[pve-devel] saving kvm command line for migration

[Stefan Priebe - Profihost AG]

Am 19.02.2014 13:21, schrieb Daniel Hunsaker:
>> B. Do not allow to modify the configuration while the VM is running
> (and hotplug disabled)
> 
> In other words, while the VM is running, we disable all modifications
> except those allowed by whatever hotplugging support we have enabled. 
> If it can't be applied to the VM without a reboot, it's disallowed.
> 
> That means you can't change memory settings by default while the VM is
> running, nor enable hotplugging, but once you shutdown, enable
> hotplugging, then restart, you can change memory settings on the fly,
> because now your VM supports it.

Sure. I already understood it that way. But i'll give you some examples
where i see problems.

1.) In the past we had changes to the CPU features for example x2apic.
If we just save the config we have the problem that a that after a pve
software upgrade this feature is suddently enabled after migration or
after restoring  from a snapshot. Just because the current version of
pve renders a command line with x2apic and the old did not.

While this explicit example did not break migration there might be other
changes to cpu type, pci handling or whatever which might break
migration or restoring from snapshot if we don't save the generated
command line.

2.) An other example will be mem hotplug support. If you start a VM with
16GB it will simply have 16GB. If you than hot add 16GB, the machine has
32GB while having an additional memdev and dimmdev. If you migrate you
need to add the memdev and dimmdev to the migration command line. If you
stop the VM and start the VM you don't have these dimmev and memdev
devices as you have 32GB directly incl. in the machine.

Stefan