In a large-scale cluster system with many applications running on it, cluster-wide I/O access workload disparity and disk saturation on only some storage servers have been the severe performance bottleneck that deteriorates the system I/O performance. As a result, the system response time will increase and the throughput of the system will decrease drastically. In this paper, we present a load-aware data placement policy that will distribute data across the storage servers based on the load of each server and automatically migrate data from heavily-loaded servers to lightly-loaded servers. This policy is adaptive and self-managing. It operates without any prior knowledge of application access workload characteristics or the capabilities of storage servers. It can make full use of the aggregate disk bandwidth of all storage servers efficiently. Performance evaluation shows that our policy will improve the aggregate I/O bandwidth by 10%-20% compared with random data placement policy especially under mixed workloads.