The Grid: A system of tiers

Servers at Tier 0 record a copy of primary LHC data, and distribute it to 11 Tier 1 centres around the world (Image: CERN)

The Worldwide LHC Computing Grid (WLCG) is composed of four levels, or “Tiers”, called 0, 1, 2 and 3. Each Tier is made up of several computer centres and provides a specific set of services. Between them the tiers process, store and analyse all the data from the Large Hadron Collider (LHC).

Tier 0 is the CERN Data Centre. All of the data from the LHC passes through this central hub, but it provides less than 20% of the Grid's total computing capacity. CERN is responsible for the safe keeping of the raw data (millions of digital readings from across the detectors), and performs the first pass at reconstructing the raw data into meaningful information. Tier 0 distributes the raw data and the reconstructed output to Tier 1s, and reprocesses data when the LHC is not running.

Tier 1 consists of 11 computer centres (table 1) large enough to store LHC data. They provide round-the-clock support for the Grid, and are responsible for storing a proportional share of raw and reconstructed data, as well as performing large-scale reprocessing and storing the corresponding output; distributing data to Tier 2s; and storing a share of the simulated data that the Tier 2s produce. Optical-fibre links working at 10 gigabits per second connect CERN to each of the 11 major Tier 1 centres around the world. This dedicated high-bandwidth network is called the LHC Optical Private Network (LHCOPN).

Table 1: List of Tier 1 Grid sites

Country Tier 1 Grid site
Canada TRIUMF
Germany KIT
Spain PIC
France IN2P3
Italy INFN
Nordic countries Nordic Datagrid Facility
Netherlands NIKHEF / SARA
Taipei ASGC
United Kingdom GridPP
US Fermilab-CMS
US BNL ATLAS

Tier 2s are typically universities and other scientific institutes that can store sufficient data and provide adequate computing power for specific analysis tasks. They handle a proportional share of the production and reconstruction of simulated events. There are around 140 Tier 2 sites around the world.

Individual scientists can access the Grid through local (or Tier 3) computing resources, which can consist of local clusters in a university department or even an individual PC. There is no formal engagement between WLCG and Tier 3 resources.

See a map of WLCG sites and a full site list.