Received: from localhost (unknown [200.46.204.191]) by postgresql.org (Postfix) with ESMTP id 570DF2E0114 for ; Wed, 26 Dec 2007 18:40:15 -0400 (AST) Received: from postgresql.org ([200.46.204.71]) by localhost (mx1.hub.org [200.46.204.191]) (amavisd-maia, port 10024) with ESMTP id 00446-05 for ; Wed, 26 Dec 2007 18:40:08 -0400 (AST) Received: from westnet.com (westnet.com [216.187.52.2]) by postgresql.org (Postfix) with ESMTP id 5DF602E2D97 for ; Wed, 26 Dec 2007 18:40:08 -0400 (AST) Received: from westnet.com (localhost [127.0.0.1]) by westnet.com (8.14.0/8.14.0) with ESMTP id lBQMe7E0012281 for ; Wed, 26 Dec 2007 17:40:07 -0500 (EST) Received: from localhost (gsmith@localhost) by westnet.com (8.14.0/8.13.2/Submit) with ESMTP id lBQMe7ui012278 for ; Wed, 26 Dec 2007 17:40:07 -0500 (EST) X-Authentication-Warning: westnet.com: gsmith owned process doing -bs Date: Wed, 26 Dec 2007 17:40:06 -0500 (EST) From: Greg Smith X-X-Sender: gsmith@westnet.com To: "'pgsql-performance'" Subject: Re: With 4 disks should I go for RAID 5 or RAID 10 In-Reply-To: Message-ID: References: <0f8201c847f6$08e15090$8f01010a@iptel.com.ar> <87zlvxnpkl.fsf@mid.deneb.enyo.de> <4772CA58.5050406@mark.mielke.cc> <4772CE34.2040608@mark.mielke.cc> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed X-Virus-Scanned: Maia Mailguard 1.0.1 X-Archive-Number: 200712/393 X-Sequence-Number: 28209 On Wed, 26 Dec 2007, david@lang.hm wrote: > yes, the two linux software implementations only read from one disk, but I > have seen hardware implementations where it reads from both drives, and if > they disagree it returns a read error rather then possibly invalid data (it's > up to the admin to figure out which drive is bad at that point). Right, many of the old implementations did that; even the Wikipedia article on this subject mentions it in the "RAID 1 performance" section: http://en.wikipedia.org/wiki/Standard_RAID_levels The thing that changed is on modern drives, the internal error detection and correction is good enough that if you lose a sector, the drive will normally figure that out at the firmware level and return a read error rather than bad data. That lowers of the odds of one drive becoming corrupted and returning a bad sector as a result enough that the overhead of reading from both drives isn't considered as important. I'm not aware of a current card that does that but I wouldn't be surprised to discover one existed. -- * Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD