Received: from localhost (unknown [200.46.208.211]) by mail.postgresql.org (Postfix) with ESMTP id 053526327A8 for ; Wed, 25 Mar 2009 13:16:43 -0300 (ADT) Received: from mail.postgresql.org ([200.46.204.86]) by localhost (mx1.hub.org [200.46.208.211]) (amavisd-maia, port 10024) with ESMTP id 03977-01 for ; Wed, 25 Mar 2009 13:16:40 -0300 (ADT) X-Greylist: from auto-whitelisted by SQLgrey-1.7.6 Received: from EXHUB018-3.exch018.msoutlookonline.net (exhub018-3.exch018.msoutlookonline.net [64.78.17.18]) by mail.postgresql.org (Postfix) with ESMTP id 6275C632361 for ; Wed, 25 Mar 2009 13:16:37 -0300 (ADT) Received: from EXVMBX018-1.exch018.msoutlookonline.net ([64.78.17.47]) by EXHUB018-3.exch018.msoutlookonline.net ([64.78.17.18]) with mapi; Wed, 25 Mar 2009 09:16:34 -0700 From: Scott Carey To: Greg Smith , Mark Kirkwood CC: "pgsql-performance@postgresql.org" Date: Wed, 25 Mar 2009 09:16:33 -0700 Subject: Re: Raid 10 chunksize Thread-Topic: [PERFORM] Raid 10 chunksize Thread-Index: AcmtIOWwbuLDb20CSXq4e6fyiR6mVgARCpk9 Message-ID: In-Reply-To: Accept-Language: en-US Content-Language: en X-MS-Has-Attach: X-MS-TNEF-Correlator: acceptlanguage: en-US Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-Virus-Scanned: Maia Mailguard 1.0.1 X-Spam-Status: No, hits=0 tagged_above=0 required=5 tests=AWL=0.000 X-Spam-Level: X-Archive-Number: 200903/367 X-Sequence-Number: 33289 On 3/25/09 1:07 AM, "Greg Smith" wrote: > On Wed, 25 Mar 2009, Mark Kirkwood wrote: >=20 >> I'm thinking that the raid chunksize may well be the issue. >=20 > Why? I'm not saying you're wrong, I just don't see why that parameter > jumped out as a likely cause here. >=20 If postgres is random reading or writing at 8k block size, and the raid array is set with 4k block size, then every 8k random i/o will create TWO disk seeks since it gets split to two disks. Effectively, iops will be cu= t in half.