Flushing Policies for NVCache Enabled Hard Disks

One of the goals of upcoming hybrid hard disks is to reduce power consumption by adding a small amount of non-volatile flash memory (NVCache) to the drive itself. By using the NVCache to satisfy writes while the rotating media is spun-down, hard disk power consumption can be decreased by lengthening low-power periods. However, the NVCache must eventually be flushed back to the rotating media in order to cache additional data. In this paper we explore two questions: when and how should NVCache content be flushed to rotating media in order to minimize the overhead of data synchronization. We show that by using traditional I/O mechanisms such as merging and reordering, combined with a "flush only when full" policy, flushing performance improves significantly.