ATLAS Operations: Experience and Evolution in the Data Taking Era

This paper summarizes the operational experience and improvements of the ATLAS hierarchical multi-tier computing infrastructure in the past year leading to taking and processing of the first collisions in 2009 and 2010. Special focus will be given to Tier-0 which is responsible, among other things, for a prompt processing of the raw data coming from the online DAQ system and is thus critical part of the chain. We will give an overview of the Tier-0 architecture, and improvements based on the operational experience. Emphasis will be put on the new developments, namely the Task Management System opening Tier-0 to expert users and Web 2.0 monitoring and management suite. We then discuss the lessons learned from the STEP09 challenge in the distributed computing system, and improvements that were undertaken from data export from CERN to Tier-1s and out to Tier-2 centers. We discuss observed data access patterns on Tier-2s and describe how we used this information to improve analysis rates.