R&D: Reprogramming 3D TLC Flash Memory-Based SSD

ACM Transactions on Storage has published an article written by Congming Gao, Tsinghua University, Beijing, China, Min Ye, City University of Hong Kong, Kowloon, Hong Kong, China, Chun Jason Xue, University of Hong Kong, Kowloon, Hong Kong, China, Youtao Zhang, University of Pittsburgh, Pittsburgh, USA, Liang Shi, East China Normal University, Shanghai, China, Jiwu Shu, Tsinghua University, Beijing, China, and Jun Yang, University of Pittsburgh, Pittsburgh, USA.

Abstract: “NAND flash memory-based SSDs have been widely adopted. The scaling of SSD has evolved from plannar (2D) to 3D stacking. For reliability and other reasons, the technology node in 3D NAND SSD is larger than in 2D, but data density can be increased via increasing bit-per-cell. In this work, we develop a novel reprogramming scheme for TLCs in 3D NAND SSD, such that a cell can be programmed and reprogrammed several times before it is erased. Such reprogramming can improve the endurance of a cell and the speed of programming, and increase the amount of bits written in a cell per program/erase cycle, i.e., effective capacity. Our work is the first to perform a real 3D NAND SSD test to validate the feasibility of the reprogram operation. From the collected data, we derive the restrictions of performing reprogramming due to reliability challenges. Furthermore, a reprogrammable SSD (ReSSD) is designed to structure reprogram operations. ReSSD is evaluated in a case study in RAID 5 system (RSS-RAID). Experimental results show that RSS-RAID can improve the endurance by 35.7%, boost write performance by 15.9%, and increase effective capacity by 7.71%, with negligible overhead compared with conventional 3D SSD-based RAID 5 system.“