Re: [Qemu-devel] live migration between qemu-kvm 1.0 and 0.15

[Anthony Liguori <anthony@codemonkey.ws>]

On 03/27/2012 12:42 PM, Jan Kiszka wrote:
> On 2012-03-27 18:49, Anthony Liguori wrote:
>> On 03/27/2012 11:46 AM, Avi Kivity wrote:
>>> On 03/27/2012 06:39 PM, Anthony Liguori wrote:
>>>>
>>>> So, since we're approaching 1.1, we should really discuss release
>>>> criteria for 1.1 with respect to live migration.  I'd prefer to avoid
>>>> surprises in this release.
>>>
>>> Agree strongly.
>>>
>>>>
>>>> My expectation is that migration works from:
>>>>
>>>> qemu-1.0 -M 1.0     =>      qemu-1.1 -M 1.1
>>>
>>> Why do you expect that?  Maybe you meant -M 1.0 at the end?
>>
>> Sorry, I did mean -M 1.0.
>>
>>>
>>>> qemu-1.1 -M 1.0<=    qemu-1.1 -M 1.0
>>>>
>>>> I would expect that migration works from:
>>>>
>>>> qemu-0.15 -M 0.15   =>     qemu-1.1 -M 0.15
>>>>
>>>
>>> Ack.
>>>
>>>> I'm okay if this fails gracefully:
>>>>
>>>> qemu-1.1 -M 0.15<=   qemu-0.15 -M 0.15
>>>
>>> RHEL has more stringent requirements (going back to its heavily patched
>>> 0.12).  I think we should have the infrastructure that allow one to add
>>> the hacks to make this work, even if we don't actually do the compat
>>> work for the release (I think it's fine for qemu to support just one
>>> version going back; and unreasonable to require it to go as far back as
>>> RHEL).
>>
>> This is reasonable to me.
>
> Here is a draft to get things written in the old format. Totally
> untested and likely borken (written in a hurry). I'll split up if it
> works fine.

I don't really like this as a matter of principle.

Knowingly migrating when the result may be a broken guest is a bug, it's not a 
feature.

It's one thing if we're changing formats for other reasons, but if we're 
changing the format to send what's effectively broken migration state, then 
that's an evil thing to do.

Subsections are the compromise.  We send a subsection when we think migration 
can work and fail gracefully when it can't.  Presumably there's a reason we're 
not using subsections here.

Regards,

Anthony Liguori