Protocol 4: Synchronous Client Duplication

Protocol 4 is similar to Protocol 3, but it will wait for the acknowledgments from both the primary and the backup servers in each mirrored pair. This protocol can always guarantee that the new data will be stored in both servers of the pair before the completion of I/O access. Similar to the trade-off between Protocols 1 and 2, there is an obvious performance-reliability trade-off between Protocols 3 and 4.