Received: from localhost (unknown [200.46.204.187]) by postgresql.org (Postfix) with ESMTP id BF7892E007C for ; Tue, 4 Dec 2007 08:40:41 -0400 (AST) Received: from postgresql.org ([200.46.204.71]) by localhost (mx1.hub.org [200.46.204.187]) (amavisd-maia, port 10024) with ESMTP id 13414-06 for ; Tue, 4 Dec 2007 08:40:24 -0400 (AST) Received: from oxford.xeocode.com (unknown [62.232.55.118]) by postgresql.org (Postfix) with ESMTP id C312C2E0033 for ; Tue, 4 Dec 2007 08:40:34 -0400 (AST) Received: from localhost ([127.0.0.1] helo=oxford.xeocode.com) by oxford.xeocode.com with esmtp (Exim 4.67) (envelope-from ) id 1IzX4i-0001LK-DM; Tue, 04 Dec 2007 12:40:32 +0000 To: "Matthew" Cc: Subject: Re: RAID arrays and performance In-Reply-To: (matthew@flymine.org's message of "Tue\, 4 Dec 2007 12\:23\:02 +0000 \(GMT\)") User-Agent: Gnus/5.11 (Gnus v5.11) Emacs/22.1 (gnu/linux) X-Draft-From: ("nnimap+mail01.enterprisedb.com:INBOX.performance" 3452) References: From: Gregory Stark Organization: EnterpriseDB Date: Tue, 04 Dec 2007 12:40:32 +0000 Message-ID: <87lk8aiqf3.fsf@oxford.xeocode.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Virus-Scanned: Maia Mailguard 1.0.1 X-Archive-Number: 200712/31 X-Sequence-Number: 27847 "Matthew" writes: > Does Postgres issue requests to each random access in turn, waiting for > each one to complete before issuing the next request (in which case the > performance will not exceed that of a single disc), or does it use some > clever asynchronous access method to send a queue of random access > requests to the OS that can be distributed among the available discs? Sorry, it does the former, at least currently. That said, this doesn't really come up nearly as often as you might think. Normally queries fit mostly in either the large batch query domain or the small quick oltp query domain. For the former Postgres tries quite hard to do sequential i/o which the OS will do readahead for and you'll get good performance. For the latter you're normally running many simultaneous such queries and the raid array helps quite a bit. -- Gregory Stark EnterpriseDB http://www.enterprisedb.com Ask me about EnterpriseDB's On-Demand Production Tuning