Hello,
I have upgraded our Ceph cluster from Nautilus to Octopus (15.2.15) over the
weekend. The upgrade went well as far as I can tell.
Earlier today, noticing that our CephFS data pool was approaching capacity, I
removed some old CephFS snapshots (taken weekly at the root of the filesystem),
keeping only the most recent one (created today, 2022-02-21). As expected, a
good fraction of the PGs transitioned from active+clean to active+clean+snaptrim
or active+clean+snaptrim_wait. In previous occasions when I removed a snapshot
it took a few days for snaptrimming to complete. This would happen without
noticeably impacting other workloads, and would also free up an appreciable
amount of disk space.
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.
Outside of users' working hours, I decided to resume snaptrimming by setting
osd_pg_max_concurrent_snap_trims back to 1. Much to my surprise, nothing
happened. All PGs remained (and still remain at time of writing) in the state
active+clean, even after restarting some of them. This definitely seems
abnormal, as I mentioned earlier, snaptrimming this FS previously would take in
the order of multiple days. Moreover, if snaptrim were truly complete, I would
expect pool usage to have dropped by appreciable amounts (at least a dozen
terabytes), but that doesn't seem to be the case.
A du on the CephFS root gives:
# du -sh /mnt/pve/cephfs
31T /mnt/pve/cephfs
But:
# ceph df
<snip>
--- POOLS ---
POOL ID PGS STORED OBJECTS USED %USED MAX AVAIL
cephfs_data 7 512 43 TiB 190.83M 147 TiB 93.22 3.6 TiB
cephfs_metadata 8 32 89 GiB 694.60k 266 GiB 1.32 6.4 TiB
<snip>
ceph pg dump reports a SNAPTRIMQ_LEN of 0 on all PGs.
Did CephFS just leak a massive 12 TiB worth of objects...? It seems to me that
the snaptrim operation did not complete at all.
Perhaps relatedly:
# ceph daemon mds.choi dump snaps
{
"last_created": 93,
"last_destroyed": 94,
"snaps": [
{
"snapid": 93,
"ino": 1,
"stamp": "2022-02-21T00:00:01.245459+0800",
"name": "2022-02-21"
}
]
}
How can last_destroyed > last_created? The last snapshot to have been taken on
this FS is indeed #93, and the removed snapshots were all created on previous
weeks.
Could someone shed some light please? Assuming that snaptrim didn't run to
completion, how can I manually delete objects from now-removed snapshots? I
believe this is what the Ceph documentation calls a "backwards scrub" - but I
didn't find anything in the Ceph suite that can run such a scrub. This pool is
filling up fast, I'll throw in some more OSDs for the moment to buy some time,
but I certainly would appreciate your help!
Happy to attach any logs or info you deem necessary.
Regards,
LRT
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