From owner-freebsd-current@FreeBSD.ORG Mon Mar 31 21:34:09 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 0A762106566C for ; Mon, 31 Mar 2008 21:34:08 +0000 (UTC) (envelope-from ivoras@gmail.com) Received: from wr-out-0506.google.com (wr-out-0506.google.com [64.233.184.225]) by mx1.freebsd.org (Postfix) with ESMTP id 85D618FC1C for ; Mon, 31 Mar 2008 21:34:08 +0000 (UTC) (envelope-from ivoras@gmail.com) Received: by wr-out-0506.google.com with SMTP id 50so914837wra.13 for ; Mon, 31 Mar 2008 14:34:07 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=beta; h=domainkey-signature:received:received:message-id:date:from:sender:to:subject:cc:in-reply-to:mime-version:content-type:content-transfer-encoding:content-disposition:references:x-google-sender-auth; bh=khtxrb8OE1OXWZFzGrl4TJpyCFawlXRp8A2B6kC2DDo=; b=UtXOSpL7tiGXlmD5n6kvMpLckOPHEqDAFbUwy3VJr/3S8ElkyKDhkS46FPbteG/uhpqeEtVAnw3q4mlNZWLzCjLCc4zlQgAW08gUTKMp8HtEDbZk1a+33nfgsYA8ooCy5/fdLbqkoAvjbn19dmq7axKUy96zxeg37w/ePgCkRL0= DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=beta; h=message-id:date:from:sender:to:subject:cc:in-reply-to:mime-version:content-type:content-transfer-encoding:content-disposition:references:x-google-sender-auth; b=JuHp4VvRaPFwZ50tyxD89/C0QOzsxXtkSzD4OJeeRpr1NlI3htC1v5Uq1uHaWAoXpvLYI2eWTJujYGy93NoPCtySF8nNS7yXg2lWTkWXoMuG25nL+9wVwjvaQiUsIftC4PH1Yg2SxVzSMrUv/oM8EQQcmS2qeR9pxr+SlDJoYXc= Received: by 10.140.134.15 with SMTP id h15mr3816199rvd.48.1206999244174; Mon, 31 Mar 2008 14:34:04 -0700 (PDT) Received: by 10.141.212.1 with HTTP; Mon, 31 Mar 2008 14:34:04 -0700 (PDT) Message-ID: <9bbcef730803311434s48d3269cs1e8ae0fd1eb7ffc3@mail.gmail.com> Date: Mon, 31 Mar 2008 23:34:04 +0200 From: "Ivan Voras" Sender: ivoras@gmail.com To: "Scott Long" In-Reply-To: <47F15772.5010104@samsco.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Content-Disposition: inline References: <47F147D8.3030905@samsco.org> <9bbcef730803311409ha25effam9dd522c9084783ad@mail.gmail.com> <47F15772.5010104@samsco.org> X-Google-Sender-Auth: 140fb1432db9f5da 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 21:34:09 -0000 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?