Hello,
Is 256 good value in our case ? We have 80TB of data with more than 300M files.
You want at least as many PGs that each of the OSDs host a portion of the OMAP data. You want to spread out OMAP to as many _fast_ OSDs as possible.
I have tried to find an answer to your question: are more metadata PGs better? I haven't found a definitive answer. This would ideally be tested in a non-prod / pre-prod environment and tuned
to individual requirements (type of workload). For now, I would not blindly trust the PG autoscaler. I have seen it advise settings that would definately not be OK. You can skew things in the
autoscaler with the "bias" parameter, to compensate for this. But as far as I know the current heuristics to determine a good value do not take into account the importance of OMAP (RocksDB)
spread accross OSDs. See a blog post about autoscaler tuning [1].
It would be great if tuning metadata PGs for CephFS / RGW could be performed during the "large scale tests" the devs are planning to perform in the future. With use cases that take into
consideration "a lot of small files / objects" versus "loads of large files / objects" to get a feeling how tuning this impacts performance for different work loads.
Gr. Stefan
[1]: https://ceph.io/en/news/blog/2022/autoscaler_tuning/
Thanks for the information, I agree that autoscaler seem to not be able to handle my use case.
(thanks to icepic.dz@xxxxxxxxx too)
By the way, since I have set PG=256, I have much less SLOW requests than before, even I still have, the impact on my users has been reduced a lot.
# zgrep -c -E 'WRN.*(SLOW_OPS|SLOW_REQUEST|MDS_SLOW_METADATA_IO)' floki.log.4.gz floki.log.3.gz floki.log.2.gz floki.log.1.gz floki.log
floki.log.4.gz:6883
floki.log.3.gz:11794
floki.log.2.gz:3391
floki.log.1.gz:1180
floki.log:122
If I have the opportunity, I will try to run some benchmark with multiple value of the PG on cephfs_metadata pool.
Best regards,
--
Yoann Moulin
EPFL IC-IT
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