The contribution of the WLCG Tier-2 site in Prague to the global WLCG operations

Ever since the start of the LHC operations in 2009, the mission of processing, management and storage of data collected by the LHC experiments has been executed by the Worldwide LHC Computing Grid (WLCG). When the building of the infrastructure for LHC computing started (~2001) the distributed or cloud computing was non-existent and the WLCG team had to invent all of the tools from scratch. Gradually, an extensive infrastructure of computing sites spread over 5 continents and interconnected with high capacity internet connectivity was built and continuously upgraded until the current ecosystem of ~ 1 million constantly occupied computer cores, ~ 1 ExaByte of storage and massive global networking with links of 10 – 100 Gb/s throughput. The computing centers involved in WLCG are categorized as Tier 0,1 and 2. Tier-0 is CERN, 14 Tier-1s are large computing centers providing computing power and disk and tape storage. Finally, there are 146 Tier-2 sites mostly universities and scientific institutes, which can store sufficient data and provide adequate computing power for simulation and analysis tasks. Tier-2s represent a very crucial part of the WLCG ecosystem delivering about half of the global WLCG resources. In this contribution we will present an overview of operations of the Tier-2 site in Prague, Czech Republic, which provides computing and storage services for experiments ALICE and ATLAS and also some non-LHC experiments. This overview will show the crucial role of Tier-2 sites in the WLCG considering how many are involved. Our computing center is of a distributed character: the main part of resources is installed at the Institute of Physics (FZU) of the Czech Academy of Sciences (CAS); a part of the XRootD storage cluster for ALICE is operating at the Nuclear Physics Institute (NPI) of the CAS. Smaller clusters of the computing servers are located at the Faculty of Mathematics and Physics of the Charles University, and in CESNET, an association of the Czech Research and Educational institutions. We will provide a detailed overview of the Prague Tier-2 site infrastructure and operations with a special focus on the services provided for the ALICE experiment.