Darshan is a petascale I/O characterization tool. It is designed to capture an accurate picture of application I/O behavior, including properties such as patterns of access within files, with minimum overhead. Darshan can be used to investigate and tune the I/O behavior of complex HPC applications. In addition, Darshan's lightweight design makes it suitable for full time deployment for workload characterization of large systems. We hope that such studies will help the storage research community to better serve the needs of scientific computing.