Date: Thu, 19 Aug 2004 01:03:12 -0600 From: Scott Long <scottl@samsco.org> To: "Greg 'groggy' Lehey" <grog@FreeBSD.org> Cc: Wilko Bulte <wb@freebie.xs4all.nl> Subject: Re: RAID-3? Message-ID: <412450B0.80701@samsco.org> In-Reply-To: <20040819064926.GQ85432@wantadilla.lemis.com> References: <20040819062228.GO85432@wantadilla.lemis.com> <11555.1092897238@critter.freebsd.dk> <20040819064926.GQ85432@wantadilla.lemis.com>
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Greg 'groggy' Lehey wrote: > On Thursday, 19 August 2004 at 8:33:58 +0200, Poul-Henning Kamp wrote: > >>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. > > > Agreed. I don't think it is. > > >>The main features of RAID3 are the always full stripe access which >>keeps your disk heads running in tandem which has desirable >>performance characteristica. > > > ... for single accessors. > > But a single IDE drive nowadays can transfer 40 MB a second. A 5 disk > RAID-3 array should thus be able to transfer 160 MB a second. What do > you need that for? > Video streaming and recoding would find this quite useful I would think. But regardless, it's not about thoroughput, it's about having predictable latency. I can't stress this enough! > >>Also the fact that you can trivially add ECC instead of mere parity >>is a big plus. > > > Ah, but that would be RAID-2. Or something similar. > > >>Raid5 with two bit ECC (sometimes called raid6) > > > I thought RAID-6 was RAID-5 with two identical parity disks. Not so? > > >>is a royal nightmare to code (see the raidframe paper) > > > Does this define RAID-6, or just describe the pain? > There is no formal definition of RAID-6. There are various competing companies that have tried to position their products as the de-facto RAID-6, but that isn't terribly useful here. > >>whereas RAID3 in 4+2 or 8+3 is pretty trivial because of the >>full-stripe access pattern. > > > Sure, easy coding is good. And having written a RAID-5 > implementation, I can believe what a nightmare that an ECC version > might provide. > Ah, but that is the simplicity of RAID-3. Your ECC/FEC/Parity calculation is relatively easy and deterministic to code since you are always writing to all disks at the same time. I'll concede that a general-purpose PC has challenges in meeting the strict interpretation of RAID-3, but what Pawel has meets enough of the common definition that I think that it's Close Enough and the vast majority of users will get what they expect from it. Scott
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