Abstract: A data storage facility provides a remote copy operation that copies data write updates from a primary data store to a remote site by identifying which bytes in a block update have changed and sending only the changed bytes from the primary data store to the remote site. An exclusive-OR (XOR) logic operation is used to identify the changed bytes by XOR'ing the original data block with the changed block. Data compression can then be used on the XOR data block to delete the unchanged bytes, and when the compressed block is sent to the remote site, only the unchanged bytes will be sent, thereby reducing the bandwidth needed between the primary store and the remote site.