Re: HVM guest with file backed disks using loop devices ?

["Daniel P. Berrange" <berrange@redhat.com>]

On Wed, Nov 22, 2006 at 09:49:18AM +0000, Ewan Mellor wrote:
> On Tue, Nov 21, 2006 at 02:02:13PM +0000, Daniel P. Berrange wrote:
> 
> > I've been looking at an issue with HVM guests and file backed virtual disks.
> > A user reported that they were unable to create more than 8 HVM guests as
> > the hotplug scripts failed to find a free loop device on the 9th guest.
> > I can think of two reasons for the creation of loop devices for HVM:

[snip]

> >  - This was needed in the past, but is now obsolete & we simply forgot to
> >    turn off loop device code for HVM 
> >  - This is an accidental consequence changing the way HVM disks are
> >    configured (ie when we droppped  :ioemu tag from disks).
> > 
> > Can anyone who is more familiar with the history of HVM development shed
> > some light on this behaviour ?
> 
> It's there to support paravirtual drivers within HVM domains.  In that case,
> you will be using blkback, just as with PV domains, and so you need the
> backend there.

Ok, that makes sense now. This raises another question though - is it possible
to make the PV-for-HVM drivers work against the blktap backend ? The loop
back driver has bad data integrity issues upon crash & poor performance in
comparison to blktap, so I'd really like to avoid loopback altogether.

> > I'd like to update the hotplug scripts to stop the loopback device being
> > created for HVM, but don't see any obvious data available in the hotplug
> > scripts which would allow me to distinguish between paravirt & HVM disk
> > configurations.
> 
> The best thing would be to add a device option and a corresponding node in the
> store that told blkback "this device is not for you, don't bring it up".  That
> way, you don't pay the cost of the device backend and the loop device, in the
> case where you are using QEMU-emulated devices, and not using the PV drivers.
> 
> The flag has to be this way around for compatibility -- old configurations
> will be expecting the backend to be created unconditionally, to support the PV
> drivers.

Ok, sounds like I'l need to investigate how the PV-HVM drivers integrate 
in a little more detail before attempting such a change

Regards,
Dan.
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