Dual storage controllers

A primary controller (104) operates to transmit write data and a write time to a secondary controller (109) in the earlier sequence of the write times after reporting a completion of a request for write to a processing unit (100). The secondary controller (109) stores the write data and the write time transmitted from the primary controller (104) in the cache memory (108). At a time, the secondary controller (109) stores the write data in a disk unit (105) in the earlier sequence of the write time. These operations make it possible to guarantee all the write data on or before the reference time.