From owner-freebsd-questions@FreeBSD.ORG Thu Jan 25 20:15:50 2007 Return-Path: X-Original-To: freebsd-questions@freebsd.org Delivered-To: freebsd-questions@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52]) by hub.freebsd.org (Postfix) with ESMTP id D901416A402 for ; Thu, 25 Jan 2007 20:15:50 +0000 (UTC) (envelope-from cswiger@mac.com) Received: from mail-out4.apple.com (mail-out4.apple.com [17.254.13.23]) by mx1.freebsd.org (Postfix) with ESMTP id C1C2D13C44B for ; Thu, 25 Jan 2007 20:15:50 +0000 (UTC) (envelope-from cswiger@mac.com) Received: from relay8.apple.com (a17-128-113-38.apple.com [17.128.113.38]) by mail-out4.apple.com (8.13.8/8.13.8) with ESMTP id l0PKFoRc027086; Thu, 25 Jan 2007 12:15:50 -0800 (PST) Received: from relay8.apple.com (unknown [127.0.0.1]) by relay8.apple.com (Symantec Mail Security) with ESMTP id 6964C40078; Thu, 25 Jan 2007 12:15:50 -0800 (PST) X-AuditID: 11807126-a3e85bb000000245-8b-45b90ff63814 Received: from [17.214.13.96] (unknown [17.214.13.96]) (using TLSv1 with cipher AES128-SHA (128/128 bits)) (No client certificate requested) by relay8.apple.com (Apple SCV relay) with ESMTP id 54C7440018; Thu, 25 Jan 2007 12:15:50 -0800 (PST) In-Reply-To: <25E0702D-C3A3-4B6B-BC56-D1BC5C1347F5@cyberlifelabs.com> References: <25E0702D-C3A3-4B6B-BC56-D1BC5C1347F5@cyberlifelabs.com> Mime-Version: 1.0 (Apple Message framework v752.2) Content-Type: text/plain; charset=US-ASCII; delsp=yes; format=flowed Message-Id: <0C1E63BE-0E2B-4ABC-952C-3EDC95CF8D8A@mac.com> Content-Transfer-Encoding: 7bit From: Chuck Swiger Date: Thu, 25 Jan 2007 12:15:50 -0800 To: Milo Hyson X-Mailer: Apple Mail (2.752.2) X-Brightmail-Tracker: AAAAAA== Cc: freebsd-questions@freebsd.org Subject: Re: RAID Performance Questions X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 25 Jan 2007 20:15:50 -0000 On Jan 25, 2007, at 10:50 AM, Milo Hyson wrote: > The write times of both RAID configurations are slower than the > single drive (which is expected due to having to write to multiple > drives). However, I wasn't expecting such a drastic reduction > (about 50%). The read times, although faster, are only marginally > so in per-char transfer. They're a bit better in block performance, > but still not what I would expect. It would seem to me that a read > spread across four drives should see more than a 45% performance > increase. The highest rate recorded here is only a quarter of the > PCI bus-speed, so I doubt that's a bottleneck. CPU load peaks at > 50%, so I don't see that being a problem either. Single-byte accesses are a worst-case scenario for RAID throughput; the block rates are generally more applicable to the performance you'll see for decently-written applications and many use-case scenarios. If you've got a UPS or battery-backup option for the RAID card enabled, consider turning on write-back mode rather than write- thru mode, which ought to improve write performance pretty significantly. Still, you also ought to consider that a 3-disk RAID-5 configuration is very much not ideal from either an efficiency or performance standpoint-- you want more like 5 or 6 drives being used, in which case your performance numbers ought to increase some. This is also somewhat true of the 4-disk RAID-10 config; using 6 or all 8 drives would likely improve performance compared with striping against only two disks. > I also ran some performance tests with a stock build of PostgreSQL > 8.0 to get a different angle on things. [ ... ] > Any performance benefit of RAID in these tests is almost > nonexistent. Am I doing something wrong? Am I expecting too much? > Any advice that can be offered in this area would be much appreciated. Most databases dislike any form of RAID except plain old RAID-1 mirroring, but absolutely hate RAID-5. Databases can do OK with big RAID-10 combinations, too, but ask any experienced DBA what they'd like, and they'd rather have as many RAID-1 spindles available as possible compared with any other drive arrangement. -- -Chuck