Date: Tue, 1 Feb 2000 19:07:30 +1030 From: Greg Lehey <grog@lemis.com> To: Wilko Bulte <wilko@yedi.iaf.nl> Cc: "Justin T. Gibbs" <gibbs@narnia.plutotech.com>, scsi@FreeBSD.ORG Subject: Re: hardware vs software stripping Message-ID: <20000201190730.R76348@freebie.lemis.com> In-Reply-To: <20000131191957.A906@yedi.iaf.nl> References: <Pine.BSF.4.10.10001301401360.60037-100000@server.b0x.com> <200001311432.HAA32638@narnia.plutotech.com> <20000131191957.A906@yedi.iaf.nl>
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On Monday, 31 January 2000 at 19:19:57 +0100, Wilko Bulte wrote: > On Mon, Jan 31, 2000 at 07:32:31AM -0700, Justin T. Gibbs wrote: >> In article <20000131104827.A62824@freebie.lemis.com> you wrote: >>> >>> I suppose you mean striping. RAID-5 doesn't stripe at the byte level, >>> it stripes at the block level. RAID-3 stripes at the byte level. >> >> I've heard you say this several times, but it is simply not true. >> RAID-3 is the same as RAID4 without the optimization for partial >> stripe writes. In otherwords, in RAID-3, you must read or write >> a full stripe where RAID-4 adds the ability to perform RMW operations >> on the parity block of the stripe for sub-stripe updates. Pluto >> uses a RAID-3 system in its video server products and it is certainly >> not striped on a byte level. (Just as an aside, given the minimum >> 512 byte sector size of most magnetic media, striping an a per byte >> basis would be really wasteful). > > FWIW the Compaq HSx arrays try hard to distinguish full stripe > writes on RAID5 and switches to RAID3 behaviour. This is as Justin > says all on block level (or rather chunk level where a chunk is a > number of 512byte blocks). Well, see my other message. I'd like to call this RAID-4. But it's an interesting optimization, and I have a request to implement it in Vinum. It should work just as well on RAID-5 as on RAID-4. The key is that the write transfers must go exactly across the stripe: then you can just calculate the parity block and write it without having to do pre-reads of the area. > For RAID3 to work well you would like to have synchronised disk > spindles too (but try to find disks that can do that these days, > they are not too common. And multiple diskvendors in one RAIDset > don't mix well with spindle sync). RAID3, I think.., is mostly for > specialised use (like video, or loading giant datasets on a > numbercruncher) these days That's OK, there are plenty of requirements for that, as Justin has shown. Greg -- Finger grog@lemis.com for PGP public key See complete headers for address and phone numbers To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-scsi" in the body of the message
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