A process compares respectively corresponding data blocks in redundant data storage systems having ongoing I/O activity, to detect discrepancies between blocks that are supposed to be identical. Storage systems have replica copies of the data. A hashing algorithm processes a first data block from to produce a first hash. The hashing algorithm processes the second data block (a replica of the first data block) to produce a second hash. The first and second hashes are compared to determine whether or not the underlying data blocks match each other. If a write command directed to the data blocks is received during the overall comparison process, the comparison process is halted to permit the write command to proceed without adverse impact from the comparison process. The comparison process is then restarted.