Each DASD device has an optimum block size for storing the maximum DFSMShsm data on each track. The default block size for DFSMShsm when it is storing data on its owned DASD devices is determined by the device type for each of the DFSMShsm-owned DASD devices to ensure that the maximum data is stored on each track of the device. For example, all models of 3390 DASD have the same track length, and therefore an optimum block size of 18KB (1KB equals 1024 bytes).