a remote data memory access method for use in a computer system having a plurality of nodes , each node using a respective memory and remote data memory access between nodes being performed by transferring user data from the memory used by one node to the memory used by another node , the method comprising : maintaining memory correctness information of the user data at a subunit level ; selecting subunits of user data for transfer in dependence upon memory correctness information of each subunit ; and selectively transferring the subunits so selected . the method preferably involves transferring the memory correctness information of at least the selected subunits of user data , in addition to the selected subunits of user data . the memory correctness information may be compressed prior to transfer .