From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from lists.gentoo.org (pigeon.gentoo.org [208.92.234.80]) by finch.gentoo.org (Postfix) with ESMTP id 0523C1381F3 for ; Fri, 21 Jun 2013 17:57:29 +0000 (UTC) Received: from pigeon.gentoo.org (localhost [127.0.0.1]) by pigeon.gentoo.org (Postfix) with SMTP id CB547E0ADB; Fri, 21 Jun 2013 17:57:22 +0000 (UTC) Received: from mail-vb0-f51.google.com (mail-vb0-f51.google.com [209.85.212.51]) (using TLSv1 with cipher ECDHE-RSA-RC4-SHA (128/128 bits)) (No client certificate requested) by pigeon.gentoo.org (Postfix) with ESMTPS id 055A0E0A9F for ; Fri, 21 Jun 2013 17:57:21 +0000 (UTC) Received: by mail-vb0-f51.google.com with SMTP id x17so6171973vbf.38 for ; Fri, 21 Jun 2013 10:57:21 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:sender:in-reply-to:references:date :x-google-sender-auth:message-id:subject:from:to:content-type; bh=FW0Nl6in0Z8emoaeZ2HM0UZ34Xeyw+qf471HCwoTOPI=; b=ha+KPatNX485AcTP/cmRPTU4tKDyQHIYtBpaiVpuC7gROuQTfmDyvWi+c0Zv/9/xYN 3YdkPFv8UpVxzT1JmIoMWfxZWaz18w908wZKWFakAMCdhvWZL/IRjgnT184is9bw3Pv/ 6/HSdOAtUwdAjA1/h7uyi7EEDuO+htEl44Y7fQyl/o1OJuF0UreSzTQKtlzzWThQnPhZ S2HrhiiH+e/Bbn4zYIYYSDgvmfYwXUNiokBihwSVxxfkfwezBlpJA0wyP8v3fHB2Q5fE kOFXwdR+caQULHr/9qM/XsveC7+2MgeqAjOadVJ0BO2QE4QoeEFG6LtxAbXooll0xEF6 E7zQ== Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-Id: Gentoo Linux mail X-BeenThere: gentoo-amd64@lists.gentoo.org Reply-to: gentoo-amd64@lists.gentoo.org MIME-Version: 1.0 X-Received: by 10.221.21.74 with SMTP id qr10mr6044339vcb.25.1371837441044; Fri, 21 Jun 2013 10:57:21 -0700 (PDT) Sender: freemanrich@gmail.com Received: by 10.52.180.98 with HTTP; Fri, 21 Jun 2013 10:57:20 -0700 (PDT) In-Reply-To: References: Date: Fri, 21 Jun 2013 13:57:20 -0400 X-Google-Sender-Auth: 8oeU5vo9NG9Q4DQNC8eT82Lh8KM Message-ID: Subject: Re: [gentoo-amd64] Re: Is my RAID performance bad possibly due to starting sector value? From: Rich Freeman To: gentoo-amd64@lists.gentoo.org Content-Type: text/plain; charset=ISO-8859-1 X-Archives-Salt: 8706ef0e-9a19-4391-a7a8-168aecf6ea2f X-Archives-Hash: 3148852bbf63a3f090aaf290b640ed69 On Fri, Jun 21, 2013 at 1:40 PM, Mark Knecht wrote: > One place where I wanted to double check your thinking. My thought > is that a RAID1 will _NEVER_ outperform the hdparm -tT read speeds as > it has to read from three drives and make sure they are all good > before returning data to the user. That isn't correct. In theory it could be done that way, but every raid1 implementation I've heard of makes writes to all drives (obviously), but reads from only a single drive (assuming it is correct). That means that read latency is greatly reduced since they can be split across two drives which effectively means two heads per "platter." Also, raid1 typically does not include checksumming, so if there is a discrepancy between the drives there is no way to know which one is right. With raid5 at least you can always correct discrepancies if you have all the disks (though as Duncan pointed out in practice this only happens if you do an explicit scrub on mdadm). With btrfs every block is checksummed and so as long as there is one good (err, consistent) copy somewhere it will be used. Rich