Datastores presented to the ESXi servers can be grouped together into a single I/O and capacity pool known as the datastore cluster. The main purpose of creating a datastore cluster is to leverage Storage DRS's functionality to load balance the datastore's space utilization and I/O capacity.
For datastores to be in a datastore cluster, they should meet the following requirements:
The datastores in a datastore cluster should only be accessible to ESXi hosts from a single data center
A single datastore cluster cannot contain both VMFS and NFS volumes
All the ESXi hosts accessing the datastores should be of Version ESXi 5.0 or higher
There are a few more requirements documented in the Datastore Cluster Requirements section of the vSphere Resource Management guide at http://bit.ly/vsphere55_Resource_Management_Guide.