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