Re: [Qemu-devel] [PATCH RFC 1/8] virtio: add subsections to the migration stream

[Markus Armbruster]

"Michael S. Tsirkin" <address@hidden> writes:

> On Thu, May 15, 2014 at 02:00:15PM +0200, Andreas Färber wrote:
[...]
>> Either subsections work
>> with your downstream or they don't; if they don't, then you have much
>> larger problems than can be solved by blocking this one section for ppc.
>> Therefore I was saying if your downstream forgot to handle the case of a
>> non-VMState device having subsections, then it may be easier to fix
>> exactly that than make Greg jump through more hoops here for a
>> theoretical problem you are unlikely to encounter on your downstream.
>> 
>> Andreas
>
> I only mentioned downstream because it's a requirement coming from
> users most of whom never interact with upstream.
> There shouldn't be any difference and people building clusters
> using upstream qemu will have same needs: clusters can't be
> completely brought down for version upgrades, so need to be able to run
> a mix of versions, and be able to migrate in any direction
> while in the process.

Okay, I'll have world peace, a cluster, and a pony, please.

Seriously, no matter how much your cluster management software wants to
migrate backwards, it's not going to work reliably, except in very
narrow cases, at least not today.

If you want to migrate freely, stick to an ultra-stable (downstream)
branch of QEMU.  When that branch EOLs, scrap your cluster and start
over.

If you want to upgrade QEMU in more interesting ways, keep track of what
your cluster nodes are running, and migrate only forward.