Received: from maia.hub.org (maia-3.hub.org [200.46.204.243]) by mail.postgresql.org (Postfix) with ESMTP id 57121B5D80A for ; Thu, 18 Aug 2011 04:08:07 -0300 (ADT) Received: from mail.postgresql.org ([200.46.204.86]) by maia.hub.org (mx1.hub.org [200.46.204.243]) (amavisd-maia, port 10024) with ESMTP id 88949-02 for ; Thu, 18 Aug 2011 07:07:57 +0000 (UTC) X-Greylist: from auto-whitelisted by SQLgrey-1.7.6 Received: from mail.catalyst.net.nz (bertrand.catalyst.net.nz [202.78.240.40]) by mail.postgresql.org (Postfix) with ESMTP id B1BEAB5D803 for ; Thu, 18 Aug 2011 04:07:56 -0300 (ADT) Received: from localhost (localhost [127.0.0.1]) by mail.catalyst.net.nz (Postfix) with ESMTP id 6216232FAC for ; Thu, 18 Aug 2011 19:07:53 +1200 (NZST) X-Virus-Scanned: Debian amavisd-new at catalyst.net.nz Received: from mail.catalyst.net.nz ([127.0.0.1]) by localhost (bertrand.catalyst.net.nz [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id UJx2VXd7gMwL for ; Thu, 18 Aug 2011 19:07:52 +1200 (NZST) Received: from [192.168.1.11] (122-60-64-230.jetstream.xtra.co.nz [122.60.64.230]) (Authenticated sender: mark.kirkwood) by mail.catalyst.net.nz (Postfix) with ESMTPSA id A9BC132FA9 for ; Thu, 18 Aug 2011 19:07:52 +1200 (NZST) Message-ID: <4E4CBA47.5070600@catalyst.net.nz> Date: Thu, 18 Aug 2011 19:07:51 +1200 From: Mark Kirkwood User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.18) Gecko/20110617 Thunderbird/3.1.11 MIME-Version: 1.0 To: pgsql-performance@postgresql.org Subject: Re: Raid 5 vs Raid 10 Benchmarks Using bonnie++ References: <959BD2FA-0312-458B-BD5F-ED358D2C47ED@darkstatic.com> <75A85649-2B95-43D5-8404-4776D2E580EC@darkstatic.com> <4E4C2FB4.4010409@2ndQuadrant.com> <207E881B-73CD-4C31-A863-2C238838C41F@darkstatic.com> <4E4CA4BC.9050908@ringerc.id.au> In-Reply-To: <4E4CA4BC.9050908@ringerc.id.au> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Virus-Scanned: Maia Mailguard 1.0.1 X-Spam-Status: No, hits=-3.037 tagged_above=-10 required=5 tests=BAYES_00=-1.9, RP_MATCHES_RCVD=-1.137 X-Spam-Level: X-Archive-Number: 201108/223 X-Sequence-Number: 44670 On 18/08/11 17:35, Craig Ringer wrote: > On 18/08/2011 11:48 AM, Ogden wrote: >> Isn't this very dangerous? I have the Dell PERC H700 card - I see >> that it has 512Mb Cache. Is this the same thing and good enough to >> switch to nobarrier? Just worried if a sudden power shut down, then >> data can be lost on this option. >> >> > Yeah, I'm confused by that too. Shouldn't a write barrier flush data > to persistent storage - in this case, the RAID card's battery backed > cache? Why would it force a RAID controller cache flush to disk, too? > > If the card's cache has a battery, then the cache is preserved in the advent of crash/power loss etc - provided it has enough charge, so setting 'writeback' property on arrays is safe. The PERC/SERVERRAID cards I'm familiar (LSI Megaraid rebranded models) all switch to write-though mode if they detect the battery is dangerously discharged so this is not normally a problem (but commit/fsync performance will fall off a cliff when this happens)! Cheers Mark