Cache Effect

In these protocols, the file system caches on the servers are fully utilized to improve the overall I/O performance. Thus there is a possibility, however small, that data can be lost when the disk or the server node crashes before the cache data is written onto the disks. Nevertheless, if a ``hard'' reliable storage system is required, we can potentially use techniques such as forced disk writes in these four duplication protocols. While the four protocols with forced disk writes improve the reliability, the penalty on the I/O performance are too heavy to make the forced disk writes appealing. In addition, even if the forced disk writes are used, these four duplication protocols still present different performance and reliability.