Comment 2 for bug 1484631

I think this is not a bug, Failed to delete the snapshot of the volume with rbd backend, because of the config of cinder-volume/cinder.conf there is a field "rbd_flatten_volume_from_snapshot".
The default value is False, when you create snapshots of a volume, will make a chain clone,cannot be deleted because you create a new volume or virtual machine with this snapshot.
On the other hand,if "rbd_flatten_volume_from_snapshot = True". When you create a volume of time will make a full copy,this is actually done with a flatten operation.So there is no dependency between this volume and the snapshot,and you can delete the snapshot.
If you want a deeper understanding of this operation, you can look at the create_volume_from_source interface until rbd driver.

I hope this helps you.