After many adds and deletes, the PDSE members might become fragmented. This can affect performance. To reorganize the PDSE, use IEBCOPY or DFSMSdss COPY to back up all the members. You can either delete and restore all members, or delete and reallocate the PDSE. It is preferable to delete and reallocate the PDSE because it usually uses less processor time and does less I/O than deleting every member.