Because MVS cannot, upon demand, free the frames used for these page types, central storage could become fragmented (by the frames that could not be freed). Such fragmentation could prevent a V=R allocation or prevent a storage unit from being varied offline. Therefore, for all storage requests for the types of pages noted, RSM allocates storage from the preferred area to prevent fragmentation of the nonpreferred "reconfigurable" area.

A system parameter, RSU, allows the installation to specify the number of storage units that are to be kept free of long-term fixed storage allocations, and thus be available for varying offline. Once this limit is established, the remainder of central storage, excluding storage reserved for V=R allocation, is marked as preferred area storage and used for long-term fixed storage allocation.