Re: Ceph PG Number

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On Wed, 30 Nov 2016, Piotr Dałek wrote:
> On Wed, Nov 30, 2016 at 04:22:10PM +0800, Zhen Cao wrote:
> > Hi,
> > I am reading the RADOS paper,but i could not understand why more PGs
> > per OSD result in a more balanced distribution.I konw the mapping
> > procedure between oid and osd,but it is neverless difficult to
> > understand for me.
> > Thanks for your help!
> 
> In a nuthshell - because each PG corresponds to multiple objects.
> And because of this, it is possible that some PGs will aggregate more
> objects than other PGs, resulting in lack of acutal balance. Also see here:
> http://docs.ceph.com/docs/jewel/rados/operations/placement-groups/ 

More generally, it's a [pseudo-]random process that puts objects in PGs 
and PGs on OSDs.  If you have a small number of objects, you'll have a 
small number of PG and a large variance (one PG may have 2 objects, 
another 4).  If it's a large number, things will converge toward an 
average (e.g., very close to ~1,000,000 objects per PG).  Same goes for 
PGs distributed to OSDs.  Larger numbers mean a tighter variance.

sage

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