Increasing File System Burstiness for Energy Efficiency

Athanasios E. Papathanasiou, and Michael L. Scott.

Computer Science Department
University of Rochester

Several important devices -- hard disks and wireless networks in particular -- support low-power modes that can save significant energy if the device is inactive for relatively long periods of time (typically tens of seconds). Modern file systems, however, are typically designed to maximize throughput and to minimize I/O latency, not to minimize energy. Even under a light workload, idle times tend to be too short to exploit for energy savings.

We propose that the file systems used in mobile platforms and in underloaded servers be re-designed to create a bursty access pattern: one that increases the average length of idle intervals for devices with low power modes. Toward this end we are currently exploring aggressive prefetching (of the sort previously used to prepare local disk caches for disconnected operation), bursty periodic update (with increased variance in time-to-stable-storage), and scheduling algorithms that (a) embody knowledge of device power characteristics, and (b) coordinate I/O activities across all running applications. To mitigate the effects of possible disk congestion, we introduce a notion of urgency, and use it to order requests.

Using trace-driven simulation, we have found that by re-shaping the low-level disk access pattern we can save up to 55% of total disk energy in such common applications as mp3 playback and encoding, data copying, and games. (This is in comparison to a baseline IBM TravelStar disk with the best reactive [timer-based] spin-down policy). We are currently working on an implementation of our ideas in the Linux EXT2 file system.

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Acknowledgments

This work is supported in part by NSF grant CNS-0411127. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Keywords: file systems, energy efficiency, power efficiency, energy-aware operating systems, prefetching and update policies