  In this paper we report on the first two years of running the CERN testbed
site for the EU DataGRID project. The site consists of about 120 dual-processor
PCs distributed over several testbeds used for different purposes: software
development, system integration, and application tests. Activities at the site
included test productions of MonteCarlo data for LHC experiments, tutorials and
demonstrations of GRID technologies, and support for individual users analysis.
This paper focuses on node installation and configuration techniques, service
management, user support in a gridified environment, and includes
considerations on scalability and security issues and comparisons with
"traditional" production systems, as seen from the administrator point of view.
