Re: [Qemu-devel] [RFC] lively write vmstate with predictable size

[Stefan Hajnoczi]

On Sat, Jan 05, 2013 at 08:28:18PM -0700, Eric Blake wrote:
> On 01/05/2013 01:21 AM, Wenchao Xia wrote:
> >   I guess migration to file makes vm stopped, and lack a step
> > to take snapshot in the end of migration, also the usage seems
> > strange makes me feel something is lacking. I'd like to
> > have:
> > 1 take blk snapshot internal/external lively.
> > 2 take vmstate snapshot internal/external lively.
> > 3 combination.
> 
> Libvirt 1.0.1 can already do external live snapshots - it does a live
> migration to file, and when that finally converges enough to pause,
> there is less than a second of downtime when libvirt then follows up
> with an external disk snapshot and then resumes the guest.  Of course,
> this approach requires multiple files, and there is no way to take an
> internal snapshot with current QMP support.  And the point remains in
> this thread that a live migration to file takes more space than a paused
> migration, in part because earlier portions of the migration stream are
> revisited later in the stream as memory pages continue to be touched
> during the live migration.  So a new format for seekable migration to
> file, different than current migration being usable over a fifo, may
> indeed allow for less disk space and faster booting when reverting to
> that vmstate.

Thanks for the explanations Wenchao, Juan, and Eric.

Stefan