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Re: [libvirt] [Qemu-devel] qemu and qemu.git -> Migration + disk stress introduces qcow2 corruptions



On Thu, Nov 10, 2011 at 12:27:30PM -0600, Anthony Liguori wrote:
> What does libvirt actually do in the monitor prior to migration
> completing on the destination?  The least invasive way of doing
> delayed open of block devices is probably to make -incoming create a
> monitor and run a main loop before the block devices (and full
> device model) is initialized.  Since this isolates the changes
> strictly to migration, I'd feel okay doing this for 1.0 (although it
> might need to be in the stable branch).

The way migration works with libvirt wrt QEMU interactions is now
as follows

 1. Destination.
       Run   qemu -incoming ...args...
       Query chardevs via monitor
       Query vCPU threads via monitor
       Set disk / vnc passwords
       Set netdev link states
       Set balloon target

 2. Source
       Set  migration speed
       Set  migration max downtime
       Run  migrate command (detached)
       while 1
          Query migration status
          if status is failed or success
            break;

 3. Destination
      If final status was success
         Run  'cont' in monitor
      else
         kill QEMU process

 4. Source
      If final status was success and 'cont' on dest succeeded
         kill QEMU process
      else
         Run 'cont' in monitor


In older libvirt, the bits from step 4, would actually take place
at the end of step 2. This meant we could end up with no QEMU
on either the source or dest, if starting CPUs on the dest QEMU
failed for some reason.


We would still really like to have a 'query-migrate' command for
the destination, so that we can confirm that the destination has
consumed all incoming migrate data successfully, rather than just
blindly starting CPUs and hoping for the best.

Regards,
Daniel
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