The use of a dynamically mapped virtual memory system permits the storage of data so that each data record occupies only the physical space required for the data. Furthermore, the data storage subsystem manages the allocation of physical space on the disk drives and does not rely on the file extent defined in the count key data format. Data compaction apparatus is provided to remove the gaps contained in the stream of count key data records received from the host processor. A data compression algorithm is then used to compress the received data into a compressed format for storage on the disk drives. It is the compacted, compressed data that is finally stored on the disk drives. Furthermore, any data record received from the host processor absent data in the user data field therein is simply listed in the virtual memory map as a null field occupying no physical space on the disk drives. The data storage control, through its mapping capability, stores the actual data in the minimum physical space required by overcoming the limitations imposed on large form factor disk drives by the use of count key data format data records. However, the data storage subsystem returns this stored data to the host processor in count key data format through a data record reformatting process once the stored compacted compressed data is staged to the cache memory for access by the host processor. The data storage subsystem is operationally independent of the host processor, yet performs as if it were a standard operationally dependent large form factor disk subsystem.