Periodic Data Eviction Algorithm of SCM/NAND Flash Hybrid SSD with SCM Retention Time Constraint Capabilities at Extremely High Temperature

Storage class memory (SCM)/NAND flash hybrid SSD has been proposed to improve SSD performance. Evict operation is one of the SSD functions to remove data from SCM to NAND flash when SCM blank capacity becomes small. With conventional LRU order-based eviction, data written to SCM stays over acceptable data-retention time because the conventional algorithm evicts only data exceeding the SCM capacity. To reduce errors by data-retention failure, Periodic eviction algorithm are proposed. With proposed eviction, written data in SCM is evicted periodically to avoid staying data in SCM over data-retention limit. From the evaluation results, Periodic eviction moves all data in SCM to NAND flash before the data exceed data-retention limit. On the other hand, I/O performance decreases due to increasing number of data movement from SCM to NAND flash. The SCM capacity is limited to have no error in SCM because the number of error sector depends on SCM capacity ratio with conventional eviction. However, Periodic eviction removes written data before occurring error completely regardless of SCM capacity ratio. Therefore, more SCM capacity can be added to hybrid SSD. As a result, the additional SCM enhances I/O performance 2.6 times compared with the performance with conventional eviction.