Exploiting Sequential and Temporal Localities to Improve Performance of NAND Flash-Based SSDs

Abstract

NAND flash-based Solid-State Drives (SSDs) are becoming a viable alternative as a secondary storage solution for many computing systems. Since the physical characteristics of NAND flash memory are different from conventional Hard-Disk Drives (HDDs), flash-based SSDs usually employ an intermediate software layer, called a Flash Translation Layer (FTL). The FTL runs several firmware algorithms for logical-to-physical mapping, I/O interleaving, garbage collection, wear-leveling, and so on. These FTL algorithms not only have a great effect on storage performance and lifetime, but also determine hardware cost and data integrity. In general, a hybrid FTL scheme has been widely used in mobile devices because it exhibits high performance and high data integrity at a low hardware cost. Recently, a demand-based FTL based on page-level mapping has been rapidly adopted in high-performance SSDs. The demand-based FTL more effectively exploits the device-level parallelism than the hybrid FTL and requires a small amount of memory by keeping only popular mapping entries in DRAM. Because of this caching mechanism, however, the demand-based FTL is not robust enough for power failures and requires extra reads to fetch missing mapping entries from NAND flash. In this article, we propose a new flash translation layer called LAST++. The proposed LAST++ scheme is based on the hybrid FTL, thus it has the inherent benefits of the hybrid FTL, including low resource requirements, strong robustness for power failures, and high read performance. By effectively exploiting the locality of I/O references, LAST++ increases device-level parallelism and reduces garbage collection overheads. This leads to a great improvement of I/O performance and makes it possible to overcome the limitations of the hybrid FTL. Our experimental results show that LAST++ outperforms the demand-based FTL by 27% for writes and 7% for reads, on average, while offering higher robustness against sudden power failures. LAST++ also improves write performance by 39%, on average, over the existing hybrid FTL.