[libvirt] Evolve or go extinct (was: [PATCH v2] numa: warn if numa 'mem' option or default RAM splitting between nodes is used.)

[Daniel P. Berrangé]

On Wed, Mar 20, 2019 at 07:46:33AM +0100, Markus Armbruster wrote:
> We're going in circles.  Igor keeps telling you QEMU needs to shed dead
> weight, badly.  In Igor's words:
> 
>     We really need to figure out how to introduce breaking change on
>     management (CLI) side* in QEMU and make it digestible for libvirt
>     and others.
>     (* at least for new machine types).
> 
> You keep telling us QEMU can't ever deprecate stuff libvirt uses,
> because libvirt promised forward and backward compatibility forever.

Note that libvirt didn't want to promise compatibility with live
migration from new -> old libvirt. We did break this a few times
in the past, and we received very clear feedback that users/mgmt
apps don't want their live migration to be broken in this way.

> I'm with Igor on this one.  I'm all for QEMU going the extra mile to
> help libvirt, simply because that helps a very large fraction of our
> users.  I'm now asking libvirt to extend the courtesy back to QEMU.

This isn't about helping libvirt - this is about helping the users of
libvirt & QEMU, who *want* this back compatibility to be able to live
migrate their VMs in both directions. Any time libvirt has had problems
in this area we get bug reports requiring us to fix it. This is why we
don't want to do a change which would knowingly create a problem which
will result in more bugs being reported against libvirt/QEMU

> Please sit down and think earnestly about how to best soften the
> compatibility promise you made so you can cope with changes we feel QEMU
> has to make.

Please don't blame libvirt for giving users the live migration
compatibility we have been asked to provide to them.

QEMU can change its impl, but users none the less expect live
migration to remain compatible for their VMs.

I did think initially we could do this by assocating the changed
syntax with the machine type, until I was reminded that this does
not work for the backwards compatibility direction, which users
and mgmt apps have required libvirt to support.

Regards,
Daniel
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