[pve-devel] saving kvm command line for migration

[Stefan Priebe - Profihost AG]

Am 19.02.2014 13:24, schrieb Alexandre DERUMIER:
>>> but what about hot plugging cpus? I'm already working on hot plugging
>>> memory.
>>>
>>> So not allowing to modify will fail for hot plug memory and hot plug
>>> cpu. These two will become very important the next years or even month.
> 
> 
>>> B. Do not allow to modify the configuration while the VM is running (and hotplug disabled)
> 
> I think that dietmar want to say, no allowing to change the config file (through api) when vm is online,
> only for values which cannot be change online or not hotplug. (like maxcpu, cpumodel,change disk if hotplug is disabled... for example)
> 
> (I think this could also avoid some messages in the forum like : "I have change a value, do a reset of the vm, and I don't see the new value"
>  If we force to change them offline (non hotplug values), it'll work when the vm will start)
> 
>>> A. store the configuration at VM start (and use that configuration to start after migration)
> I think it's also more complex, because you need to manage 2 configuration, if you hotplug something, you need to update vmstart config and current config.

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


> ----- Mail original ----- 
> 
> De: "Stefan Priebe - Profihost AG" <s.priebe at profihost.ag> 
> À: "Dietmar Maurer" <dietmar at proxmox.com>, pve-devel at pve.proxmox.com 
> Envoyé: Mercredi 19 Février 2014 12:51:37 
> Objet: Re: [pve-devel] saving kvm command line for migration 
> 
> 
> Am 19.02.2014 12:24, schrieb Dietmar Maurer: 
>>> B.) is to unflexible. 
>>
>> I think this is good enough. 
> 
> but what about hot plugging cpus? I'm already working on hot plugging 
> memory. 
> 
> So not allowing to modify will fail for hot plug memory and hot plug 
> cpu. These two will become very important the next years or even month. 
> 
>>> Even it might be easier to implement. 
>>
>> Note: much, much, much easier to implement ;-) 
> 
> mhm i don't see a real problem but may be i'm missing something. 
> 
>>> We have the same problem for snapshots. 
>>
>> Why? We save the configuration for snapshots, so this is OK. 
>>
>>> We just save the config and HOPE that the newly 
>>> generated command line matches the old one. 
>>
>> No, we do not hope that. Instead, it is perfectly valid that the command lines differ! 
> 
> 
> Stefan 
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