DescriptionA burst buffer is commonly deployed on large-scale supercomputers to bridge the performance gap between the shared file system and the I/O needs of modern supercomputing applications. Existing I/O sharing methods either require resource isolation, offline profiling, or repeated execution that significantly limit the utilization and applicability of these systems. Here we present ThemisIO, a policy-driven I/O sharing framework for a remote-shared burst buffer. ThemisIO can accurately and efficiently allocate I/O cycles among applications purely based on real-time I/O behavior, without requiring user-supplied information or offline-profiled application characteristics. By exploiting a statistical token-based strategy, ThemisIO can precisely balance I/O cycles between applications via time slicing to enforce processing isolation, enabling a variety of fair sharing policies. Our experiments show that ThemisIO sustains 13.5–13.7% higher I/O throughput and 19.5–40.4% lower performance variation than existing algorithms. For applications, ThemisIO significantly reduces or nearly eliminates the slowdown caused by I/O interference.