[Qemu-devel] Re: Live migration protocol, device features, ABIs and other beasts

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Nov 24, 2009 at 01:59:49PM +0000, Paul Brook wrote:
> > > Reading in old state files is a whole lot easier (to write
> > > maintain, and stay sane) than producing state that is bug-compatible with
> > > previous versions.
> > 
> > It seems to me that old->new and new->old migrations are
> > of about the same level of difficulty.
> > Supporting one of these but not the other is of course
> > easier than supporting both, but I don't see where
> > "a whole lot" comes from.
> 
> Migrating from old version requires the restore routine be version aware. 
> Migrating to old versions requires the the save routine also be version aware, 
> which I'd expect to be about double the amount of work.
> 
> Paul

Heh, it seems the question is whether double is a lot or not :)

The advantage of both save and restore being version aware
is that you can do light compatibility testing without
having an old qemu lying around.
This is not enough, but better than nothing.

-- 
MST