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

[Eric Blake]

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).

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

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