Date: Wed, 2 Feb 2000 08:22:14 +1030 From: Greg Lehey <grog@lemis.com> To: "Justin T. Gibbs" <gibbs@FreeBSD.org> Cc: "Justin T. Gibbs" <gibbs@plutotech.com>, Gary Palmer <gjp@in-addr.com>, scsi@FreeBSD.org, up@3.am, Wilko Bulte <wilko@yedi.iaf.nl> Subject: Re: hardware vs software stripping Message-ID: <20000202082214.S76348@freebie.lemis.com> In-Reply-To: <200002011423.HAA15471@caspian.plutotech.com> References: <20000201190440.Q76348@freebie.lemis.com> <200002011423.HAA15471@caspian.plutotech.com>
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On Tuesday, 1 February 2000 at 7:23:37 -0700, Justin T. Gibbs wrote: >>> 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. >> >> I'm not sure I follow you here. Are you saying that the data layout >> is the same and the difference is in the implementation of the >> software? That doesn't seem to justify a separate level. > > In RAID3, there is no restriction on the per-drive blocking factor. That doesn't correspond to any of the definitions I have seen. Where did you get it from? > In RAID4, it is supposed to be a multiple of your transaction size Where do you get the term "transaction" from? I haven't seen it in any RAID documentation. In ufs, there is no fixed size. > so you can perform partial read (assuming you don't need parity > verification) When would you need parity verification for reads? > and RMW operations to update the parity for updating contiguous > transactions that are not as large as the stripe. I'd call both of these RAID-4, considering that RAID doesn't use the term "transaction". >>> Pluto uses a RAID-3 system in its video server products and it is >>> certainly not striped on a byte level. >> >> So how exactly is it striped? > > Our stripe size is 1-2MB with the per-drive stripe component size > varying depend on the number of drives in the system. So what's the difference from RAID-4? I can accept the fact that this is the way you use the term "RAID-3", but it conflicts with all documentation I have seen, and you haven't presented any other evidence. 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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