[9372] in linux-scsi channel archive
Re: shared SCSI buses
daemon@ATHENA.MIT.EDU (Kurt Garloff)
Fri Aug 11 20:18:13 2000
Date: Fri, 11 Aug 2000 18:29:50 +0200
From: Kurt Garloff <garloff@suse.de>
To: David Teigland <teigland@sistina.com>,
Mark Veteikis <mark@iphase.com>,
Chris Meadors <clubneon@hereintown.net>,
Martin Peschke <peschke@fh-brandenburg.de>
Cc: linux-scsi@vger.rutgers.edu, gfs-devel@sistina.com
Message-ID: <20000811182950.O16909@D250.suse.de>
Mail-Followup-To: Kurt Garloff <garloff@suse.de>,
David Teigland <teigland@sistina.com>,
Mark Veteikis <mark@iphase.com>,
Chris Meadors <clubneon@hereintown.net>,
Martin Peschke <peschke@fh-brandenburg.de>,
linux-scsi@vger.rutgers.edu, gfs-devel@sistina.com
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On Fri, Aug 11, 2000 at 11:16:04AM -0500, David Teigland wrote:
> On Fri, Aug 11, 2000 at 09:35:19AM -0500, Mark Veteikis wrote:
> - Hot-swapping SCA disks on the bus should be relatively reliable if it's=
done
> with care. It sounds like if any transfer is happening during a swap y=
ou're
> in serious danger of crashing everthing. The scsi drivers can be promp=
ted to
> add or remove devices. I wonder if multiple hosts put a wrench in thin=
gs
> here.
If the electrical problems are solved by using SCA, it should not be
dangerous to have a tranfer going on on the bus to another device at the
same time. I have no experience with SCA myself.
> - The other important issue is hosts which crash at any time, including d=
uring
> a transfer. It sounds like the drivers on other machines will currently
> start a reset-war, but the drivers could be improved to avoid this and
> hopefully keep using the devices as they were.
I can only comment on the Linux SCSI drivers that I have hacked on. They all
are old EH. As faar as I can see, they are not proe to reset wars. When they
see a bus reset (they monitor for this), they reset all their negotiated
settings (Sync, Wide, ...) and return all SCSI commands back to the
mid-layer wit DID_RESET. The mid-layer will just requeue them (in the order
it got it, so the low-level authors need to make sure to pass them back in
the right order).
As I have multiple host adapters on the same bus (in one machine though), I
have this situation and never got reset wars.
A crash during a sync transfer is indeed bad, as the bus will be kept busy
and you will need at least one reset to clear it. If the host adapter on a
crashed machine keeps the bus locked and does ignore the SCSI bus reset,
you're lost of course, but I don't think that it's that likely.
> - A similar problem for devices which crash abruptly.
>=20
> - How about adding machines to the bus and then booting them up?=20
If the SCSI adapter in the new machine does strange things to the bus on
power up, you may have problems; but apart from that, there should not be
any problems.
That's theory. Some buggy firmwares out there may not handle many initiators
correctly. THe hard disks that I tested so far, all behaved correctly, thou=
gh.
> By the look of things here, it is not reasonable to use GFS with multiple=
hosts
> on a shared SCSI bus if you're interested in HA. If any machine or disk
> crashes, all your devices are probably in trouble. Stopping all machines=
' I/O
> (and maybe unmounting everyone) to add or remove storage would also be=20
> prohibitive.=20
I would not be so negative.
Regards,
--=20
Kurt Garloff <garloff@suse.de> Eindhoven, NL
GPG key: See mail header, key servers Linux kernel development
SuSE GmbH, Nuernberg, FRG SCSI, Security
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