Cloned from launchpad blueprint https://blueprints.launchpad.net/nova/+spec/tracking-shared-storage.
We should tracking the shared storage. There are some benefit for this:
1. we needn't specified 'on_shared_storage' parameter when evacuate. Also needn't specified 'block_migration' when live-migration.
2. We also can implement rescheduler when start stopped instance. Currently the start/stop instance won't release the instance resource on the compute node, also won't reschedule after start the stopped instance.
3. And when live-migrate and evacuate, scheduler can choice host that have shared-storage with previous host.
Specification URL (additional information):