On 07/12/2013 09:53 AM, Gandalf Corvotempesta wrote:
2013/7/12 Mark Nelson <mark.nelson@xxxxxxxxxxx>:
At large numbers of PGs it may not matter very much, but I don't think it
would hurt either!
Basically this has to do with how ceph_stable_mod works. At
non-power-of-two values, the bucket counts aren't even, but that's only a
small part of the story and may ultimately only have a small effect on the
distribution unless the PG count is small.
In case of 12 OSDs for each node, and a cluster made with 18 storage
nodes are you suggesting:
(12*18*100) / 3 = 7200 PGs, that rounded to an exponent of 2 means 8192 ?
Well, our official recommendation on the website is PGS = OSDS * 100 /
replicas. I think the thought is that with sufficient numbers of OSDs
the behaviour of ceph_stable_mod shouldn't matter (much). At some point
I'd like to do a little more involved of an analysis to see how PG
distribution changes, but for now I wouldn't really expect a dramatic
difference between 7200 and 8192 PGs.
Mark
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