[4912] in linux-scsi channel archive
Re: dual-attached disk
daemon@ATHENA.MIT.EDU (Simon Park)
Fri Oct 16 01:02:43 1998
Date: Thu, 15 Oct 1998 20:05:44 +0100 (BST)
From: Simon Park <si@nemesis.demon.co.uk>
To: Andy Poling <andy@globalauctions.com>
cc: Robert Dobozy <robo@idata.sk>, linux-scsi@vger.rutgers.edu
In-Reply-To: <Pine.LNX.3.96.981015000704.19200U-100000@roadrunner.realbig.com>
On Thu, 15 Oct 1998, Andy Poling wrote:
> On Wed, 14 Oct 1998, Robert Dobozy wrote:
> > > this. I don't have a lot of experience with using dual-hosted disks -
> > > the SCSI specification does allow for "reservations" so that one host can
> > > block others from using a device for some period of time, but I have no
> > > idea whether this is usable in practice.
> >
> > Yes it can be. If you want to share disk, but not in parallel. One machine
> > can reserve this disk and use it. Other machine will get reservation
> > conflict when accessing this device. When first machine is finished, it
> > must unreserve device.
>
> That could be a problem when the machine with the reservation is
> down/crashed...
>
In an dual HA setup the other box would come to the eventual conclusion
that it's peer was dead and initiate a fail-over. As part of the fail-over
the SCSI bus is reset which releases the reservations. After the bus reset
the live machine reserves the shared devices and starts to provide
service.
If the presumed-dead machine miraculously comes back to life without a
reboot then it will find the shared devices reserved by its peer and be
unable to fiddle with them.
Cheers
Si
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