I want to back up some vms externally. I used to do this by snapshotting and saving the image from glance. When I used volume backed instances this doesn't seem to work.

I have deployed Openstack with Cinder. When I boot an instance, I select boot from image (Creates new volume)

After making changes to the instance, I hit create snapshot. This creates a snapshot visible under the volumes page in horizon, and also creates an entry in the glance image list.

If I try to save/export that image from glance I get a file with 0 bytes i.e.

openstack image save 84b481fe-aeec-4d36-9ebd-8e2df286c82c --file /tmp/test.img ls -l /tmp/test

Is this a bug with cinder ? Or am I missing some vital command or flag?