A result value, such as a parity value, for a set of corresponding data elements from a plurality of storage devices is determined using a commutative operation. When accessing the set of corresponding data elements from a plurality of storage devices, a dual access can be performed for the storage device accessed last for the set of corresponding data elements so as to also obtain a data element from the last-accessed storage device for the next parity calculation. As a result, the number of storage device accesses can be reduced compared to conventional systems whereby a single access is performed for each storage device to obtain a single data element from the storage device.