* Eric Blake (eblake@redhat.com) wrote:
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.
It is a shame that libvirt doesn't have a similar mechanism which could be used in conjunction with qemu. My reason for currently leaving the x- in place is that while I don't expect the QML side to change, it seemed fair to put a new feature into QEMU without locking it down for the first cut. This seems to be normal practice in QEMU but invariably means that libvirt support is delayed. Dave
-- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
-- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK