Date: Mon, 31 Jan 2000 19:19:57 +0100 From: Wilko Bulte <wilko@yedi.iaf.nl> To: "Justin T. Gibbs" <gibbs@narnia.plutotech.com> Cc: Greg Lehey <grog@lemis.com>, scsi@FreeBSD.ORG Subject: Re: hardware vs software stripping Message-ID: <20000131191957.A906@yedi.iaf.nl> In-Reply-To: <200001311432.HAA32638@narnia.plutotech.com>; from gibbs@narnia.plutotech.com on Mon, Jan 31, 2000 at 07:32:31AM -0700 References: <Pine.BSF.4.10.10001301401360.60037-100000@server.b0x.com> <200001311432.HAA32638@narnia.plutotech.com>
next in thread | previous in thread | raw e-mail | index | archive | help
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
Wilko
--
Wilko Bulte Arnhem, The Netherlands
WWW : http://www.tcja.nl
To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe freebsd-scsi" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20000131191957.A906>