From owner-freebsd-scsi Tue Feb 1 6:23:49 2000 Delivered-To: freebsd-scsi@freebsd.org Received: from caspian.plutotech.com (caspian.plutotech.com [206.168.67.80]) by builder.freebsd.org (Postfix) with ESMTP id D195D3DD8 for ; Tue, 1 Feb 2000 06:23:42 -0800 (PST) Received: from caspian.plutotech.com (localhost [127.0.0.1]) by caspian.plutotech.com (8.9.3/8.9.1) with ESMTP id HAA15471; Tue, 1 Feb 2000 07:23:37 -0700 (MST) (envelope-from gibbs@caspian.plutotech.com) Message-Id: <200002011423.HAA15471@caspian.plutotech.com> X-Mailer: exmh version 2.1.0 09/18/1999 To: Greg Lehey Cc: "Justin T. Gibbs" , Gary Palmer , scsi@FreeBSD.org, up@3.am, Wilko Bulte Subject: Re: hardware vs software stripping In-reply-to: Your message of "Tue, 01 Feb 2000 19:04:41 +1030." <20000201190440.Q76348@freebie.lemis.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Date: Tue, 01 Feb 2000 07:23:37 -0700 From: "Justin T. Gibbs" Sender: owner-freebsd-scsi@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.org >> 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. In RAID4, it is supposed to be a multiple of your transaction size so you can perform partial read (assuming you don't need parity verification) and RMW operations to update the parity for updating contiguous transactions that are not as large as the stripe. >> 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. >> (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). > >Agreed, unless you use a PLA to split the data. > >Obviously, the manufacturer of your RAID-3 box uses the term >differently from the way it's defined above. Pluto makes its own RAID hardware. 8-) -- Justin To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-scsi" in the body of the message