[4891] in linux-scsi channel archive
Re: dual-attached disk
daemon@ATHENA.MIT.EDU (Robert Dobozy)
Thu Oct 15 00:04:26 1998
Date: Wed, 14 Oct 1998 16:51:44 +0200 (MET DST)
From: Robert Dobozy <robo@idata.sk>
To: linux-scsi@vger.rutgers.edu
In-Reply-To: <Pine.LNX.3.96.981014004001.10449B-100000@andante.jic.com>
On Wed, 14 Oct 1998, Eric Youngdale wrote:
Hello everybody,
> > thoth@purplefrog.com wrote:
> > I've tried this with the 2.0.30 kernel (RedHat 4.2) and the AHA-2940U
> > host
> > adapters and it doesn't work very well.
>
> Doesn't surprise me at all that it doesn't work well.
Yes, this is IMHO best way to destroy your filesystem. You can't attach
disk (filesystem) without somebody who will control that on software
layer (some kind of lock daemon). Imagine, that kernel can reorder I/O
operation. And as I know this can be done by disk (or scsi adapter -
currently I'm not 100% sure), so You don't when file (or meta data) is in
consistent state and you can give access to other machine. And when two
machines wants to access the same file (and wants to write into it) ....
> 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. Second machine than can use it. With this
configuration scsi reset from time to time appears (especially when
switching on and rebooting). Currently I started to work on something like
this. This should be part of other project. I'm not sure if this can be
made with current linux kernel (or 2.1 kernels) but I'm investigating it.
That's the reason why I'm in this list :-)).
Regards
Robert
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