diff --git a/stackrc b/stackrc
index 8f595eff68..088cfeed26 100644
--- a/stackrc
+++ b/stackrc
@@ -639,11 +639,17 @@ if [[ "$DOWNLOAD_DEFAULT_IMAGES" == "True" ]]; then
     DOWNLOAD_DEFAULT_IMAGES=False
 fi
 
-# Staging Area for New Images, have them here for at least 24hrs for nodepool
-# to cache them otherwise the failure rates in the gate are too high
+# Staging area for new images.  These images are cached by a run of
+# ./tools/image_list.sh during CI image build (see
+# project-config:nodepool/elements/cache-devstack/extra-data.d/55-cache-devstack-repos).
+#
+# To avoid CI failures grabbing the images, new images should be here
+# for at least 24hrs (nodepool builds images at 14:00UTC) so the they
+# are in the cache.
 PRECACHE_IMAGES=$(trueorfalse False PRECACHE_IMAGES)
 if [[ "$PRECACHE_IMAGES" == "True" ]]; then
-
+    # required for trove devstack tests; see
+    #  git.openstack.org/cgit/openstack/trove/tree/devstack/plugin.sh
     IMAGE_URL="http://tarballs.openstack.org/trove/images/ubuntu/mysql.qcow2"
     if ! [[ "$IMAGE_URLS"  =~ "$IMAGE_URL" ]]; then
         IMAGE_URLS+=",$IMAGE_URL"