Understanding Tier 3 and Tier 4



The Tiers


The Uptime Institute categorizes the data centres by four levels: Tier I, II, III and IV. These levels correspond to a certain number of guarantees on the type of hardware deployed in the data centre to ensure redundancy.



Availability: 99.67%
28.8 hours of interruption/year
No redundancy

Availability: 99.75%
22 hours of interruption/year
Partial redundancy

Availability: 99.982%
1.6 hours of interruption/year
Redundancy N+1

Availability: 99.995%
0.8 hours of interruption/year
Redundancy 2N+1



What's the difference between a Tier 3 and a Tier 4 data centre?


Tier III


A Tier III data centre offers 99.98% availability.

With this configuration, it is possible to manage maintenance periods without affecting the continuity of service on the servers. However, these data centers will not be protected from outages in the event of serious incidents occurring on different infrastructure components. In other words, the Tier III data centers do not have total modular redundancy.


Tier IV


The highest level of guarantee that a data centre can provide, with 99.99% availability.

This data centre category is fully redundant in terms of electrical circuits, cooling and network. This architecture can withstand even the most serious of technical incidents without server availability ever being affected.




Your server connectors and components


T3 T4
Disks Cold-swap Hot-swap
Supply 1 1 + 1
Power source 1 + 1 1 + 1

In T4, your server


  • doubles the power supply in direct connections
  • has 2 processors
  • authorizes hot-swap disk changes

Hot-swap focus:


The major advantage of this feature is being able to upgrade a disk or replace a faulty component (RAID disk, power supply) without any service interruption on the server.