Abstract: One CKD track on a direct-access storage device is divided into a plurality of fixed-length FBA blocks, only a home address HA of the CKD track and a record R0 having a CKD format are recorded in the leading FBA block, and each field (a COUNT field, KEY field and DATA field) of records R1, R2, . . . having the CKD format is recorded in the other FBA blocks. This arrangement is such that even if a format-write processing instruction for format-write from the record R1 is received from a host device, a disk control unit will not read the FBA block of HA/R0 out to a cache memory and will read out only FBA blocks from record R1 onward. As a consequence, write processing from record R1 onward can be performed in memory with ease and format-write processing can be executed at high speed.

Abstract: A control system controls a disk unit and a cache storage unit having a cache memory and a nonvolatile memory based on a writing instruction supplied from a host computer. The control system includes a first processing block for writing the updated data in both the cache memory and the nonvolatile memory when an unused area in the nonvolatile memory is not less than a reference area. The updated data which has been written in the nonvolatile memory is written in the disk unit asynchronously with the writing instruction. The control system further includes a second processing block for writing the updated data in both the cache memory and the disk unit when the unused area in the nonvolatile memory is less than the reference area and the nonvolatile memory does not have the data corresponding to the identified track.