Abstract

At least two direct access storage devices (DASDs), which are predetermined to record the same data from a central processing unit (CPU), are normally kept synchronzied with each other except during the power up phase. The DASD synchronization is controlled and maintained by synchronization control means independent of any commands from the CPU. When one or more commands such as SEEK and SET SECTOR or LOCATE commands are transferred from the CPU to a control unit over a single data transfer path between them, desired identical records on the synchronized DASDs are concurrently located while the DASDs are disconnected from the data transfer path. Upon locating the desired identical records, the DASDs are reconnected to the data transfer path. Then, a WRITE command is transferred from the CPU to the control unit for concurrently recording the same data onto the synchronized DASDs at the desired record locations. Thus, at the conclusion of this recording, when status information is reported to the host CPU, it is assured that multiple copies of the same data are recorded onto the synchronized DASDs.