",
+		snprintf(tmpstr, sizeof(tmpstr), "MYSERIAL%04d",
 			 softc->num_luns);
 		strncpy((char *)cbe_lun->serial_num, tmpstr,
 			MIN(sizeof(cbe_lun->serial_num), sizeof(tmpstr)));
@@ -2338,7 +2338,7 @@ ctl_be_block_create(struct ctl_be_block_softc *softc, struct ctl_lun_req *req)
 			sizeof(params->serial_num)));
 	}
 	if ((params->flags & CTL_LUN_FLAG_DEVID) == 0) {
-		snprintf(tmpstr, sizeof(tmpstr), "MYDEVID%4d", softc->num_luns);
+		snprintf(tmpstr, sizeof(tmpstr), "MYDEVID%04d", softc->num_luns);
 		strncpy((char *)cbe_lun->device_id, tmpstr,
 			MIN(sizeof(cbe_lun->device_id), sizeof(tmpstr)));
 
diff --git a/sys/cam/ctl/ctl_backend_ramdisk.c b/sys/cam/ctl/ctl_backend_ramdisk.c
index 3b982a7c0e3..3730327653c 100644
--- a/sys/cam/ctl/ctl_backend_ramdisk.c
+++ b/sys/cam/ctl/ctl_backend_ramdisk.c
@@ -1096,7 +1096,7 @@ ctl_backend_ramdisk_create(struct ctl_be_ramdisk_softc *softc,
 	cbe_lun->lun_config_status = ctl_backend_ramdisk_lun_config_status;
 	cbe_lun->be = &ctl_be_ramdisk_driver;
 	if ((params->flags & CTL_LUN_FLAG_SERIAL_NUM) == 0) {
-		snprintf(tmpstr, sizeof(tmpstr), "MYSERIAL%4d",
+		snprintf(tmpstr, sizeof(tmpstr), "MYSERIAL%04d",
 			 softc->num_luns);
 		strncpy((char *)cbe_lun->serial_num, tmpstr,
 			MIN(sizeof(cbe_lun->serial_num), sizeof(tmpstr)));
@@ -1110,7 +1110,7 @@ ctl_backend_ramdisk_create(struct ctl_be_ramdisk_softc *softc,
 			    sizeof(params->serial_num)));
 	}
 	if ((params->flags & CTL_LUN_FLAG_DEVID) == 0) {
-		snprintf(tmpstr, sizeof(tmpstr), "MYDEVID%4d", softc->num_luns);
+		snprintf(tmpstr, sizeof(tmpstr), "MYDEVID%04d", softc->num_luns);
 		strncpy((char *)cbe_lun->device_id, tmpstr,
 			MIN(sizeof(cbe_lun->device_id), sizeof(tmpstr)));