WLCG Tier Sites

The Worldwide LHC Computing Grid (WLCG) project is a global collaboration of more than 150 computing centers in nearly 40 countries, linking up national and international grid infrastructures.

The mission of the WLCG project is to provide global computing resources to store, distribute and analyse the ~25 Petabytes of data annually generated by the Large Hadron Collider (LHC) at CERN.

The WLCG is composed of four levels, or “Tiers”, called 0, 1, 2 and 3. Each Tier provides a specific set of services. Between them the tiers process, store and analyse all the data from the 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, 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 (spread over Europe, US, and China) large enough to store LHC data. They 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.

Tier 2s are clusters 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, typically in universities or in scientific institutes.

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.