From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeff Mitchell Subject: Re: Questions about journals, performance and disk utilization. Date: Tue, 22 Jan 2013 16:58:02 -0500 Message-ID: <50FF0B6A.6070408@gmail.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; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from mail-ve0-f178.google.com ([209.85.128.178]:48445 "EHLO mail-ve0-f178.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754493Ab3AVV6G (ORCPT ); Tue, 22 Jan 2013 16:58:06 -0500 Received: by mail-ve0-f178.google.com with SMTP id oz10so726547veb.9 for ; Tue, 22 Jan 2013 13:58:05 -0800 (PST) In-Reply-To: <50FF0B2E.6010301@inktank.com> Sender: ceph-devel-owner@vger.kernel.org List-ID: To: Mark Nelson Cc: Stefan Priebe , martin , "ceph-devel@vger.kernel.org" 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. Yeah, that's why I said *if* it matters, once someone runs suitable benchmarks, please provide a resilvering mechanism :-) I'd be interested in figuring out the right way to migrate an RBD from one pool to another regardless. --Jeff