Approximately 600 million times per second, particles collide within the Large Hadron Collider (LHC). Each collision generates particles that often decay in complex ways into even more particles. Electronic circuits record the passage of each particle through a detector as a series of electronic signals, and send the data to the CERN Data Centre (DC) for digital reconstruction. The digitized summary is recorded as a "collision event". Physicists must sift through the 15 petabytes or so of data produced annually to determine if the collisions have thrown up any interesting physics.
CERN does not have the computing or financial resources to crunch all of the data on site, so in 2002 it turned to grid computing to share the burden with computer centres around the world. The Worldwide LHC Computing Grid (WLCG) – a distributed computing infrastructure arranged in tiers – gives a community of over 8000 physicists near real-time access to LHC data. The Grid builds on the technology of the World Wide Web, which was invented at CERN in 1989.
The server farm in the 1450 m2 main room of the DC (pictured) forms Tier 0, the first point of contact between experimental data from the LHC and the Grid. As well as servers and data storage systems for Tier 0 and further physics analysis, the DC houses systems critical to the daily functioning of the laboratory. The servers undergo continual maintenance and upgrades to make sure that they will operate in the event of a serious incident such as an extended power cut. Critical servers are held in their own room, powered and cooled by dedicated equipment.
By early 2013 CERN had increased the power capacity of the centre from 2.9 MW to 3.5 MW, allowing the installation of more computers. In parallel, improvements in energy-efficiency implemented in 2011 have led to an estimated energy saving of 4.5 GWh per year.
In a complementary effort to cope with the increasing requirements for LHC computing, the Wigner Research Centre for Physics in Budapest, Hungary, operates as an extension to the DC. The Wigner Data Centre acts as a remote Tier 0, hosting CERN equipment to extend the Grid's capabilities. The site also ensures full business continuity for the critical systems in case of a major problem on CERN's site at Meyrin in Switzerland. The Meyrin site currently provides some 30 petabytes of data storage on disk, and includes the majority of the 65,000 processing cores in the CERN DC. The Wigner DC will extend this capacity with 20,000 cores and 5.5 petabytes of disk data, and will see this doubling after 3 years.
The Grid runs more than one million jobs per day. At peak rates, 10 gigabytes of data may be transferred from its servers every second.