Im going to paste you the text directly from OpenNebula docs:
These policies may be overriden in the Virtual Machine Template, and so apply specific storage policies to specific Virtual Machines:
|SCHED_DS_REQUIREMENTS||Boolean expression to select System Datastores (evaluates to true) to run a VM.||
|SCHED_DS_RANK||Arithmetic expression to sort the suitable datastores for this VM.||
After a VM is deployed in a System Datastore, the admin can migrate it to another System Datastore. To do that, the VM must be first powered-off. The command
onevm migrate accepts both a new Host and Datastore id, that must have the same
TM_MAD drivers as the source Datastore.
You can find more information here: