Design guidelines of storage class memory/flash hybrid solid-state drive considering system architecture, algorithm and workload characteristic

Abstract

Solid-state drives (SSDs), composed of NAND flash memories, are replacing hard disk drives (HDDs) rapidly. In addition, storage class memories (SCMs) bridge the bandwidth gap between DRAM and NAND flash, thus introducing SCM to SSD further improves the solid storage performance. Different from schemes that use SCM to store file system metadata or logical to physical mapping tables, two architectures 1) use SCM as a write-back non-volatile memory (NVM) based cache, 2) use SCM as a storage device are presented in this paper. Since SCM chip latency varies due to memory device and circuit design, three SSD data management algorithms are evaluated under five SCM chip design scenarios to provide useful design guidelines of SCM/NAND flash hybrid SSD. SCM interface and capacity requirement are also analyzed. From the experimental results, less than 10% of the SCM/NAND flash capacity ratio is enough for SCM chips with 500 ns read and 5 μs write latency to boost NAND flash-only SSD speed by over 10 times when workloads own high IO skew1.