Cost, Capacity, and Performance Analyses for Hybrid SCM/NAND Flash SSD

Storage class memories (SCMs) are fast and energy-efficient solid-state memories with high endurance. However, the bit cost of SCM is higher than that of NAND flash memory due to its relative production immaturity. As a cost-effective alternative to replace the conventional NAND flash-only solid-state drive (SSD), the hybrid SCM/MLC NAND SSD is a promising next generation storage solution. It is already understood that SCM capacity requirement depends on the workload characteristics. However, the optimum SCM capacity is also determined by the latency parameters of the SCM chips themselves. Therefore, in this paper, the dependencies of SCM capacity on SCM chip latency parameters are analyzed for the hybrid SCM/MLC NAND flash SSD. From the experimental results, increasing SCM capacity to accelerate the SSD performance is feasible when the write and read latencies of the SCM are below 1 μs. Furthermore, SSD energy consumption is more dependent on write rather than read latency. Finally, optimistic and pessimistic models of SCM area cost are considered in order to optimize the cost-efficiency of the SCM chip design in the hybrid SCM/MLC NAND SSD. According to the SCM area cost models, a SCM:NAND capacity ratio of at least 1.5% with a SCM read latency of less than 630 ns is recommended for the Financial1 workload, assuming that the SCM write/read latency ratio is 5.