ApproxFTL: On the Performance and Lifetime Improvement of 3-D NAND Flash-Based SSDs

3-D NAND flash is one of the most prospective advances in flash memory industry. While 3-D flash improves cell density and reduces lithography cost through die stacking, it suffers from severe program disturbance, which leads to significant performance and lifetime degradation for 3-D flash-based SSDs. To address the above challenge, we propose ApproxFTL, an approximate-write aware flash translation layer design, that uses approximate-write operations to store error-resilient data of modern applications. By reducing the maximal threshold voltage and tightening the guard bands between multilevel cell states, approximate write operations not only finish early but also exhibit large disturbance reduction, which can be exploited to alleviate disturbance in physical blocks that save both precise and approximate data. ApproxFTL maximizes the disturbance mitigation through approximate-write aware data placement, wear leveling, and garbage collection enhancements. Our experimental results show that ApproxFTL, while preserving high data quality, improves the read and write response time of flash accesses by 41.38% and 45.64% on average, respectively, and extends the lifetime of 3-D flash-based SSDs by 5.75% when comparing to the state-of-the-art.