From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gregory Farnum Subject: Re: Questions about journals, performance and disk utilization. Date: Tue, 22 Jan 2013 14:00:52 -0800 Message-ID: <211179F2EB514DFA98FD55D7F2311A80@inktank.com> References: <58f5e24e5ac1a7bfff8fc6b90719ec75@skytech.dk> <50FF0197.6020202@inktank.com> <50FF0410.3030308@gmail.com> <50FF098C.8050707@profihost.ag> <50FF0B2E.6010301@inktank.com> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Return-path: Received: from mail-da0-f54.google.com ([209.85.210.54]:63567 "EHLO mail-da0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752758Ab3AVWBD (ORCPT ); Tue, 22 Jan 2013 17:01:03 -0500 Received: by mail-da0-f54.google.com with SMTP id n2so3423424dad.13 for ; Tue, 22 Jan 2013 14:01:02 -0800 (PST) In-Reply-To: <50FF0B2E.6010301@inktank.com> Content-Disposition: inline Sender: ceph-devel-owner@vger.kernel.org List-ID: To: Mark Nelson Cc: Stefan Priebe , Jeff Mitchell , martin , "=?utf-8?Q?ceph-devel=40vger.kernel.org?=" On Tuesday, January 22, 2013 at 1:57 PM, Mark Nelson wrote: > On 01/22/2013 03:50 PM, Stefan Priebe wrote: > > Hi, > > Am 22.01.2013 22:26, schrieb Jeff Mitchell: > > > Mark Nelson wrote: > > > > It may (or may not) help to use a power-of-2 number of PGs. It's > > > > generally a good idea to do this anyway, so if you haven't set up your > > > > production cluster yet, you may want to play around with this. Basically > > > > just take whatever number you were planning on using and round it up (or > > > > down slightly). IE if you were going to use 7,000 PGs, round up to 8192. > > > > > > > > > > > > As I was asking about earlier on IRC, I'm in a situation where the docs > > > did not mention this in the section about calculating PGs so I have a > > > non-power-of-2 -- and since there are some production things running on > > > that pool I can't currently change it. > > > > > > > > Oh same thing here - did i miss the doc or can someone point me the > > location. > > > > Is there a chance to change the number of PGs for a pool? > > > > Greets, > > Stefan > > > > Honestly I don't know if it will actually have a significant effect. > ceph_stable_mod will map things optimally when pg_num is a power of 2, > but that's only part of how things work. It may not matter very much > with high PG counts. IIRC, having a non-power of 2 count means that the extra PGs (above the lower-bounding power of 2) will be twice the size of the other PGs. For reasonable PG counts this should not cause any problems. -Greg