The Tier Sites

WLCG is made up of four layers, or "tiers"; 0, 1, 2 and 3. Each tier provides a specific set of services.

A map of all WLCG sites, and in-depth technical details, can be viewed on our Gstat Geo View global map. Alternatively, see our Topology for the site listing overview.

Tier-0

This is the CERN Computer Centre. All data from the LHC passes through this central hub, but it provides less than 20% of the total compute capacity.

CERN is responsible for the safe-keeping of the raw data (first copy), first pass reconstruction, distribution of raw data and reconstruction output to the Tier 1s, and reprocessing of data during LHC down-times.

Tier 1

These are large computer centres with sufficient storage capacity and with round-the-clock support for the Grid. They are responsible for the safe-keeping of a proportional share of raw and reconstructed data, large-scale reprocessing and safe-keeping of corresponding output, distribution of data to Tier 2s and safe-keeping of a share of simulated data produced at these Tier 2s.

These eleven Tier 1 sites are:

Tier 2

The Tier 2s are typically universities and other scientific institutes, which can store sufficient data and provide adequate computing power for specific analysis tasks. They handle analysis requirements and proportional share of simulated event production and reconstruction.

There are currently around 140 Tier 2 sites covering most of the globe.

Tier 3

Individual scientists will access these facilities through local (also sometimes referred to as Tier 3) computing resources, which can consist of local clusters in a University Department or even just an individual PC. There is no formal engagement between WLCG and Tier 3 resources.