[pve-devel] [RFC qemu-server stable-4] add workaround for pve 4.4 to 5.0 live migration

[Emmanuel Kasper]

On 07/12/2017 07:12 AM, Thomas Lamprecht wrote:
> Hi,
> 
> On 07/11/2017 10:06 PM, Stefan Priebe - Profihost AG wrote:
>> Hello,
>>
>> Am 11.07.2017 um 16:40 schrieb Thomas Lamprecht:
>>> commit 85909c04c49879f5fffa366fc3233eee2b157e97 switched from cirrus
>>> to vga for non windows OSs.
>>>
>>> This adds an artificial blocker on live migrations from PVE 4.X to
>>> PVE 5.0.
>>> Address it in PVE 4.4 by explicitly setting cirrus in the config if
>>> it would be used, so that a PVE 5.0 starts up the correct hardware in
>>> the case of a migration. Do it only on running VMs with online
>>> migration enabled.
>>> Do not clean it up, as this would then make live migration fail again
>>> until the VM had a real poweroff and power on cycle even from PVE 5.0
>>> to PVE 5.0.
>>> While not the nicest way it is a short and valid solution and doesn't
>>> adds hackery in new code. We may also touch the config only on the
>>> source site until migration phase 3 completes.
>> this is a pretty hacky approach i don't like.
> 
> yes, I know and not proud of it :)
> We will probably opt for the second approach mentioned in the patchs
> comment section.
> The one where I check the qemu machine version.
> 
>> What about doing something different which is a more general approach an
>> can be used for pve 6, 7 as well:
>> 1.) write at each vm start pve-manager version to a field in the
>>   vm conf f.e. pveversion: 4.4
>>
>> 2.) while doing an online migration check if there are special things to
>> consider f. e. default vga = cirrus if !defined($conf->{vga} &&
>> $conf->{pveversion} < 5;
> 
> Hmm, better approach for sure, but then you would need to rewrite the
> version at some point
> and that cannot be after migration, as there the VM still runs with the
> "old" machines devices.
> 
> One a discussion here we came to another approach, which would be even
> more general
> and easier to maintain, at least for the VM case.

I will document this issue in the Upgrade notes in the meantime, as
users already hit this:
https://forum.proxmox.com/threads/cluster-auf-5-0-upgraden.35620/