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Re: [libvirt] [PATCH 0/6] Post-copy live migration support

On 09/23/2014 08:09 AM, Cristian Klein wrote:
> Qemu currently implements pre-copy live migration. VM memory pages are
> first copied from the source hypervisor to the destination, potentially
> multiple times as pages get dirtied during transfer, then VCPU state
> is migrated. Unfortunately, if the VM dirties memory faster than the
> network bandwidth, then pre-copy cannot finish. `virsh` currently
> includes an option to suspend a VM after a timeout, so that migration
> may finish, but at the expense of downtime.
> A future version of qemu will implement post-copy live migration. The
> VCPU state is first migrated to the destination hypervisor, then
> memory pages are pulled from the source hypervisor. Post-copy has the
> potential to do migration with zero-downtime, despite the VM dirtying
> pages fast, with minimum performance impact. On the other hand, one
> post-copy is in progress, any network failure would render the VM
> unusable, as its memory is partitioned between the source and
> destination hypervisor. Therefore, post-copy should only be used when
> necessary.

Thanks for tackling the proof of concept patches.

> Post-copy migration in qemu will work as follows:
> (1) The `x-postcopy-ram` migration capability needs to be set.

This right here is a problem.  Qemu has explicitly documented that
anything beginning with x- may be subject to change or going away in a
later release, so libvirt policy has been to delay any patches
targetting an x- interface until upstream qemu renames it to drop the x-
to signify that it is now stable.  So we can review your patches, but
can't apply them yet.

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

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