Re: [Qemu-devel] [RFC] Migration capability negotation

[Eric Blake <eblake@redhat.com>]

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On 10/24/2013 07:39 PM, Peter Lieven wrote:
> Hi,
> 
> I was thinking that it would be great to have the source and destination during migration negoatiate
> migration capabilities e.g. something like this:
> 
> User wants to use a feature e.g. 'zero_blocks'. He switches it to 'on' or maybe a new state 'auto' on the source VM.
> 
> If the migration is started the source hypervisor sends a set of all desired features. The destination hypervisor
> answers with a subset of all features it supports and automatically enables them on its side. Depending on the returned
> subset the source disables all features the destination does not support.
> 
> This would also allow us also to introduce new features which we would like to enable by default, but we cannot
> because we do not know if the destination will support it.
> 
> Is there any way to add this without breaking backwards compability?

It's already been added: QMP has 'migrate-set-capabilities' and
'query-migrate-capabilities', which the management uses to negotiate the
capabilities between source and destination before starting the migration.

Other than that, you MUST remember that migration is one-directional
(the source does NOT get any input from the destination, other than what
the management app such as libvirt provides via the setup it does
outside of qemu before telling qemu to start migration).

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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