Ceph » RADOS

See https://www.spinics.net/lists/ceph-users/msg71061.html

This time around, after a few hours of snaptrimming, users complained of high IO
latency, and indeed Ceph reported "slow ops" on a number of OSDs and on the
active MDS. I attributed this to the snaptrimming and decided to reduce it by
initially setting osd_pg_max_concurrent_snap_trims to 1, which didn't seem to
help much, so I then set it to 0, which had the surprising effect of
transitioning all PGs back to active+clean (is this intended?). I also restarted
the MDS which seemed to be struggling. IO latency went back to normal
immediately.

In the code, when osd_pg_max_concurrent_snap_trims is 0, PrimaryLogPG::AwaitAsyncWork::react(const DoSnapWork&) calls pg->snap_mapper.get_next_objects_to_trim looking for 0 snaps to trim. But pg->snap_mapper.get_next_objects_to_trim returns ENOENT in this case, then DoSnapWork erases the remaining snap_to_trim.