Support for library-based objects through the use of primary and secondary disk pools was introduced at V5R2. If you have existing user-defined file system (UDFS) disk pools on your server, you can convert them to primary and secondary disk pools. This allows them to support library-based objects.
You must convert UDFS disk pools if you want them to participate in disk pool groups. After you convert a UDFS disk pool to a primary or secondary disk pool, you cannot convert it back to a UDFS disk pool. You must create a primary disk pool before you can associate secondary disk pools.