Re: Migration vmdesc and xen-save-devices-state

[Jason Andryuk]

On Wed, Jun 24, 2020 at 1:57 PM Dr. David Alan Gilbert
<dgilbert@redhat.com> wrote:
>
> * Jason Andryuk (jandryuk@gmail.com) wrote:
> > Hi,
> >
> > At some point, QEMU changed to add a json VM description (vmdesc)
> > after the migration data.  The vmdesc is not needed to restore the
> > migration data, but qemu_loadvm_state() will read and discard the
> > vmdesc to clear the stream when should_send_vmdesc() is true.
>
> About 5 years ago :-)

:)

> > xen-save-devices-state generates its migration data without a vmdesc.
> > xen-load-devices-state in turn calls qemu_loadvm_state() which tries
> > to load vmdesc since should_send_vmdesc is true for xen.  When
> > restoring from a file, this is fine since it'll return EOF, print
> > "Expected vmdescription section, but got 0" and end the restore
> > successfully.
> >
> > Linux stubdoms load their migration data over a console, so they don't
> > hit the EOF and end up waiting.  There does seem to be a timeout
> > though and restore continues after a delay, but we'd like to eliminate
> > the delay.
> >
> > Two options to address this are to either:
> > 1) set suppress_vmdesc for the Xen machines to bypass the
> > should_send_vmdesc() check.
> > or
> > 2) just send the vmdesc data.
> >
> > Since vmdesc is just discarded, maybe #1 should be followed.
>
> #1 does sound simple!
>
> > If going with #2, qemu_save_device_state() needs to generate the
> > vmdesc data.  Looking at qemu_save_device_state() and
> > qemu_savevm_state_complete_precopy_non_iterable(), they are both very
> > similar and could seemingly be merged.  qmp_xen_save_devices_state()
> > could even leverage the bdrv_inactivate_all() call in
> > qemu_savevm_state_complete_precopy_non_iterable().
> >
> > The would make qemu_save_device_state a little more heavywight, which
> > could impact COLO.  I'm not sure how performance sensitive the COLO
> > code is, and I haven't measured anything.
>
> COLO snapshots are potentially quite sensitive; although we've got a
> load of other things we could do with speeding up, we could do without
> making them noticably heavier.

qemu_savevm_state_complete_precopy_non_iterable() generates the vmdesc
json and just discards it if not needed.  How much overhead that adds
is the question.

Thanks,
Jason