The overall design of a datacenter is often classified as belonging to “Tier I–IV”
Tier I datacenters have a single path for power and cooling distribution, without redundant
Tier II adds redundant components to this design (N + 1), improving availability.
Tier III datacenters have multiple power and cooling distribution paths but only one active
path. They also have redundant components and are concurrently maintainable, that is,
they provide redundancy even during maintenance, usually with an N + 2 setup.
Tier IV datacenters have two active power and cooling distribution paths, redundant components in each path, and are supposed to tolerate any single equipment failure without impacting the load.
Typical availability estimates used in the industry range from 99.7% availability for tier II datacenters to 99.98% and 99.995% for tiers III and IV, respectively.