The problem is that rsync creates and renames files a lot. When doing
this with small files it can be very heavy for the MDS.
Perhaps run rsync with --in-place to prevent it from re-creating partial
files to a temp entity named .dfg45terf.~tmp~ and then renaming it into the
correct filename, and instead just edit the data in the file it currently
syncs.
I think there is an option to have the ~tmp~ files elsewhere (as in local
non-ceph path) and then copying them into the correct destination, if small
edits to large files still make odd IO patterns that cephfs doesn't like
when syncing a partially correct file.
--
May the most significant bit of your life be positive.