To start multiple KVM guests from one qcow2 image with transient disk option

[Masayoshi Mizuma]

On Thu, Jan 07, 2021 at 09:05:42AM +0100, Peter Krempa wrote:
> On Tue, Jan 05, 2021 at 15:12:55 +0100, Peter Krempa wrote:
> > On Mon, Jan 04, 2021 at 15:30:19 -0500, Masayoshi Mizuma wrote:
> > > On Sat, Dec 19, 2020 at 11:30:39PM -0500, Masayoshi Mizuma wrote:
>  
>  [...]
>  
>  >   {"execute":"cont"}
> > 
> > So that is a no-go. Some disk bus-es such as IDE don't support hotplug:
> > 
> > https://gitlab.com/libvirt/libvirt/-/blob/master/src/qemu/qemu_hotplug.c#L1074
> > 
> > You could try to just instantiate the backend of the disk as read-only,
> > and then create a writable overlay. You just need to make sure that the
> > disk will be writable and that it works even for IDE/SATA which doesn't
> > support read-only:
> > 
> > https://gitlab.com/libvirt/libvirt/-/blob/master/src/qemu/qemu_validate.c#L2634
> 
> Okay, I've actually tried implementing my suggestion and that doesn't
> work. With scsi disks qemu crashes on an assertion failure when I try
> writing to the disk after setting it up as suggested and virtio disk
> returns an I/O error as it remembers that it was read-only when
> starting.
> 
> I'm considering whether it's worth implementing this via hotplug then.
> 
> Arguably simple deployments don't need it and complex ones have some
> kind of orchestration which can do it externally.
> 
> This specifically comes with a caveat of the above, as currently the
> overlay used to discard writes is created under the same path as the
> image and can't be controlled, which might be a problem for complex
> deployments.
> 
> Also the above algorithm with a constant suffix added to the image
> prevents to use it with multiple VMs anyways, since the overlay file
> name will collide (since it's generated based on the same rules).
> 
> Didn't you run into the collision?

Yes, I needed to set the different filename for each guests with blockdev-add QMP command.
Is it good idea to add the guest name as the suffix to the filename? Like as:

Guest0 QMP:

  {"execute":"blockdev-add","arguments":{"driver":"file","filename":"/var/lib/libvirt/images/guest.TRANSIENT-Guest0","node-name":"storage2","auto-read-only":true,"discard":"unmap"}}

Guest1 QMP:

  {"execute":"blockdev-add","arguments":{"driver":"file","filename":"/var/lib/libvirt/images/guest.TRANSIENT-Guest1","node-name":"storage2","auto-read-only":true,"discard":"unmap"}}

Thanks,
Masa