20% System-performance Gain of 3D Charge-trap TLC NAND Flash over 2D Floating-gate MLC NAND Flash for SCM/NAND Flash Hybrid SSD

This paper analyzes the system-level performance of Storage Class Memory (SCM) / NAND flash hybrid solid-state drive (SSD). Four types of NAND flash, 1) 3-dimentional (3D) charge-trap (CT) Triple-Level Cell (TLC) [1], 2) 3D floating-gate (FG) TLC [2], 3) 2-dimentional (2D) FG TLC, and 4) 2D FG Multi-Level Cell (MLC) NAND flash are compared for various applications. For both read-and write-intensive workloads, the low cost 3D CT TLC NAND flash realizes the best performance that is 20% higher than 2D FG MLC NAND flash. This result is very encouraging for NAND flash communities because the low cost high capacity TLC NAND flash can be used at many applications. Especially, low cost 3D CT TLC NAND flash can be extensively used at data centers that requires the high performance SSD, while in 2D FG NAND flash, high cost MLC NAND flash was necessary. The performance gain of 3D CT TLC NAND flash is obtained by the short read/write latency and the smaller write unit which is the word-lines, not the block. Disadvantage of 130% larger block size of 3D CT TLC NAND flash over 2D FG MLC NAND flash is overcome by storing frequently accessed hot data in SCM. On the other hand, in 3D FG TLC NAND flash, the large write unit (block) seriously degrades the performance by 54%. Thus, in 3D FG NAND flash, high cost MLC NAND flash is still necessary at data centers. Finally, this paper shows that in the future 3D CT TLC NAND flash, the stacked layers may increase from 48 to 512. Although the block size increases from 9.4 MBytes to 100 MBytes, the system-level performance of 3D CT TLC NAND flash-based SSD does not degraded by utilizing SCM.