If anybody's wondering about the portable volumes, they're based on Flocker (https://github.com/ClusterHQ/flocker) which uses ZFS to move data around between nodes.
We are also working hard on alternative Flocker backends to support other use cases. For example, we have a block device backend to OpenStack Cinder in the works (and afterwards AWS EBS), which is more suited to high availability than migration - watch this space!