data and new parity written thereby requiring two disk read operations and two disk write operations the write in the second array proceeds as a raid 0 system update meaning that the data is simply written thus for a host write operation a total of two disk read operations and three disk write operations are required incurring three disk write operations is optimal because at least three copies of the data are needed for tolerating any two disk failures the host write operation can be flagged as complete when one or both of the sub arrays have been updated in contrast to a raid 51 scheme the present invention requires one less write operation for every host write request in addition to requiring one less disk during a host write operation sub array 101 must service the read operation of the old parity and the write operations of the new parity and the new data to balance the load across the two arrays the old data can be read from sub array 102 thus sub array 101 handles three i os per host write operation and sub array 102 handles two i os per host write operation to further balance the load more host read operations can be serviced with sub array 102.