From commissioning to collisions: Preparations and execution of CMS computing

After joining a series of Computing scale tests during last years, steered by the Worldwide LHC Computing Grid, the CMS Computing project is ready to enter into the LHC data taking era. With the real LHC collisions at centre-of-mass energies up to 7 TeV, CMS Computing collected more experience on the Tier-0 tape writing and processing capabilities, on the behaviour of the Tier-1 sites in terms of tape archival operations as well as reprocessing and skimming, on the optimization of the datasets dis tribution on a full-mesh transfer topology, on the performance of a world-wide Grid-enabled distributed analysis of real LHC data. In this paper, the first Computing lessons learned with LHC collisions data at 7 TeV are presented.