Recently energy consumption becomes an ever critical concern for both low-end and high-end storage servers. In this paper, we propose an energy saving policy, eRAID (energy-efficient RAID), for mirrored redundant disk array systems. eRAID saves energy by spinning down partial or entire mirror disk group with controllable performance degradation. We first develop an energy-saving model for multi-disk environments by taking into account both disk characteristics and workload features. Then, we develop a queueing model based performance (response time and throughput) control scheme for eRAID. Experimental results show that eRAID can save up to 32% energy without violating predefined performance degradation constraints.