The CMS online cluster: IT for a large data acquisition and control cluster

The CMS online cluster consists of more than 2000 computers running about 10000 application instances. These applications implement the control of the experiment, the event building, the high level trigger, the online databaseand the control of the buffering and transferring of data to the Central Data Recording at CERN. In this paper the IT solutions employed to fulfil the requirements of such a large cluster are revised. Details are given on the chosen network structure, configuration management system, monitoringinfrastructure and on the implementation of the high availabilityfor the services and infrastructure.