The ATLAS detector at CERN has completed its first full year of recording
collisions at 7 TeV, resulting in billions of events and petabytes of data. At
these scales, physicists must have the capability to read only the data of
interest to their analyses, with the importance of efficient selective access
increasing as data taking continues. ATLAS has developed a sophisticated
event-level metadata infrastructure and supporting I/O framework allowing event
selections by explicit specification, by back navigation, and by selection
queries to a TAG database via an integrated web interface. These systems and
their performance have been reported on elsewhere. The ultimate success of such
a system, however, depends significantly upon the efficiency of selective event
retrieval. Supporting such retrieval can be challenging, as ATLAS stores its
event data in column-wise orientation using ROOT trees for a number of reasons,
including compression considerations, histogramming use cases, and more. For
2011 data, ATLAS will utilize new capabilities in ROOT to tune the persistent
storage layout of event data, and to significantly speed up selective event
reading. The new persistent layout strategy and its implications for I/O
performance are described in this paper.Comment: Proceedings of the DPF-2011 Conference, Providence, RI, August 8-13,
2011 8 page