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Re: shared disk (DOS)

From: Peter Maydell
Subject: Re: shared disk (DOS)
Date: Sat, 10 Apr 2021 16:57:26 +0100

On Sat, 10 Apr 2021 at 16:42, Tomas By <tomas@basun.net> wrote:
> On Sat, 10 Apr 2021 17:24:03 +0200, Peter Maydell wrote:
> > If you are trying to have multiple guests running simultaneously
> > which are all using the same disk, then that *is* a distributed
> > filesystem setup (multiple clients, one disk).
> Well...
> "GlusterFS aggregates various storage servers over Ethernet or
> Infiniband RDMA interconnect into one large parallel network file
> system."

Yes, but there's no DOS support for it, is there? (Also,
it's a network filesystem, not a thing that goes on a disk.)

> > What that message
> > is saying is effectively that you have two choices:
> >  (1) [...]
> >  (2) accept that you are running a multi-machine cluster which is
> >      sharing a disk and use a filesystem type and guest setup that
> >      works that way
> Okay, #2 sounds fine...

...except that it's probably a lot of work and might not
even be possible if your guest is DOS rather than Linux.

> > For DOS in particular, I don't expect it is likely to have support
> > for multi-cluster disk use.
> I'm not sure I follow the last bit there, but sharing a disk is what
> SHARE.EXE is for:
> "Locking files became necessary when MS-DOS began allowing files to be
> accessed simultaneously by multiple programs, either through
> multitasking or networking."

No, SHARE.EXE is just "if you have a networked fileserver,
then you need this". It doesn't help you in getting or setting
up that networked fileserver in the first place.

> > I suppose in theory you could set it up the way you'd have set up
> > a shared disk in hardware back in the day: have a file server
> > and get all the DOS guests to use it as a networked disk.
> Well, you did not need a file server, only a network.
> I think it worked the same on one single machine when Windows (or
> OS/2) started to allow multi-tasking.
> So how do I set this up under QEMU? Which format for the shared disk?

This is a guest configuration question. There is no magic
"tell QEMU to use this format for a shared disk image and it
will all work" option. You'd need to run *one* guest connected
to the disk, and have it serve access to that disk over the
network, that your other guests access only as a network drive.

-- PMM

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