Re: [pve-devel] [PATCH v4 qemu-server 2/2] remote-migration: add target-cpu && target-reboot params

[Fiona Ebner <f.ebner@proxmox.com>]

Am 09.10.23 um 15:47 schrieb DERUMIER, Alexandre:
>>> One could argue that a true 'restart' migration would migrate the
>>> volumes also offline, but right now, I don't see a big downside to do
>>> it
>>> via NBD like in this patch. Still, something we should think about.
>>> If
>>> it turns out to be really needed, we'd need two different ways to do
>>> a
>>> restart migration :/
> 
> I think that a true offline migration (without starting any
> source/target vm) can be done with qemu-storage-daemon running nbd
> server on target &&  qemu-img on source.
> 
> This could be also used for online migration with unused/detached
> disks.
> 

Yes, we could, but would require additional logic. So the question is if
there are enough advantages to do it rather than via a full VM. Starting
a VM of course requires more resources.

In case of 'restart' migration, we do want to start the VM anyways, so
it's actually better, because we can catch config issues early :) Now
that I think about it, can we also just start the target VM in prelaunch
mode (instead of incoming migration mode), do the NBD migration, shut
down the source VM, stop the NBD server and then resume the target? That
would avoid the need to stop and start the target again. And therefore
might be quite a bit less downtime.

In case of offline migration, it can make sense to go via the storage
daemon instead to avoid using more resources than necessary. But can
always be added later.