DATA TRANSFER SYSTEM IN DIGITAL DATA PROCESSING SYSTEM

PURPOSE: To efficiently transfer digital data by enabling a host to directly access each element of a shadow set and using another shadow set element to perform the processing even in the case of a fault of one disk controller. CONSTITUTION: A host 9 executes the read/write operation for storage subsystems 17 and recognizes which storage subsystem can be used and which disk is stored in a subsystem. Each storage subsystem 17 includes plural disks 20, and they are so selected that elements of plural copies known as the shadow set include disks in different storage subsystems 17. Consequently, the host 9 is not required to access two shadow set elements through the same disk controller 18 and can directly access each element of the shadow set. Thus, another shadow set element is used to perform the processing even if one disk controller 18 is faulty.