[ceph-users] Is there any way to merge an rbd image's full backup and a diff?

Hi, I'm developing RBD images' backup system. In my case, a backup data must be stored at least two weeks. To meet this requirement, I'd like to take backups as follows: 1. Take a full backup by rbd export first. 2. Take a differencial backups everyday. 3. Merge the full backup and the oldest (taken two weeks ago) diff. As a result of evaluation, I confirmed there is no problem in step 1 and 2. However, I found that step 3 couldn't be accomplished by `rbd merge-diff <full backup> <diff>` because `rbd merge-diff` only accepts a diff as a first parameter. Is there any way to merge a full backup and a diff? Thanks, Satoru