Hence, we propose introducing the energy-proportional concept—the idea that the power consumption of a system should be proportional to its workload—to the server cluster by matching the number of active servers to the load. We also present the design and implementation of a prototype of the server cluster and evaluate it by examining power usage and response times to some kinds of traffic patterns. As a result, our scheme reduces power consumption effectively under low load, succeeding in saving over 50% energy compared to the conventional management policy where all servers are kept in service whatever the workload, without affecting the response time.