Date: Wed, 18 Aug 2004 23:44:01 -0700 From: John-Mark Gurney <gurney_j@resnet.uoregon.edu> To: "Greg 'groggy' Lehey" <grog@FreeBSD.org> Cc: Wilko Bulte <wb@freebie.xs4all.nl> Subject: Re: RAID-3? Message-ID: <20040819064401.GN99980@funkthat.com> In-Reply-To: <20040819063843.GP85432@wantadilla.lemis.com> References: <20040817004407.GA81257@wantadilla.lemis.com> <20040817074633.GO30151@darkness.comp.waw.pl> <20040817112900.GA31635@freebie.xs4all.nl> <20040817124020.GK88156@wantadilla.lemis.com> <20040817131612.GT30151@darkness.comp.waw.pl> <20040819024359.GA85432@wantadilla.lemis.com> <41244217.6010102@samsco.org> <20040819062228.GO85432@wantadilla.lemis.com> <20040819062848.GM99980@funkthat.com> <20040819063843.GP85432@wantadilla.lemis.com>
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Greg 'groggy' Lehey wrote this message on Thu, Aug 19, 2004 at 16:08 +0930:
> On Wednesday, 18 August 2004 at 23:28:48 -0700, John-Mark Gurney wrote:
> > Greg 'groggy' Lehey wrote this message on Thu, Aug 19, 2004 at 15:52 +0930:
> >>> Your quoted text also seems a bit subjective as there are very valid
> >>> reasons for RAID-3, especially if one is looking for consistent
> >>> low-latency transactions like in video recorders and servers.
> >>
> >> Well, I did use *exactly* this example. I also pointed out that the
> >> relative performance of modern disk subsystems is adequate for a
> >> single streaming video channel.
> >>
> >> Low latency depends on the number of concurrent accesses. RAID-3
> >> handles concurrent access poorly, exactly because it accesses all
> >> disks for each transfer.
> >
> > One thing that RAID-3 has is that you never have to do a READ/MODIFY
> > cycle when you do writes. Until we implement a write-through cache
> > geom module, raid-5 will continue to substandard performance.
>
> Even then, RAID-5 might have higher bandwidth under some
> circumstances.
Pick your tool, and you can always find a good example and a bad
example of how to use the tool. Doesn't mean it's bad.
> My real question about RAID-3 remains: what use is it? This isn't
> nit-picking, it's certainly not a criticism of pjd. I just don't see
> any practical use on FreeBSD machines.
I originaly was working on a RAID-3 module (which is possibly where pjd
got his idea) that used Luigi's FEC code. The advantage of this code was
the fact that you could have n parity disks beyond the m data disks. The
advantage of this was that you could loose any n disks, and your data is
still recoverable. Unlike with RAID-4/5 implementations where if you
happen to loose a second disk (due to a power surge or something) while
rebuilding, you'd be SOL. That type of redundancy is good thing to have.
--
John-Mark Gurney Voice: +1 415 225 5579
"All that I will do, has been done, All that I have, has not."
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