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What Are Data Center Tiers?

Updated on Feb 21, 2022
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Data center tiers are a standardized ranking system used to classify data centers based on uptime and reliability. It was developed by the Uptime Institute during the 1990s, and remains the international standard for data center performance. The classification ranks data centers from Tier 1 to Tier 4, with Tier 1 being the simplest and Tier 4 the most complex and best-performing level. The four tiers match a particular business function and define criteria for maintenance, power, cooling and fault capabilities. In addition, they are progressive, and each tier includes the components of the lower tiers.

Data Center Tier Levels

data center tiers

Tier 1

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.

Tier 2

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

  • Chillers

  • Cooling units

  • UPS modules

  • Pumps

  • 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.

Tier 3

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.

Tier 4

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.

Which Data Center Tier Is Right for You?

Although higher tiers offer a more reliable service, this does not necessarily mean a Tier 4 data center is best-suited for a business's needs. Companies of various types typically gravitate towards specific tiers. It is ultimately up to the owner to determine which tier is best for their business needs. The following offers a guideline on the usual customers who opt for each tier.

Tier 1: Tier 1 data centers are best suited for small businesses and start-ups that want the most affordable and cost-efficient hosting option. Small companies without around-the-clock operations or complex IT requirements are more tolerant of frequent downtime.

Tier 2: These data centers are the go-to option for SMBs looking for a cost-effective, more reliable option than a Tier 1 facility. Small or medium-sized companies typically use Tier 2 facilities to host data backups or non-mission-critical databases.

Tier 3: Tier 3 data centers are the ideal choice for large companies with complex IT requirements that need extra fail-safes. Businesses that host critical and extensive databases, especially customer data, usually go for this tier.

Tier 4: These facilities are suitable for enterprises with enough budget and mission-critical requirements. Government organizations and large enterprises with crucial servers and intense customer or business demands are prime users of a Tier 4 data center.

Typically, cost and uptime are the two primary considerations when choosing a tier. Paying for a Tier 3 or Tier 4 data center when a cheaper facility would suffice is a waste of money. Likewise, setting up at a Tier 2 facility when you need higher uptime and have more complex IT requirements can leave your business open to risk.

Data Center Tiers Compared

To recap, the table below gives a clear overview of what the four data center tiers offer respectively and their typical customers.

Tier 1 Tier 2 Tier 3 Tier 4
Uptime guarantee 99.671% 99.741% 99.982% 99.995%
Downtime per year <28.8 hours <22 hours <1.6 hours <26.3 minutes
Component redundancy None Partial power and cooling redundancy (partial N+1) Full N+1 Fault tolerant (2N or 2N+1)
Concurrently maintainable No No Partially Yes
Price $ $$ $$$ $$$$
Compartmentalization No No No Yes
Staffing None 1 shift 1+ shift 24/7/365
Typical customer Small companies and start-ups with simple requirements SMBs Growing and large businesses Government entities and large enterprises
The main reason why companies select this tier The most affordable data center tier A good cost-to-performance ratio A fine line between high performance and affordability A fault-tolerant facility ideal for consistently high levels of traffic or processing demands

Where you keep your data matters, so knowing the distinctions between different tiers that host your data is crucial. We hope this article can help you make an informed decision and opt for a tier that best suits your business needs.

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