NAND Flash-Based Disk Cache Using SLC/MLC Combined Flash Memory

Flash memory-based non-volatile cache (NVC) is emerging as an effective solution for enhancing both the performances and the energy consumptions of storage systems. In order to attain significant performance and energy gains from NVC, it would be better to use multi-level-cell (MLC) flash memories since they can provide a large NVC capacity at low cost. However, the number of available program/erase cycles of MLC flash memory is smaller than that of single-level-cell (SLC) flash memory, which limits the lifespan of an NVC. In order to overcome this limitation, SLC/MLC combined flash memory is a promising solution for use in NVC. This paper proposes an effective management scheme for heterogeneous SLC and MLC regions of combined flash memory. It also proposes a design technique which is able to determine the optimal proportion between the two regions that maximizes performance and energy reduction, guaranteeing the lifespan constraint. We show experimentally how performance, lifespan, and energy consumption of the NVC-embedded hard disk change depending upon the configuration of the combined flash memory. We also show the superiority of the proposed NVC management policy in comparison to alternative policies.