Hello,
There are two things that might help you here. One is to try the new
"rocksdb_cf_compaction_on_deletion" feature that I added in Reef and we
backported to Pacific in 16.2.13. So far this appears to be a huge win
for avoiding tombstone accumulation during iteration which is often the
issue with threadpool timeouts due to rocksdb. Manual compaction can
help, but if you are hitting a case where there's concurrent iteration
and deletions with no writes, tombstones will accumulate quickly with no
compactions taking place and you'll eventually end up back in the same
place. The default sliding window and trigger settings are fairly
conservative to avoid excessive compaction, so it may require some
tuning to hit the right sweet spot on your cluster. I know of at least
one site that's using this feature with more aggressive settings than
default and had an extremely positive impact on their cluster.
The other thing that can help improve compaction performance in general
is enabling lz4 compression in RocksDB. I plan to make this the default
behavior in Squid assuming we don't run into any issues in testing.
There are several sites that are using this now in production and the
benefits have been dramatic relative to the costs. We're seeing
significantly faster compactions and about 2.2x lower space requirement
for the DB (RGW workload). There may be a slight CPU cost and read/index
listing performance impact, but even with testing on NVMe clusters this
was quite low (maybe a couple of percent).
Mark
On 9/7/23 10:21, J-P Methot wrote:
Hi,
Since my post, we've been speaking with a member of the Ceph dev team.
He did, at first, believe it was an issue linked to the common
performance degradation after huge deletes operation. So we did do
offline compactions on all our OSDs. It fixed nothing and we are going
through the logs to try and figure this out.
To answer your question, no the OSD doesn't restart after it logs the
timeout. It manages to get back online by itself, at the cost of
sluggish performances for the cluster and high iowait on VMs.
We mostly run RBD workloads.
Deep scrubs or no deep scrubs doesn't appear to change anything.
Deactivating scrubs altogether did not impact performances in any way.
Furthermore, I'll stress that this is only happening since we upgraded
to the latest Pacific, yesterday.
On 9/7/23 10:49, Stefan Kooman wrote:
On 07-09-2023 09:05, J-P Methot wrote:
Hi,
We're running latest Pacific on our production cluster and we've
been seeing the dreaded 'OSD::osd_op_tp thread 0x7f346aa64700' had
timed out after 15.000000954s' error. We have reasons to believe
this happens each time the RocksDB compaction process is launched on
an OSD. My question is, does the cluster detecting that an OSD has
timed out interrupt the compaction process? This seems to be what's
happening, but it's not immediately obvious. We are currently facing
an infinite loop of random OSDs timing out and if the compaction
process is interrupted without finishing, it may explain that.
Does the OSD also restart after it logged the timeouts?
You might want to perform an offline compaction every $timeperiod to
fix any potential RocksDB degradation. That's what we do. What kind
of workload do you run (i.e. RBD, CephFS, RGW)?
Do you also see these timeouts occur during deep-scrubs?
Gr. Stefan
--
Best Regards,
Mark Nelson
Head of Research and Development
Clyso GmbH
p: +49 89 21552391 12 | a: Minnesota, USA
w: https://clyso.com | e: mark.nelson@xxxxxxxxx
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