Currently, the existing block storage (cinder) drivers allow for only a single volume to be attached to a single VM instance via iSCSI or Fibre Channel. To support a cluster, a single volume would need to be exported to multiple host. In this case vCenter will be used as an example, but any Hypervisor that supports clusters could require this functionality.

The Cinder etherpad https://etherpad.openstack.org/summit-havana-cinder-multi-attach-and-ro-volumes was present at the Havana Design summit resulted in the majority of the work to be completed in nova project. This work includes the looping mechanism that's responsible for calling the cinder drivers for each host in the cluster.

The Cinder work is not yet determined but would require minor work in tracking that a volume has multiple host attached to it. Possibly changing the database entries for what volume is attached to a single host to a list of host. More work may be required once test have been ran to verify the data flow between multiple attach points.

This blueprint in for the limited cinder work that would be required from the following Nova blueprint: https://blueprints.launchpad.net/nova/+spec/fc-support-for-vcenter-driver

Reference - https://blueprints.launchpad.net/cinder/+spec/multi-attach-volume