Hi *,
are there any known limitations or impacts of (too) many objects per
PG? We're dealing with a performance decrease on Nautilus (I know, but
it can't be upgraded at this time) while pushing a million emails
(many small objects) into the cluster. At some point, maybe between
600.000 and 900.000 emails or so, the client requests became slower
(CephFS kernel clients) although all entities look fine, the MDS
daemons are not overloaded, the OSDs not fully utilized (HDDs with
shared rocksDB on SSDs). The HDD OSDs currently have around 100 PGs
per OSD with 45 GB per PG (quite a lot) and around 180.000 objects per
PG. The main data pool has an EC profile with k=4 m=5.
We want to increase the pg_num anyway, we expect a general performance
increase after the pg splitting, but we're still wondering where the
limits are. For example, if we increase mon_max_pg_per_osd to more
than 250 we could split the PGs even more to reduce PG size and number
of objects per PG, if the OSDs can cope with it utilization-wise, but
I'm not sure if that's a good idea. Are there any other tunables we
could tweak instead to reduce the performance impact?
Any comments or references are highly appreciated!
Thanks,
Eugen
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