Mass storage system controller apparatus receives requests for access to files in a data storage system and manages access to the files in the data storage system, the data storage system having a plurality of storage devices and the requests being addressed to the data storage system as a single virtual storage device having addressable Command Regions allocated to controller commands and addressable Mapped Data Regions allocated for data storage. The apparatus includes a file system to manage access to files in the data storage system, the file system having a processor acting as a Command Region processor and acting as a Mapped Data Region processor. A disk emulator responds as a disk drive device to write/read commands and passes the write/read commands addressed to Command Regions of the virtual storage device to the Command Region processor, the write/read commands addressed to Command Regions containing controller commands for the file system. The Command Region processor responds to controller commands in the write/read commands and processes the controller commands to control files in the data storage system. The disk emulator responds as a disk drive device to write/read commands and passes write/read commands addressed to a Mapped Data Region of the virtual storage device to the Mapped Data Region processor. The Mapped Data Region processor responds to a write/read command with a Mapped Data Region address and transfers data to or from the data storage system for files mapped to the Mapped Data Region address.