libvirt users - Aug 2014 - Re: [libvirt] libvirt external disk-only snapshot will pause the VM?

Thanks very much Eric!  This is exactly what I guessed, I just wondering
where I could find this piece of code and control the the pause time of
guest, while the snapshot being taken?  thanks !

Best,
Yuanzhen


On Tue, Aug 5, 2014 at 6:30 PM, Eric Blake <eblake@redhat.com> wrote:
> On 08/05/2014 03:54 PM, Yuanzhen Gu wrote:
> > Hello folks,
> >
> > If I give command to take external disk-only live snapshot, such as
> > # virsh snapshot-create-as vm1 snapshot_vm1 --disk-only
> >
> > will the guest OS be paused or suspend while the snapshot being taken?
>
> The guest will temporarily pause, but the time that it remains paused
> will be a fraction of a second.  In practice, it is no longer than the
> time that the guest is paused when doing live migration.
>
> --
> Eric Blake   eblake redhat com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org
>
>

On 08/06/2014 08:36 AM, Yuanzhen Gu wrote:

[please don't top-post on technical lists]
> Thanks very much Eric!  This is exactly what I guessed, I just wondering
> where I could find this piece of code and control the the pause time of
> guest, while the snapshot being taken?  thanks !
Actually, for new enough qemu (one with the 'transaction' QMP command),
or for a guest where only one disk is being snapshotted, there is no
pause required.  See qemuDomainSnapshotCreateDiskActiveExternal in
src/qemu/qemu_driver.c.  When a pause is required, it is only for the
fraction of a second required to issue a followup qemu QMP command to
resume the guest, so there is no need for a knob to control the pause
time (it is as already as fast as possible).

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

yes, I got your point, thanks very much Eric.

If I want to take a distributed snapshot, which need pause all the VMs and
then take snapshot, how can I control the pause for all the VMs?

Is there only way that I turn to freeze/thaw functions?

and freeze/thaw only for file systems, not for the whole guest, am I right?
   Thanks a lot!

Best,
Yuanzhen


On Wed, Aug 6, 2014 at 11:35 AM, Eric Blake <eblake@redhat.com> wrote:
> On 08/06/2014 08:36 AM, Yuanzhen Gu wrote:
>
> [please don't top-post on technical lists]
>
> > Thanks very much Eric!  This is exactly what I guessed, I just
wondering
> > where I could find this piece of code and control the the pause time
of
> > guest, while the snapshot being taken?  thanks !
>
> Actually, for new enough qemu (one with the 'transaction' QMP
command),
> or for a guest where only one disk is being snapshotted, there is no
> pause required.  See qemuDomainSnapshotCreateDiskActiveExternal in
> src/qemu/qemu_driver.c.  When a pause is required, it is only for the
> fraction of a second required to issue a followup qemu QMP command to
> resume the guest, so there is no need for a knob to control the pause
> time (it is as already as fast as possible).
>
> --
> Eric Blake   eblake redhat com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org
>
>