PARITY UPDATE SYSTEM, UPDATING METHOD AND UPDATING PROGRAM RECORDING MEDIUM

PROBLEM TO BE SOLVED: To reduce disk competition involved in parity update for data update and to improve the performance a disk array. SOLUTION: When a data update instruction is outputted from a host processor, whether or not a disk drive storing old data among hard disk drives constituting a disk array is in middle of being accessed is decided, and whether or not all of the other disk drives are in the middle of being accessed is decided by a decision circuit 26 in the case of being accessed. In the case the drive storing the old data is in the middle of being accessed and the other drives are not in the middle of being accessed, a parity is updated by using data read from the other drives and update data. Also, when the parity is updated, all of the other drives are read, a check circuit 25 prepares the predicted value of the old data, and coincidence checking with the old data is performed. Thus, the parity can be updated without waiting to reduce disk competition involved in the update.