Ceph » rbd

rbd export-diff can export the variation of src image between two snapshots: snap1, snap2.
Supose you have already done the snap1 import to dest image, then you can use import-diff to dest image, and will get dest@snap2, and dest@snap2 is exactly same as src@snap2.

However, this works on the assumption: the dest image's header version does not change since the latest snap.
For example, before import the diff of snap1 and snap2 to dest image, we make some writes to dest image, so the dest header is not same as dest@snap1. The following import diff operation of variation of snap1 and snap2 become meaningless. Because in this situation, dest@snap2 is not same as src@snap2.

In summary, if src@snap1==dest@snap1, then export-diff src@snap1_between_snap2 to import-diff dest, we should get dest@snap2==src@snap2, no matter dest header version changed or not.