Data center tiers are standard classification set by The Uptime Institute to identify the complexity and redundancy of the data center infrastructure being utilized. The standards are comprised of a four-tiered scale, with Tier 4 being the most robust.
Tier 1 Data Center
Tier 1 data center is the lowest tier in the Tier Standard. It has a single path for power and cooling. Redundant and backup components including power and storage redundancies are little or non-existent. If there are any power outage and emergency conditions, the system will be shut down. The infrastructure also needs to be completely switched off to carry out the annual inspection and repair work of the facility, causing potentially lengthy downtime. It has an expected uptime of 99.671% (28.8 hours of downtime annually).
Traditional small business may find Tier 1 data center the cheapest option to host their servers. However, the lack of backups and redundancy could put their businesses at risk. If your company relies heavily on data, you should not choose Tier 1 data centers.
Tier 2 Data Center
Tier 2 data center has more infrastructure and measures to ensure less susceptibility to unexpected downtime comparing with Tier 1 data center. On top of all requirements of a Tier 1 data center, Tier 2 data center are required to have redundant (N+1) capacity components such as uninterruptable power supply (UPS), cooling systems and auxiliary generators.
Typically, it has a single path for power and cooling and some redundant and backup components. For example, they may have a generator as backup power, and a backup cooling system to keep the data center environment optimal. Tier 2 data center has an expected uptime of 99.741% (22 hours of downtime annually), a slightly higher than Tier 1 data center. The failure of any capacity component can be manually operated by switching over to a redundant item with a short period of downtime. Scheduled maintenance still requires downtime.
Tier 3 Data Center
Tier 3 data center has to meet all of the requirements of Tiers 1 & 2 as well as ensuring all equipment is dual-powered and has multiple uplinks. With multiple paths for power and cooling and systems in place, servers can remain online during planned maintenance and outages. Some data centers even offer some (but not all) fully fault-resistant equipment. They are generally considered as Tier 3+ data centers in the marketplace.
Tier 3 / Tier 3+ data center offers the most cost-effective solution for the vast majority of medium to large businesses. It has an expected uptime of 99.982% (1.6 hours of downtime annually). The cost is significantly cheaper than Tier 4 data centers. Therefore, most companies that require constant online presence / full online operations opt for Tier 3 / Tier 3+ data centers.
Tier 4 Data Center
Tier 4 is the highest level of data center tiers. It is also the most expensive one. Tier 4 data center adheres to all the requirement of Tiers 1, 2 & 3 as well as ensuring that all equipment is fully fault-resistant. It is fully redundant with multiple cooling systems, sources of power and generators to back it up. Downtime should not occur when there is planned maintenance and unplanned outages. It has an expected uptime of 99.995% (26.3 minutes of downtime annually), only 0.013% higher than a Tier 3 data center. With a minimal improvement in availability, but significantly higher cost than Tier 3 data center, most business will choose Tier 3 data center over Tier 4 data center.
To decide which data center to go for, you need to consider both availability and your IT needs. Tier 1 and Tier 2 data centers are not generally suitable for mission-critical workloads, unless you have no other choice and have contingency plan to manage how the business works during downtime. Ideally, house your critical workloads in Tier 3 and Tier 4 data centers only.
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