From owner-freebsd-scsi Mon Jan 31 13:28:56 2000 Delivered-To: freebsd-scsi@freebsd.org Received: from richard2.pil.net (richard2.pil.net [207.8.164.9]) by hub.freebsd.org (Postfix) with SMTP id 501CE14F21 for ; Mon, 31 Jan 2000 13:28:52 -0800 (PST) (envelope-from up@3.am) Received: (qmail 17022 invoked by uid 1825); 31 Jan 2000 21:28:41 -0000 Received: from localhost (sendmail-bs@127.0.0.1) by localhost with SMTP; 31 Jan 2000 21:28:41 -0000 Date: Mon, 31 Jan 2000 16:28:41 -0500 (EST) From: X-Sender: up@richard2.pil.net To: Wilko Bulte Cc: scsi@FreeBSD.ORG Subject: Re: hardware vs software stripping In-Reply-To: <20000131191957.A906@yedi.iaf.nl> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-freebsd-scsi@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.org On Mon, 31 Jan 2000, 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). > > 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 IIRC, the main difference between 3 and 5 is that 3 puts all of the parity blocks on one spindle, whereas 5 distributes them across all of the spindles. James Smallacombe PlantageNet, Inc. CEO and Janitor up@3.am http://3.am ========================================================================= To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-scsi" in the body of the message