ceph: rbd images cannot be removed, "snap is protected from removal."

Issue

• I cannot delete an rbd snapshot
• We can not remove a rbd image provided via GLANCE; it contains snapshots reported as busy:

rbd ls -p GLANCE --id glance --keyring /etc/ceph/ceph.client.glance.keyring 
0363fb9f-9c5e-4124-8898-c7df92f6df11 
3e4810d3-3572-4d7d-ae46-6624be8a6edf 
7cf3b64c-31eb-49bf-924f-ed15b8565350 
a6936c53-3b82-4b62-9d5b-848d9a8bbc5a 
bd3abfa3-1881-43c8-9622-1a8686c00dad 
eb0b1a30-6cb7-4ba5-9f9f-4aa4ecd3ca81 
root@system:~ # rbd snap purge 7cf3b64c-31eb-49bf-924f-ed15b8565350 -p GLANCE --id glance --keyring /etc/ceph/ceph.client.glance.keyring 
Removing all snapshots: 2015-06-10 13:26:59.337291 7efeeeebe760 -1 librbd: removing snapshot from header failed: (16) Device or resource busy0% complete...failed. 
rbd: removing snaps failed: (16) Device or resource busy 
root@d90-1b-0e-43-f2-31:~ # rbd snap rm 7cf3b64c-31eb-49bf-924f-ed15b8565350@snap -p GLANCE --id glance --keyring /etc/ceph/ceph.client.glance.keyring 
rbd: snapshot '2015-06-10 13:27:14.363225 7f879983c760 -1 librbd: removing snapshot from header failed: (16) Device or resource busy 
snap' is protected from removal. 
root@system:~ #