From owner-cvs-src@FreeBSD.ORG  Thu Aug 19 07:10:01 2004
Return-Path: <owner-cvs-src@FreeBSD.ORG>
Delivered-To: cvs-src@freebsd.org
Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125])
	by hub.freebsd.org (Postfix) with ESMTP
	id 4FD3416A4D0; Thu, 19 Aug 2004 07:10:01 +0000 (GMT)
Received: from critter.freebsd.dk (critter.freebsd.dk [212.242.86.163])
	by mx1.FreeBSD.org (Postfix) with ESMTP
	id 9FFB543D3F; Thu, 19 Aug 2004 07:10:00 +0000 (GMT)
	(envelope-from phk@critter.freebsd.dk)
Received: from critter.freebsd.dk (localhost [127.0.0.1])
	by critter.freebsd.dk (8.13.1/8.13.1) with ESMTP id i7J79o0A012290;
	Thu, 19 Aug 2004 09:09:50 +0200 (CEST)
	(envelope-from phk@critter.freebsd.dk)
To: Scott Long <scottl@samsco.org>
From: "Poul-Henning Kamp" <phk@phk.freebsd.dk>
In-Reply-To: Your message of "Thu, 19 Aug 2004 00:56:23 MDT."
             <41244F17.9030007@samsco.org> 
Date: Thu, 19 Aug 2004 09:09:50 +0200
Message-ID: <12289.1092899390@critter.freebsd.dk>
Sender: phk@critter.freebsd.dk
cc: John-Mark Gurney <gurney_j@resnet.uoregon.edu>
cc: src-committers@FreeBSD.org
cc: Pawel Jakub Dawidek <pjd@FreeBSD.org>
cc: cvs-src@FreeBSD.org
cc: cvs-all@FreeBSD.org
cc: Greg 'groggy' Lehey <grog@FreeBSD.org>
cc: Wilko Bulte <wb@freebie.xs4all.nl>
Subject: Re: RAID-3? 
X-BeenThere: cvs-src@freebsd.org
X-Mailman-Version: 2.1.1
Precedence: list
List-Id: CVS commit messages for the src tree <cvs-src.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/cvs-src>,
	<mailto:cvs-src-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/cvs-src>
List-Post: <mailto:cvs-src@freebsd.org>
List-Help: <mailto:cvs-src-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/cvs-src>,
	<mailto:cvs-src-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Thu, 19 Aug 2004 07:10:01 -0000

In message <41244F17.9030007@samsco.org>, Scott Long writes:
>Greg 'groggy' Lehey wrote:

>> I can see that as a great advantage, but it's not part of the RAID-3
>> definition, and I can't see why you couldn't expand RAID-5 in a
>> similar manner.  Am I missing something?
>> 
>> Greg

Yes you are missing the complexity of the code to implement it.
As far as I know, RaidFrame is the only working implementation of
RAID5 with two redundant disks.

>Yes, you are!  The advantage of RAID-3 is that there are NO
>Read-Modify-Write cycles when writing blocks.  Period.  Zippo.  None.
>Every write takes exactly the same amount of time.  There is no waiting
>for data to be read off of any disks.  That is why it's nice to
>applications that require fixed latency.  RAID-3 has no concept of
>stripe sizes becuase of this, unlike 4 and 5.
>
>Scott

Well, in RAID3 the stripe size becomes your sectorsize and if you are
using a filesystem that demands a particular sectorsize you may be
prevented from using RAID3 because of that.  UFS/FFS does not have
this problem.

The other thing is that RAID5 can be made in any configutation from
1+1 to N+1, whereas RAID3 is generally limited to 2^N+X

Poul-Henning

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.