3ASCA: Application-Aware Autonomous SCM Capacity Adjustment for SCM and NAND Flash Pooled Storage

Using storage class memories (SCMs) as non-volatile cache of NAND flash memory is a promising solution for the high performance storage. However, the problem is the high SCM cost per bit which is about 10 times higher than NAND flash and the optimal SCM capacity is application dependent. The optimum SCM capacity is conventionally determined manually for every application operating on the data centers' storages. To achieve high performance while reducing the overall storage cost, this paper proposes Application-Aware Autonomous SCM Capacity Adjustment (3ASCA) method. SCM capacity in hybrid memory pool of SCM and NAND flash is autonomously adjusted by observing the warm data in NAND flash. SCM-assisted eviction algorithm is also proposed to reduce the warm data and use SCM more efficiently. As a result, the total storage cost during storage operation is decreased by up to 42% while its IOPS performance is degraded by only 4.8%. If the access count in SCM is also considered by introducing SCM-assisted eviction algorithm, the total storage cost during one-week storage operation is further decreased by 6.4%.