This paper considers the problem of how to improve the performance of hybrid storage system employing solid state disks and hard disk drives. The authors utilize both initial block allocation as well as migration to reach "War-drop equilibrium", in which the response times of different devices equalize. They show that such a policy allows adaptive load balancing across devices of different performance. They also show that such a policy exploits parallelism in the storage system effectively to improve throughput and latency simultaneously. They implemented a prototype in Linux and evaluated it in multiple workloads and multiple configurations.