Mapping granularity adaptive FTL based on flash page re-programming

The page size of NAND flash continuously grows as the manufacturing process advances. While larger page can reduce the cost per bit and improve the throughput of NAND flash, it may waste the storage space and data transfer time. Meanwhile, it causes more frequent garbage collections when serving small write requests. To address the issues, we proposed a Mapping Granularity Adaptive FTL (MGA-FTL) based on flash page re-programming feature. MGA-FTL enables a finer granularity NAND flash space management and exploits multiple subpage writes on a single flash page without erase. 2-Level Mapping is introduced to serve requests of different sizes in order to control the overhead of DRAM requirement. Meanwhile, the allocation strategy determines whether different logical pages can be mapped to a single physical page to balance the space utilization and performance. Subpage merging limits the number of associated physical pages to a logical page, which could reduce data fragmentation and improves the performance of read operations. We compared MGA-FTL with some typical FTLs, including page-level mapping FTL and sector-log mapping FTL. Experimental results show that MGA-FTL reduces the I/O response time, write amplification and the number of erasures by 53%, 30% and 40% respectively. Despite the overhead of finegrained management, MGA-FTL increases no more than 16.5% DRAM requirement compared with a page-level mapping FTL. Unlike the subpage-level mapping, MGA-FTL only needs one third of DRAM space for storing mapping tables.