From mboxrd@z Thu Jan 1 00:00:00 1970 From: Piotr =?utf-8?B?RGHFgmVr?= Subject: Re: Ceph PG Number Date: Wed, 30 Nov 2016 09:31:05 +0100 Message-ID: <20161130083105.GM26997@predictor> References: Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Return-path: Received: from predictor.org.pl ([185.5.97.54]:50121 "EHLO predictor.org.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754165AbcK3I3F (ORCPT ); Wed, 30 Nov 2016 03:29:05 -0500 Content-Disposition: inline In-Reply-To: Sender: ceph-devel-owner@vger.kernel.org List-ID: To: Zhen Cao Cc: ceph-devel@vger.kernel.org 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@predictor.org.pl http://blog.predictor.org.pl