From owner-freebsd-current@FreeBSD.ORG  Mon Nov  1 15:58:31 2004
Return-Path: <owner-freebsd-current@FreeBSD.ORG>
Delivered-To: freebsd-current@freebsd.org
Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125])
	by hub.freebsd.org (Postfix) with ESMTP id 62F2816A4CE
	for <current@freebsd.org>; Mon,  1 Nov 2004 15:58:31 +0000 (GMT)
Received: from saturn.criticalmagic.com (saturn.criticalmagic.com
	[64.74.124.105])
	by mx1.FreeBSD.org (Postfix) with ESMTP id 0968343D3F
	for <current@freebsd.org>; Mon,  1 Nov 2004 15:58:31 +0000 (GMT)
	(envelope-from rcoleman@criticalmagic.com)
Received: from [10.40.30.144] (borg.ciphertrust.com [64.238.118.66])
	by saturn.criticalmagic.com (Postfix) with ESMTP id 0D3D13BD10;
	Mon,  1 Nov 2004 10:58:29 -0500 (EST)
Message-ID: <41865D1A.7080002@criticalmagic.com>
Date: Mon, 01 Nov 2004 10:58:18 -0500
From: Richard Coleman <rcoleman@criticalmagic.com>
Organization: Critical Magic, Inc.
User-Agent: Mozilla Thunderbird 0.7.3 (X11/20041008)
X-Accept-Language: en-us, en
MIME-Version: 1.0
To: Brad Knowles <brad@stop.mail-abuse.org>
References: <002401c4bf9c$c4fee8e0$0201000a@riker>
	<p06002002bdab24905ad8@[10.0.1.3]>
	<1099286568.4185c82881654@picard.newmillennium.net.au>
	<p06002006bdabc1160a6a@[10.0.1.3]>
In-Reply-To: <p06002006bdabc1160a6a@[10.0.1.3]>
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Transfer-Encoding: 7bit
cc: Alastair D'Silva <freebsd@newmillennium.net.au>
cc: current@freebsd.org
Subject: Re: Gvinum RAID5 performance
X-BeenThere: freebsd-current@freebsd.org
X-Mailman-Version: 2.1.1
Precedence: list
List-Id: Discussions about the use of FreeBSD-current
	<freebsd-current.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-current>,
	<mailto:freebsd-current-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-current>
List-Post: <mailto:freebsd-current@freebsd.org>
List-Help: <mailto:freebsd-current-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-current>,
	<mailto:freebsd-current-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Mon, 01 Nov 2004 15:58:31 -0000

Brad Knowles wrote:
> Keep in mind that if you've got a five disk RAID-5 array, then for 
> any given block, four of those disks are data and would have to be 
> accessed on every read operation anyway, and only one disk would be 
> parity.  The more disks you have in your RAID array, the lower the 
> parity to data ratio, and the less benefit you would get from
> checking parity in background.

My understanding is that this is true only when the system is running in 
a degraded mode.  When you blow a disk, instead of hitting that disk, 
you access all the other disks and calculate the desired block using the 
parity.  If this system is healthy, a single disk access should be 
sufficient.

If a system with a 9 disk raid5 volume (pretty common) had to hit each 
disk on a block access, it would majorly suck.

I think detecting and handling bad physical blocks is a whole other matter.

Richard Coleman
rcoleman@criticalmagic.com