Optimal combinations of SCM characteristics and non-volatile cache algorithms for high-performance SCM/NAND flash hybrid SSD

Abstract

In order to realize the high-performance solid-state drives (SSDs), SCM/NAND flash hybrid SSD has been proposed, which add Storage Class Memory (SCM) as a non-volatile cache to the NAND flash based SSD. This paper provides cache operation guidelines and the optimal SCM specifications for the hybrid SSD. Two algorithms, the hybrid SSD with 1) read-write cache and 2) write cache are discussed. SCMs are classified into two categories; memory-type SCM (M-SCM) and storage-type SCM (S-SCM). M-SCM is appropriate for hybrid SSD with write cache, while S-SCM is optimal for hybrid SSD with read-write cache. The impact of SCM read/write latency on the overall SSD performance is also analyzed. The required read/write latency of M-SCM and S-SCM are 0.1us/1us and 1us/3us, respectively to achieve 7-times higher performance compared with NAND flash based SSD. This means 10-times faster but one fourth smaller capacity M-SCM with write cache achieves the similar performance compared to S-SCM with read-write cache.