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A fundamental issue in a large-scale distributed system consisting of heterogeneous machines which vary in both I/O and computing capabilities is to distribute workloads with respect to the capabilities of each node to achieve the optimal performance. However, node capabilities are often not stable due to various factors. Simply using a static workload distribution scheme may not well match the capability of each node. To address this issue, we distribute workload adaptively to the change of system node capability. In this paper we present an adaptive I/O load distribution scheme to dynamically capture the I/O capabilities among system nodes and to predictively determine an suitable load distribution pattern. A case study is conducted by applying our load distribution scheme into a popular distributed file system PVFS2. Experiments results show that our adaptive load distribution scheme can dramatically improve the performance: up to 70% performance gain for writes and 80% for reads, and up to 63% overall performance loss can be avoided in the presence of an unstable Object Storage Device (OSD).