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To: "Greg 'groggy' Lehey" <grog@FreeBSD.org>
From: "Poul-Henning Kamp" <phk@phk.freebsd.dk>
In-Reply-To: Your message of "Thu, 19 Aug 2004 15:52:28 +0930."
             <20040819062228.GO85432@wantadilla.lemis.com> 
Date: Thu, 19 Aug 2004 08:33:58 +0200
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cc: Scott Long <scottl@samsco.org>
cc: src-committers@FreeBSD.org
cc: Pawel Jakub Dawidek <pjd@FreeBSD.org>
cc: cvs-src@FreeBSD.org
cc: cvs-all@FreeBSD.org
cc: Wilko Bulte <wb@freebie.xs4all.nl>
Subject: Re: RAID-3? 
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In message <20040819062228.GO85432@wantadilla.lemis.com>, "Greg 'groggy' Lehey"
 writes:
>On Thursday, 19 August 2004 at  0:00:55 -0600, Scott Long wrote:
>>
>> I think that you're really reading far too much into this.
>
>That depends on whether you care about accurate terminology or not.
>Or maybe it's you who is reading too much into the matter.

I think being accurate is a great thing, but accuracy of definition
should never get in the way of working code.

The main features of RAID3 are the always full stripe access which keeps
your disk heads running in tandem which has desirable performance
characteristica.

Also the fact that you can trivially add ECC instead of mere parity
is a big plus.

Raid5 with two bit ECC (sometimes called raid6) is a royal nightmare
to code (see the raidframe paper) whereas RAID3 in 4+2 or 8+3 is
pretty trivial because of the full-stripe access pattern.

Now, can we stop the definition-thumbing and let Pawel work on his
code ?   If need be, put this on your bumper-sticker:

	If you don't like RAID3 then don't use it!

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.