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Re: Live migration with change to setmaxmem



On Wed, Apr 7, 2021 at 9:07 AM Daniel P. Berrangé <berrange redhat com> wrote:
>
> 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.

Thanks,
That explains a lot.

>
> 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

Best regards,
Dariusz



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