[9121] in linux-scsi channel archive
Re: LinuxFailSafe news
daemon@ATHENA.MIT.EDU (Stephen C. Tweedie)
Mon Jul 3 18:27:58 2000
Date: Mon, 3 Jul 2000 21:54:00 +0100
From: "Stephen C. Tweedie" <sct@redhat.com>
To: James Bottomley <James.Bottomley@SteelEye.com>
Cc: "Stephen C. Tweedie" <sct@redhat.com>,
Keith Barrett <kbarrett@redhat.com>,
Brian Stevens <stevens@mclinux.com>,
linux-scsi@vger.rutgers.edu
Message-ID: <20000703215400.S3284@redhat.com>
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In-Reply-To: <200007031844.NAA00888@jb96514.il.steeleye.com>; from James.Bottomley@SteelEye.com on Mon, Jul 03, 2000 at 01:44:44PM -0500
Hi,
On Mon, Jul 03, 2000 at 01:44:44PM -0500, James Bottomley wrote:
> > Right. Cache memory on the drive is not a problem. Cache memory on a
> > controller card may be, and in the absence of SCSI reservations, you
> > need to make sure that the reads are coming from the disk and not the
> > controller at least for the quorum partition.
>
> The only case you should have a problem is with a solution based on PCI RAID
> cards connected to a JBOD (like the AMI cluster RAID card, or the DPT
> equivalent for fibre channel). I would be astonished if these cards are
> allowed to cache data on board when in cluster mode. However, if it speaks a
> reasonable SCSI dialect, turn off the WCE (Write Cache Enable) and turn on the
> RCD (Read Cache Disable) bits of the caching mode page which should make it
> behave.
Right --- that's exactly why I'm bringing this up, because I suspect
that the default behaviour is _not_ cluster-safe in all conditions.
I'd rather have an answer from somebody with more knowledge of just
how modern PCI SCSI controllers behave wrt. read caching.
--Stephen
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