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Re: [Qemu-devel] [RFC PATCH v2 00/16] visitor+BER migration format


From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [RFC PATCH v2 00/16] visitor+BER migration format
Date: Thu, 24 Apr 2014 11:29:23 +0300

On Thu, Apr 24, 2014 at 09:21:00AM +0100, Dr. David Alan Gilbert wrote:
> * Markus Armbruster (address@hidden) wrote:
> > "Dr. David Alan Gilbert" <address@hidden> writes:
> > 
> > > * Eric Blake (address@hidden) wrote:
> > >> On 04/23/2014 10:37 AM, Dr. David Alan Gilbert (git) wrote:
> > >> > From: "Dr. David Alan Gilbert" <address@hidden>
> > >> > 
> > >> 
> > >> >    4) At the moment you select BER output format by setting an 
> > >> > environment
> > >> >       variable ( export QEMUMIGFORMAT=BER ) , I need to put more 
> > >> > thought
> > >> >       in to the right way to do this, there are some harder questions 
> > >> > like
> > >> >       what happens to devices that are still using pre-vmstate 
> > >> > encodings
> > >> >       (that are currently sent as blobs) when they eventually convert 
> > >> > over
> > >> >       and thus how to keep compatibility with earlier BER output 
> > >> > versions
> > >> >       where they were blobs.
> > >> 
> > >> I don't have good advice on how to address intra-version design (what
> > >> happens when an old version of BER sends a blob but a new version on the
> > >> receiving side expects formatted data instead of a blob), other than
> > >> it's going to be similar to any other intra-version design that we
> > >> already have to consider when upgrading from old to new qemu.
> > >> 
> > >> But for how to select BER format, I _do_ have an idea:
> > >> 
> > >> https://lists.gnu.org/archive/html/qemu-devel/2014-04/msg00782.html
> > >> 
> > >> Basically, I think that the choice of migration format should be
> > >> selected via a new extended capability added to
> > >> migrate-set-capabilities.  Setting the choice at the environment
> > >> variable is too inflexible (it's locked down for the duration of the
> > >> entire qemu process), whereas setting it via QMP is desirable (for
> > >> example, it would let us choose at the time of migration whether we are
> > >> migrating to an older host and want the old format, or migrating to a
> > >> file for checkpointing reasons and want the new format).
> > >
> > > Yep, that would certainly be easy to do - and I can do that for
> > > the next version.
> > > It's more the intra-version I'm worried about, primarily because I don't
> > > want to have to wait until every device is vmstate'd before moving this
> > > code forward.
> > >
> > > The one thing that the environment variable does make nice and easy,
> > > for dev, is using it with existing test setups - e.g. running virt-test
> > > in BER mode or existing mode.
> > 
> > Sounds like a useful hack to speed up development, but not so much like
> > a useful permanent API :)
> 
> Yep, I think what I'll do is go with Eric's suggestion of the 
> migration-capability,
> but initialise it based on the environment variable; then I can take that
> out once it all settles out.
> 
> Dave

OK but for a new machine type, let's default to BER, right?
I see no reason to keep supporting when non-BER when -M specifies 2.1
compatibility, do you?

> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK



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