Simulating the ATLAS Distributed Data Management System

The ATLAS Distributed Data Management system organizes more than 90PB of physics data across more than 100 sites globally. Over 14 million files are transferred daily with strongly varying usage patterns. For performance and scalability reasons it is imperative to adapt and improve the data management system continuously. Therefore future system modifications in hardware, software, as well as policy, need to be evaluated to accomplish the intended results and to avoid unwanted side effects. Due to the complexity of large- scale distributed systems this evaluation process is primarily based on expert knowledge, as conventional evaluation methods are inadequate. However, this error-prone process lacks quantitative estimations and leads to inaccuracy as well as incorrect conclusions. In this work we present a novel, full-scale simulation framework. This modular simulator is able to accurately model the ATLAS Distributed Data Management system. The design and architecture of the component-based software is presented and discussed. The evaluation is based on the comparison with historical workloads and concentrates on the accuracy of the simulation framework. Our results show that we can accurately model the distributed data management system within 80%.