X-Original-To: pgsql-performance-postgresql.org@localhost.postgresql.org Received: from localhost (av.hub.org [200.46.204.144]) by postgresql.org (Postfix) with ESMTP id 6FF019DC950 for ; Tue, 31 Jan 2006 15:21:47 -0400 (AST) Received: from postgresql.org ([200.46.204.71]) by localhost (av.hub.org [200.46.204.144]) (amavisd-new, port 10024) with ESMTP id 31619-07 for ; Tue, 31 Jan 2006 15:21:47 -0400 (AST) X-Greylist: from auto-whitelisted by SQLgrey- Received: from noel.decibel.org (noel.decibel.org [67.100.216.10]) by postgresql.org (Postfix) with ESMTP id 0793E9DC827 for ; Tue, 31 Jan 2006 15:21:44 -0400 (AST) Received: by noel.decibel.org (Postfix, from userid 1001) id E96013983E; Tue, 31 Jan 2006 13:21:45 -0600 (CST) Date: Tue, 31 Jan 2006 13:21:45 -0600 From: "Jim C. Nasby" To: Luke Lonergan Cc: Mike Biamonte , pgsql-performance@postgresql.org Subject: Re: Huge Data sets, simple queries Message-ID: <20060131192145.GU95850@pervasive.com> References: <20060130202524.GB3920@pervasive.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Operating-System: FreeBSD 6.0-RELEASE amd64 X-Distributed: Join the Effort! http://www.distributed.net User-Agent: Mutt/1.5.11 X-Virus-Scanned: by amavisd-new at hub.org X-Spam-Status: No, score=0.1 required=5 tests=[AWL=0.100] X-Spam-Score: 0.1 X-Spam-Level: X-Archive-Number: 200601/491 X-Sequence-Number: 16969 On Tue, Jan 31, 2006 at 09:00:30AM -0800, Luke Lonergan wrote: > Jim, > > On 1/30/06 12:25 PM, "Jim C. Nasby" wrote: > > > Why divide by 2? A good raid controller should be able to send read > > requests to both drives out of the mirrored set to fully utilize the > > bandwidth. Of course, that probably won't come into play unless the OS > > decides that it's going to read-ahead fairly large chunks of the table > > at a time... > > I've not seen one that does, nor would it work in the general case IMO. In > RAID1 writes are duplicated and reads come from one of the copies. You > could alternate read service requests to minimize rotational latency, but > you can't improve bandwidth. (BTW, I did some testing that seems to confirm this) Why couldn't you double the bandwidth? If you're doing a largish read you should be able to do something like have drive a read the first track, drive b the second, etc. Of course that means that the controller or OS would have to be able to stitch things back together. As for software raid, I'm wondering how well that works if you can't use a BBU to allow write caching/re-ordering... -- Jim C. Nasby, Sr. Engineering Consultant jnasby@pervasive.com Pervasive Software http://pervasive.com work: 512-231-6117 vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461