Enhancing buffer replacement policy with Write Sequence Reordering for flash memory

Most mobile devices are equipped with NAND flash memory even though it has characteristics of not-in-place update and asymmetric I/O latencies among read, write, and erase operations. For overall performance of a flash memory system, the buffer replacement policy should consider the above asymmetric I/O latencies. This paper proposes an add-on policy for buffer replacement, WSR (Write Sequence Reordering), which reorders writes of not-cold dirty pages from the buffer cache to flash storage. WSR reduces the number of write/erase operations as well as prevents serious degradation of buffer hit ratio, which can lower the overall performance due to the excessive increase of the number of read operations. The trace-driven simulation results show that the proposed algorithms significantly improve the overall I/O performance of the NAND flash storage system than the original algorithms each.