---------- Forwarded message ---------- From: Zhen Cao <juehuiconst@xxxxxxxxx> Date: 2016-11-30 17:01 GMT+08:00 Subject: Re: Ceph PG Number To: Piotr Dałek <branch@xxxxxxxxxxxxxxxx> Thanks! I understood it. http://docs.ceph.com/docs/jewel/rados/operations/placement-groups/ said Ideally objects are evenly distributed in each placement group. Since CRUSH computes the placement group for each object, but does not actually know how much data is stored in each OSD within this placement group, the ratio between the number of placement groups and the number of OSDs may influence the distribution of the data significantly. For instance, if there was single a placement group for ten OSDs in a three replica pool, only three OSD would be used because CRUSH would have no other choice. When more placement groups are available, objects are more likely to be evenly spread among them. CRUSH also makes every effort to evenly spread OSDs among all existing Placement Groups. I got it,thanks for your help! 2016-11-30 16:31 GMT+08:00 Piotr Dałek <branch@xxxxxxxxxxxxxxxx>: > 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/ > > -- > Piotr Dałek > branch@xxxxxxxxxxxxxxxx > http://blog.predictor.org.pl -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
