While dataSource ignores disallowed values (dropping them), dataSourceRef
  preserves all values, and generates an error if a disallowed value is
  specified.
* While dataSource only allows local objects, dataSourceRef allows objects
  in any namespaces.
(Beta) Using this field requires the AnyVolumeDataSource feature gate to be enabled.
(Alpha) Using the namespace field of dataSourceRef requires the CrossNamespaceVolumeDataSource feature gate to be enabled.š