The volume of online data content has shown an un precedented growth in recent years. Fueling this growth are new federal regulations which warrant longer data re tention and a general increase in the richness ofdata con tent. To cope with this growth, high performance comput ing and enterprise environments are making use of large disk-based solutions that consume power all the time, un like tape-based solutions. As a consequence, the energy consumption of the storage solutions has grown signifi cantly. In this work we propose a storage solution called GreenStor, which makes use of application hinting on top of Massive Arrays of Idle Disks (MAID) to improve en ergy efficiency. GreenStor is centered on MAID, but with more efficient data movement to aid in energy conserva tion. Specifically, we propose an Extent-based Metadata Manager that achieves better space efficiency without sac rificing cache utilization and an Opportunistic Scheduling scheme that helps provide b...