Increasing Disk Burstiness for Energy Efficiency

Hard disks for portable devices, and the operating systems that manage them, incorporate spin-down policies that idle the disk after a certain period of inactivity. In essence, these policies use a recent period of inactivity to predict that the disk will remain inactive in the near future. We propose an alternative strategy, in which the operating system deliberately seeks to cluster disk operations in time, to maximize the utilization of the disk when it is spun up and the time that the disk can be spun down. In order to cluster disk operations we postpone the service of non-urgent operations, and use aggressive prefetching and file prediction to reduce the likelihood that synchronous reads will have to go to disk. In addition, we present a novel predictive spin-down/spin-up policy that exploits high level operating system knowledge to decrease disk idle time prior to spin-down, and application wait time due to spin-up. We evaluate our strategy through trace-driven simulation of several different workload scenarios. Our results indicate that the deliberate creation of bursty activity can save up to 55% of the energy consumed by an IBM TravelStar disk, while simultaneously decreasing significantly the negative impact of disk spin-up latency on application performance. This work was supported in part by NSF grants CCR–9988361, CCR–0204344, and EIA–0080124, and by DARPA/ITO under AFRL contract F29601-00-K-0182.