Ceph » rbd

When deep-copying from snapshot A -> B, delay the creation of snapshot B but do copy its data to the HEAD revision of the destination image. Once the deep-copy is complete, create the final B snapshot. This will be helpful for rbd-mirror snapshot-based mirroring since the lack of snapshot B in the destination (non-primary) image can be an indication that (1) the sync isn't complete yet, and (2) in the future it would would prevent other peers from attempting to sync a half-copied snapshot.