Re: shared disk (DOS)

[Peter Maydell]

On Sat, 10 Apr 2021 at 15:24, Tomas By <tomas@basun.net> wrote:
> I'd like to have several QEMU DOS images, called from Linux, that
> share a disk. Here
> https://lists.nongnu.org/archive/html/qemu-discuss/2012-07/msg00035.html
> it says that "if you plan on mounting the same shared block device in
> multiple guests, you are going to need to use a cluster aware file
> system such as ocfs2 or gfs", which sounds relevant, but I do not need
> a distributed file system, and I cannot find info about how to use
> OCFS2 with QEMU.

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). What that message
is saying is effectively that you have two choices:
 (1) don't do that, ie only have one guest accessing the disk at once
 (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

For DOS in particular, I don't expect it is likely to have support
for multi-cluster disk use.

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. That
sounds like it would be a pain to set up though.

There is one exception here -- if the guests do not need to be
able to permanently write to the disk, you could make the disk
image itself read-only, and use snapshot mode:
https://qemu-project.gitlab.io/qemu/system/images.html#snapshot-mode
Then the guest thinks it sees a writable disk, but all changes it
makes are kept in a separate temporary file and thrown away when
the guest exits.

Or you could say that "disk space is cheap", especially for small
images of the kind a DOS guest is likely to have, and duplicate
the disk image files :-)

thanks
-- PMM