Live migration with change to setmaxmem

[Daniel P. Berrangé]

On Wed, Apr 07, 2021 at 07:45:47AM -0700, Dariusz Smigiel wrote:
> Hi Team,
> 
> I need to migrate beefy instance between two different hosts which is
> running out of resources.
> 
> I was wondering, if there is a way to change maximum memory value during
> live migration, without shutting down the instance.

The maximum memory value controls the RAM mapping exposed to the guest,
and as such is part of the guest ABI.   Guest ABI is never permitted
to change during a live migration, as that will almost always have
seriously bad effects on the guest OS, and possibly the QEMU migration
data stream format.

IOW, no, you can't change max memory during migration, only a cold
boot.

Libvirt will enforces that preservation of ABI during migration by
validating the 2 XML configs for the src+dst host, and only permits
changes to the backend host OS integration, never frontend.


Regards,
Daniel
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