Re: sharing a (mostly) read-only virtual block device

[Avi Kivity <avi@redhat.com>]

On 10/16/2009 07:45 PM, Antoine Martin wrote:
> Hi,
>
> Is there an easy way that I have missed to share a virtual disk
> read-only between many guests whilst still having the ability to update
> it occasionally from the host?
>
>    

That's very fragile, since the guest won't expect the disk to change 
under its feet.  Expect oopses.

> I was hoping I could use a shared image file, and occasionally replace
> it with an updated version (move old copy to disk_image.bak, copy new
> image to disk_image), then the guests could umount/mount the drive and
> get access to the new disk image.
>
> Unfortunately qemu opens the virtual disk as soon as the guest boots, so
> the file descriptor still points to the old image.
> Note: I do not want to use the qemu monitor from the host as I want the
> guests to be in charge of when/if they get the new disk image.
>
>    

I suggest using a monitor, and have the host and guest coordinate the 
change (guest unmounts, host modifies, guest mounts).

Alternatively, export the disk from the host using nfs.

-- 
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