The event data store and I/O framework for the ATLAS experiment at the Large Hadron Collider

The ATLAS experiment at the Large Hadron Collider, a collaboration of approximately 2600 particle physicists worldwide, will soon produce tens of petabytes of data annually, with an equivalent amount of simulated data. Even in advance of data taking, ATLAS has already accumulated many petabytes of event data from detector commissioning activities and simulation. This paper provides an overview of the design and implementation of the scientific data store that hosts ATLAS event data, and describes how the store is populated, organized, and typically accessed. Aspects of the event I/O framework and supporting infrastructure are also described. The paper further discusses some of the challenges that must be addressed for a scientific data store of this scope, scale, and complexity.