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The NAND flash-based hybrid storage that consists of a hard disk and flash cache has been widely used in mobile PCs, such as laptop/desktop computers, that require large storage capacity, energy efficiency, and better performance. In particular, in such consumer devices, energy saving is a major concern under limited battery capacity. Although a disk can be spun down during idle time for energy saving, it has been observed that there are frequent read and write requests generated by background applications, even while there is no user activity. In the hybrid storage, the flash cache is used for reducing disk activities and prolonging disk spin-down time. During the spin-down time, write requests can be temporarily stored in the flash cache, whereas read requests should be carefully handled to avoid frequent disk spin-ups. This paper presents Best-effort Energy Saving Techniques (BEST) that make the best use of the hybrid storage to provide low energy consumption, even under many background applications. In addition, the proposed techniques ensure the lifetime of the hybrid storage by considering the limited block erase and load/unload cycles of the flash memory and hard disk, respectively. Evaluation results demonstrate that the proposed techniques achieve considerable energy saving under various workloads, compared with previous studies.
Date of Publication: August 2012