Dear all
We are running Ceph Pacific (16.2.15) with RadosGW and have been
getting "large omap objects" health warnings on the RadosGW index
pool. Indeed we had one bucket in particular that was positively
huge with 8'127'198 objects that had just a single shard. But we
have been seeing the message on some other buckets, too.
Eventually, we activated automatic resharding
(rgw_dynamic_resharding = true) and indeed this bucket was
resharded to now 167 shards. However, I am now getting even more
large omap object warnings. On that same bucket, too. The other
buckets have not been resharded at all. They are not in the
queue, either:
| radosgw-admin reshard list
[]
| grep 'Large omap object found' /var/log/ceph/ceph* | grep 'PG:
' | cut -d: -f 10 | cut -d. -f 4-5 | sort | uniq -c
2 7936686773.215
12 7937604172.149
10 7955243979.1209
9 7955243979.2480
13 7955243979.2481
12 7968198782.110
13 7968913553.67
11 7968913553.68
10 7968913553.69
11 7981210604.1
74 7981624399.1
217 7988881492.1
| radosgw-admin metadata list --metadata-key bucket.instance |
grep -i 7988881492
"<bucket1_name>:<pool_name>.7988881492.1",
| radosgw-admin bucket stats --bucket <bucket1_name>
{
"bucket": "<bucket1_name>",
"num_shards": 167,
[...]
"usage": {
"rgw.main": {
"size": 9669928611955,
"size_actual": 9692804734976,
"size_utilized": 9669928611955,
"size_kb": 9443289661,
"size_kb_actual": 9465629624,
"size_kb_utilized": 9443289661,
"num_objects": 8134437
}
},
Let's check another one, too:
| radosgw-admin metadata list --metadata-key bucket.instance |
grep -i 7968198782.110
"<bucket2_name>:<pool_name>.7968198782.110",
| radosgw-admin bucket stats --bucket <bucket2_name>
[...]
"num_objects": 38690
[...]
According to the documentation in [0], buckets are resharded at a
threshold of 100'000 objects per shard. For both of these, that
applies nicely, so it makes sense that they are not getting
resharded any further.
But why then am I getting these warnings?
Reading the documentation in [1], I can see that warnings are
printed at 200'000 entries per omap object. Can I assume that one
object in an RGW bucket means 1 entry in an omap object? Or is
that a missconception?
Now, here is my working theory. Please let me know if that has
any merit or if I'm completely off:
The affected buckets have versioning activated. Plus object
locking too. They get used by a backup software (Kopia) that uses
these features to provide ransomware protection. So my thinking
is that maybe with versioning active, each object in a bucket
could result in multiple omap entries, maybe one per version or
something?
If that is the case, then maybe I should reduce
`rgw_max_objs_per_shard` from 100'000 to something like 10'000 to
have the buckets resharded more aggressively?
But then again, that assumes a lot. For example, that assumes
that the num_objects counter in the bucket stats does not count
up on versioned objects. So my assumption could be completely
whack.
What do you think? What can I do to get rid of the large omap
objects? Is more resharding going to help? What else could I check?
Sincerely
Niklaus Hofer
Links:
[0] https://docs.ceph.com/en/latest/radosgw/dynamicresharding/
#confval-rgw_max_objs_per_shard
[1]
https://docs.ceph.com/en/latest/rados/operations/health-checks/
#large-omap-objects
--
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Wasserwerkgasse 7
CH-3011 Bern
Telefon: +41 31 332 53 63
www.stepping-stone.ch
[email protected]
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