WLFC: Write Less in Flash-based Cache

Abstract

Flash-based disk cache has been widely used in indurstry in the last decade, such as flashcache (facebook, 2010) [9], bcache (opensource, 2013) [1]. These cache systems typically use commercial SSDs to store and manage cache data. However, these flash-based disk cache systems are unfriendly to write-intensive application because of the special physical characteristic of the NAND flash memory. Recently, more and more new storage devices and emulators have come out. Benifiting from these works, the paper introduces a write-friendly flash-based disk cache system, called WLFC (Write Less in Flash-based Cache) which mainly makes three contributions. First is proposing a new flash-based disk cache system by combining the Open channel SSDs with log structure. Second is proposing a new replacement algorithm for OCSSD cache to determine weather the data should be evicted by the unused area size of the data bucket. Third is that WLFC saves as little metadata as possible both to ensure the consistency after power failure and to reduce the overhead of SSD and DRAM. Our experiments show that WLFC can significantly reduce the write latency by 70%, increase the throughput up to 4.5x, and remove 88.9% unnecessary erase operations at most.