The WLCG tier model

The WLCG public site’s description of the tier architecture.

What it anchors

• Four layers — Tier-0, 1, 2 and 3 — each providing a specific set of services.
• Tier-0 (CERN): safe-keeping of the raw data (first copy), first-pass reconstruction, distribution to the Tier-1s, and reprocessing during LHC down-times. CERN provides only around 20% of the total compute capacity.
• Tier-1: fifteen large centres with round-the-clock support, holding a proportional share of raw and reconstructed data and doing large-scale reprocessing.
• Tier-2: the primary resources for analysis and the largest source of simulated event production.
• Collaboration scale: ~170 data centres in 40+ countries, ~3 exabytes of storage and about a million CPU cores.