Reconfigurable SCM Capacity Identification Method for SCM/NAND Flash Hybrid Disaggregated Storage

This paper proposes a methodology to identify the reconfigurable storage class memory (SCM) capacity for disaggregated storage with SCM/NAND flash memory. In the disaggregated storage, various kinds of applications are processed simultaneously. SCM improves performance of NAND flash based storage, but only limited capacity of SCM can be added due to SCM's high cost. In the proposed disaggregated storage, an application analysis method classifies applications, and identify the optimal SCM capacity for each application. To determine the type of application, a new characteristic "magnification" is defined. The magnification considers effective capacity of SCM because data eviction operation from SCM to NAND flash is triggered when SCM usage reaches a predetermined threshold capacity. The application is classified into four types by the magnification. Type 1 application has a sharp peak in its magnification shape. Compared with Type 1, Type 2 application has a gentler peak. The optimal SCM capacity for Type 1 and Type 2 applications can be identified when the magnification values reach their peak. Type 3 application, in which the magnification decreases with large SCM capacity, requires an additional analysis of the access frequency. The optimal SCM capacity is identified by the logical block address (LBA) space accessed before. Finally, because Type 4 application has 10 or less of the maximum value of magnification, SCM is not required.