Received: from localhost (unknown [200.46.204.184]) by postgresql.org (Postfix) with ESMTP id 303AC2E0241 for ; Mon, 3 Mar 2008 11:07:45 -0400 (AST) Received: from postgresql.org ([200.46.204.71]) by localhost (mx1.hub.org [200.46.204.184]) (amavisd-maia, port 10024) with ESMTP id 97286-09 for ; Mon, 3 Mar 2008 11:07:33 -0400 (AST) X-Greylist: from auto-whitelisted by SQLgrey-1.7.5 Received: from ironport2-out.pppoe.ca (ironport2-out.pppoe.ca [206.248.154.182]) by postgresql.org (Postfix) with ESMTP id 18D6B2E0250 for ; Mon, 3 Mar 2008 11:07:41 -0400 (AST) X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: Ao8CAKeiy0fO+Igp/2dsb2JhbACsYoEA X-IronPort-AV: E=Sophos;i="4.25,438,1199682000"; d="scan'208";a="15491048" Received: from smtp.pppoe.ca ([65.39.196.238]) by ironport2-out.pppoe.ca with ESMTP; 03 Mar 2008 10:07:40 -0500 Received: from [206.248.136.41] ([206.248.136.41]) by smtp.pppoe.ca (Internet Mail Server v1.0) with ESMTP id JSK06240; Mon, 03 Mar 2008 10:07:40 -0500 Message-ID: <47CC13DB.9000406@mark.mielke.cc> Date: Mon, 03 Mar 2008 10:06:03 -0500 From: Mark Mielke User-Agent: Thunderbird 2.0.0.12 (X11/20080226) MIME-Version: 1.0 To: Matthew CC: pgsql-performance Subject: Re: How to allocate 8 disks References: <1204285894.11773.33.camel@franck-gusty> <1204296705.11773.51.camel@franck-gusty> <47C88826.4040209@paradise.net.nz> <47C99B3E.4010606@emolecules.com> <20080301130347.0832b323@commandprompt.com> <47C9D05C.3040908@emolecules.com> <47CC0FD1.7090404@mark.mielke.cc> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Virus-Scanned: Maia Mailguard 1.0.1 X-Archive-Number: 200803/29 X-Sequence-Number: 28983 Matthew wrote: > On Mon, 3 Mar 2008, Mark Mielke wrote: >> Has anybody been able to prove to themselves that RAID 0 vs RAID 1+0 >> is faster for these sorts of loads? My understanding is that RAID 1+0 >> *can* reduce latency for reads, but that it relies on random access, >> whereas RAID 0 performs best for sequential scans? Does PostgreSQL >> ever do enough random access to make RAID 1+0 shine? > Theoretically the performance of RAID 0 and RAID 10 should be > identical for reads, both seeks and throughput, assuming you have a > sensible readahead and a good controller. For writes, RAID 10 needs to > write to multiple drives, so is slower. Whether this is true in > reality is another matter, as all sorts of factors come in, not least > how good your controller is at managing the arrangement. I don't think your statement that they should be identical is true - RAID 1+0 can satisfy and given read from at least two drives. A good controller can satisfy half the reads from one side of the array, and half the reads from the other side of the array, where the first set does not have to wait for the second set, before continuing. To contrast, sequential reads of a RAID 1+0 system is almost always HALF of the speed of sequential reads of a RAID 0 system. The hardware read-ahead on the RAID 1+0 system is being wasted as even if you did leap from one side of the array to the other, each side ends up "skipping" the data served by the other side, making any caching ineffective. The question I have is not whether RAID 1+0 vs RAID 0 show different characteristics. I know they do based upon my own analysis. My question is whether PostgreSQL disk access patterns for certain loads ever benefit from RAID 1+0, or whether RAID 1+0 is always a bad choice for performance-only (completely ignore reliability) loads. Cheers, mark -- Mark Mielke