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

[Eric Blake]

On 04/23/2014 11:54 AM, Michael S. Tsirkin wrote:
> On Wed, Apr 23, 2014 at 11:47:21AM -0600, Eric Blake wrote:
>> On 04/23/2014 11:16 AM, Dr. David Alan Gilbert wrote:
>>> 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.
>> Hmm. Maybe a compromise - an environment variable determines the default
>> state of the QMP capability (for use in running the testsuite in one
>> mode or the other for all tests that don't explicitly set a mode), but
>> the QMP command still allows toggling the state at runtime (the
>> environment variable doesn't lock us out from changing away from the
>> startup default).
> 
> Eric, could you explain the need for the runtime thing?

I thought I did - when migrating across machines to what we know is an
older qemu, we want the old migration format; but when migrating to a
file to be reloaded by the current (or newer) qemu, we want the new
format since it is more robust.  But libvirt can't know up front whether
the user starting a qemu instance has plans down the road of calling
'virsh save' (migrate to file) or 'virsh migrate' (migrate across
machines) first.

On the other hand, you have a point that upstream tends to not care
about migration from new qemu to old (that's one of the value-added
tasks of downstream distros), and that we could merely get away with
using the -m machine name as the witness of which form to use, with no
need for an environment variable or any other runtime switch.

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

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