diff --git a/doc/config-reference/source/image-service/image_service_backends.rst b/doc/config-reference/source/image-service/image_service_backends.rst
index 7c0dee23a2..5c1e15653e 100644
--- a/doc/config-reference/source/image-service/image_service_backends.rst
+++ b/doc/config-reference/source/image-service/image_service_backends.rst
@@ -13,6 +13,10 @@ images:
 * Object Storage service (swift)
 * VMware ESX
 
+.. note::
+
+   You must use only ``raw`` image formats with the Ceph RBD back end.
+
 The following tables detail the options available for each.
 
 .. include:: ../tables/glance-cinder.rst