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 you're
> in serious danger of crashing everthing. The scsi drivers can be prompted to
> add or remove devices. I wonder if multiple hosts put a wrench in things
> 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 during
> 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.
>
> - How about adding machines to the bus and then booting them up?
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, though.
> 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
> prohibitive.
I would not be so negative.
Regards,
--
Kurt Garloff <[EMAIL PROTECTED]> Eindhoven, NL
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SuSE GmbH, Nuernberg, FRG SCSI, Security
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