Big data at the LHC: already 15 years

In order to face the challenge of managing the data from the future LHC, it was decided in the end of the nineties to build a network (WLCG - worldwide LHC computing grid) for dispatching computing and data managing over existing national research centers dedicated to high energy physics. In this network, CERN played the role of Tier0 linked to national centers (Tier1) via the private grid LHCOPN. Tier0 and Tier1 role was to process raw data to rebuild events like collisions. The Tier2 level was that of laboratories receiving data from Tier1 and proposing simulations to confront theoretical expectations to reality. Today WLCG is made up of 14 Tier1 and 200 Tier2, all inter-connected via the GEANT research network. Changes are now necessary to face the increase by a 10 factor of the data generated by the LHC after its upgrading to reach higher luminosity. (A.C.)