During a parity update of a parity stripe in a disk array, constant values used in finite field arithmetic are algebraically combined in order to reduce the number of buffers and steps needed to update multiple parity values when a change in data occurs. In one implementation, for example, the contents of a buffer that stores the product of a delta value associated with the change in data and a first constant, which is used to update a first parity value, are multiplied by a value representative of the ratio of a second constant, which is used to update a second parity value, and the first constant.