From owner-freebsd-current@FreeBSD.ORG Mon Mar 31 22:01:35 2008 Return-Path: Delivered-To: freebsd-current@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 38776106564A; Mon, 31 Mar 2008 22:01:35 +0000 (UTC) (envelope-from scottl@samsco.org) Received: from pooker.samsco.org (pooker.samsco.org [168.103.85.57]) by mx1.freebsd.org (Postfix) with ESMTP id E28738FC18; Mon, 31 Mar 2008 22:01:34 +0000 (UTC) (envelope-from scottl@samsco.org) Received: from phobos.samsco.home (phobos.samsco.home [192.168.254.11]) (authenticated bits=0) by pooker.samsco.org (8.13.8/8.13.8) with ESMTP id m2VM1We1065366; Mon, 31 Mar 2008 16:01:32 -0600 (MDT) (envelope-from scottl@samsco.org) Message-ID: <47F15F3C.9060100@samsco.org> Date: Mon, 31 Mar 2008 16:01:32 -0600 From: Scott Long User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X; en-US; rv:1.8.1.11) Gecko/20071128 SeaMonkey/1.1.7 MIME-Version: 1.0 To: Ivan Voras References: <47F147D8.3030905@samsco.org> <9bbcef730803311409ha25effam9dd522c9084783ad@mail.gmail.com> <47F15772.5010104@samsco.org> <9bbcef730803311434s48d3269cs1e8ae0fd1eb7ffc3@mail.gmail.com> In-Reply-To: <9bbcef730803311434s48d3269cs1e8ae0fd1eb7ffc3@mail.gmail.com> X-Enigmail-Version: 0.95.6 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-1.4 required=5.4 tests=ALL_TRUSTED autolearn=failed version=3.1.8 X-Spam-Checker-Version: SpamAssassin 3.1.8 (2007-02-13) on pooker.samsco.org Cc: freebsd-current@freebsd.org Subject: Re: Are large RAID stripe sizes useful with FreeBSD? X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Discussions about the use of FreeBSD-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 31 Mar 2008 22:01:35 -0000 Ivan Voras wrote: > On 31/03/2008, Scott Long wrote: > >> For writes, the performance penalty of smaller I/O's (assuming no RAID-5 >> effects) is minimal; most caching controllers and drives will batch the >> concurrent requests together, so the only loss is in the slight overhead >> of the extra transaction setup and completion. For reads, the penalty >> can be greater because the controller/disk will try to execute the first >> request immediately and not wait for the second part to be requested, >> leading to the potential for extra rotational and head movement delays. >> Many caching RAID controllers offer a read-ahead feature to counteract >> this. However, while my testing has shown little measurable benefit to >> this, YMMV. > > Thank you, this is the kind of explanation I hoping for. One more > thing: is TCQ (e.g. the SCSI variant) orthogonal to this? If you have a RAID controller in front of the disks then the effects of TCQ are hidden from the OS; it might ultimately make the controller complete requests faster, but the controller already looks to the OS like a disk with a really deep queue. When you're dealing directly with the disks then TCQ/NCQ is required in order for batching of concurrent requests to occur. Scott