diff --git a/devstack/lib/ceph b/devstack/lib/ceph
index 27ad69dd..4625ea99 100644
--- a/devstack/lib/ceph
+++ b/devstack/lib/ceph
@@ -44,7 +44,7 @@ CEPH_CONF_DIR=${CEPH_CONF_DIR:-/etc/ceph}
 # DevStack will create a loop-back disk formatted as XFS to store the
 # Ceph data. Set ``CEPH_LOOPBACK_DISK_SIZE`` to the disk size in
 # kilobytes.
-# Default is 10 gigabytes.
+VOLUME_BACKING_FILE_SIZE=${VOLUME_BACKING_FILE_SIZE:-8GB}
 CEPH_LOOPBACK_DISK_SIZE_DEFAULT=${CEPH_LOOPBACK_DISK_SIZE_DEFAULT:-$VOLUME_BACKING_FILE_SIZE}
 CEPH_LOOPBACK_DISK_SIZE=\
 ${CEPH_LOOPBACK_DISK_SIZE:-$CEPH_LOOPBACK_DISK_SIZE_DEFAULT}