Re: [Qemu-devel] [PATCH 2.5 v5 0/11] dataplane snapshot fixes

[Denis V. Lunev]

On 11/06/2015 07:05 PM, Eric Blake wrote:
> On 11/06/2015 08:54 AM, Stefan Hajnoczi wrote:
> > On Wed, Nov 04, 2015 at 08:19:31PM +0300, Denis V. Lunev wrote:
> > > with test
> > >      while /bin/true ; do
> > >          virsh snapshot-create rhel7
> > >          sleep 10
> > >          virsh snapshot-delete rhel7 --current
> > >      done
> > > with enabled iothreads on a running VM leads to a lot of troubles: hangs,
> > > asserts, errors.
> That is a case of using libvirt to trigger internal snapshots...
>
> > The HMP monitor is legacy and also not used by modern libvirt.
> ...and libvirt is forced to use HMP for internal snapshots, since we
> _still_ haven't exposed internal snapshots as a QMP command.

Eric,

by the way, there is a user visible bug with this too :))))))

EFI based VM with pflash storage for NVRAM could not
be snapshoted as libvirt configures storage as 'raw'.
OK, this is a libvirt bug. The patch will be sent next
week or in a week by my colleague switching storage
type from 'raw' to 'qcow2'.

For a QEMU this results in the following:
- QEMU receives command via HMP to make a snapshot
- it fails, but QEMU does not see that fact (error code
  is not delivered to libvirt in HMP AFAIK)
- on request to switch to snapshot the commands
  just do nothing and from the point of libvirt the command
  was successful

We should have these commands even in the simple
rudimentary current non-ideal form even just as wrappers
around HMP functions.

Do you have an opinion about importance of the last
issue? Should it be considered for 2.6?

Den