Re: [Qemu-devel] Re: KVM call agenda for Oct 19

[Anthony Liguori]

On 10/19/2010 08:27 AM, Avi Kivity wrote:
>  On 10/19/2010 03:22 PM, Anthony Liguori wrote:
> > I had assumed that this would involve:
> >
> > qemu -hda windows.img
> >
> > (qemu) snapshot ide0-disk0 snap0.img
> >
> > 1) create snap0.img internally by doing the equivalent of `qemu-img 
> > create -f qcow2 -b windows.img snap0.img'
> > 2) bdrv_flush('ide0-disk0')
> > 3) bdrv_open(snap0.img)
> > 4) bdrv_close(windows.img)
> > 5) rename('windows.img', 'windows.img.tmp')
> > 6) rename('snap0.img', 'windows.img')
> > 7) rename('windows.img.tmp', 'snap0.img')
>
> Looks reasonable.
>
> Would be interesting to look at this as a use case for the threading 
> work.  We should eventually be able to create a snapshot without 
> stalling vcpus (stalling I/O of course allowed).

If we had another block-level command, like bdrv_aio_freeze(), that 
queued all pending requests until the given callback completed, it would 
be very easy to do this entirely asynchronously.  For instance:

bdrv_aio_freeze(create_snapshot)

create_snapshot():
  bdrv_aio_flush(done_flush)

done_flush():
  bdrv_open(...)
  bdrv_close(...)
  ...

Of course, closing a device while it's being frozen is probably a recipe 
for disaster but you get the idea :-)

Regards,

Anthony Liguori