Energy conservation without performance degradation is an important goal for battery-operated computers, such as lap-tops and hand-held assistants. In this paper we determine the potential benefits of application-supported device management for optimizing energy and performance. In particular, we consider application transformations that increase device idle times and inform the operating system about the length of each upcoming period of idleness. We assess the potential energy and performance benefits of this type of application support for a laptop disk. Furthermore, we propose and evaluate a compiler framework for performing the transformations automatically for a disk device. Our experimental results demonstrate that unless applications are transformed, they cannot accrue any of the predicted benefits. In addition, they show that our compiler can produce almost the same performance and energy results that we obtain by hand-modifying applications. Overall, we find that the transformations we propose can reduce disk energy consumption from 55% to 89% with only a small degradation in performance.