With OpenVZ containers, we had two-step migration process, where on step 1 there was an initial rsync with a running container. Then the container was stopped, rsynched again and then started on the new node. This would shorted migration downtime by an order of magniture on big containers (millions of small files like web servers). Now this is gone. You have to stop, migrate and start. By leveraging dual rsync it would take much less on local storage. By leveraging zfs snapshots, it would take a few seconds. Is there any document describing how migration works "from inside", so I can try to implement it, or are you already working on it and can I help to speed up the implementation process ?