rbd diff is not for checking the actual disk usage of rbd, right? It should be rbd du
https://github.com/nhanhoadocs/tutorial-ceph/blob/master/docs/operating/ceph-cheat-sheet.md
rbd diff {pool-name}/{images} | awk '{SUM += $2} END {print SUM/1024/1024 " MB"}'
https://docs.ceph.com/en/pacific/man/8/rbd/
