I have been thinking about the implications of losing the snapshot chain on a RBD when doing export-diff->import-diff between two separate physical locations. As I understand it, in this scenario when you take the first snapshot again on the source, you would In effect end up copying the whole RBD image across to the other site as the diff would be based on Creation->1st Snap. If this was a large multi TB RBD, even over a reasonably fast link, this could a long time to resync.
>From what I understand the RADOS objects which RBD's are striped across, have last modified timestamps. Would it be feasible to add an option to the rbd command to export a diff of modified blocks since a certain timestamp? This way you could take a new snapshot on the source RBD and then specify a timestamp from just before the previously deleted snapshot and export the blocks to bring the 2nd copy back up to date. You could then resume the normal export-diff->import-diff procedure. Please tell me if I am thinking about this in completely the wrong way, or if this is actually a possible solution. Many Thanks, Nick
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