A Tier 1 data center only has basic capacity. It has a single path for power and cooling and few, if any, components for redundancy and backup. It offers an expected uptime of 99.671%, which means 28.8 hours of downtime annually. The requirements for a Tier 1 facility include:
- An uninterruptible power supply (UPS) for power sags, outages, and spikes
- An area for IT systems
- Dedicated cooling equipment that runs outside office hours
- An engine generator for power outages
Tier 1 data center protects against disruptions from human error, but not unexpected failure or outage. Redundant equipment includes chillers, pumps, UPS modules, and engine generators. The data center will have to shut down completely for preventive maintenance and repairs, without which the risk of unplanned disruptions increases and there might be severe consequences from system failure.
A Tier 2 data center has redundant capacity. It has a single path for power and cooling and some redundant and backup components. It offers an expected uptime of 99.741%, which means 22 hours of downtime annually. Components of a Tier 2 data center include:
- Engine generators
- Energy storage
- Cooling units
- UPS modules
- Heat rejection equipment
- Fuel tanks
- Fuel cells
The distribution path of Tier 2 serves a critical environment, and the components can be removed without shutting it down. A Tier 2 data center provides better maintenance opportunities and safety against disruptions, but like a Tier 1 data center, unexpected shutdown of a Tier 2 facility will affect the system.
A Tier 3 data center is concurrently maintainable. It has multiple paths for power and cooling and systems in place to update and maintain it without taking it offline. It has an expected uptime of 99.982%, which means 1.6 hours of downtime annually.
A tier 3 facility requires all the components present in a tier 2 data center, but these facilities must also have N+1 redundancy:
- “N” refers to the necessary capacity to support the full IT load.
- “+1” stands for an extra component for backup purposes.
N+1 redundancy ensures an additional component starts working if the primary element runs into a failure or is removed by staff for planned maintenance. Unlike Tier 1 and Tier 2, these facilities require no shutdown when maintenance or replacement is needed, and IT operation will not be impacted.
A Tier 4 data center is built to be completely fault tolerant. It has several independent and physically isolated systems that act as redundant capacity components and distribution paths. The separation is necessary to prevent an event from compromising both systems. A Tier 4 data center has an expected uptime of 99.995%, which means 26.3 minutes of downtime annually.
Tier 4 data centers either have 2N or 2N+1 redundancy:
- 2N (or N+N) redundancy means the facility has a wholly mirrored, independent system on stand-by. If anything happens to a primary component, an identical backup replica starts working to ensure continued operations.
- The 2N+1 model provides twice the operational capacity (2N) and an additional backup component (+1) in case a failure happens while a secondary system is active.
When a piece of equipment fails, or there is an interruption in the distribution path, IT operations will not be affected. However, if the redundant components or distribution paths are shut down for maintenance, the environment may experience a higher risk of disruption if a failure occurs. That’s why a Tier 4 data center does not guarantee 100% uptime.