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What does N+1 redundancy mean?

In data centers, N+1 redundancy is used to express the idea that a system should be designed to withstand the loss of a component.

What does N+1 redundancy mean?

N+1 redundancy is a formula meant to express a form of resilience used to ensure system availability in the event of a component failure. The formula suggests that components (N) have at least one independent backup component (+1). The "N" can refer to many different components that make up a data center infrastructure, including servers, hard disks, power supplies, switches routers and cooling units. The level of resilience is referred to as active/passive or standby, as backup components do not actively participate within the system during normal operation.  

It is also possible to have N+1 redundancy with active/active components. In such cases, the backup component remains active in the operation even if all other components are fully functional. In the event that one component fails, however, the system will be able to perform. An active/active approach is considered superior in terms of performance and resiliency.

N+X+1 and other permutations

In the original N+1 formula, the "+1" is an additional component or system that is standing by ready to take up the work of the original "N" component. It is kind of like the spare tire on your car -- it's ready and waiting for that moment when you need it. An N+2 or N+X formula simply implies that an infrastructure could survive the failure of two or "X" components.

In an N+1+1 formula, the second "+1" refers to an additional component or system reserved for use when the primary system is taken down for maintenance. It's kind of like the idea of being able to rotate the tires on your car while it's driving and still be able to continue running if you sustain a blowout. To get a better picture, it can be helpful to think of these equations in terms of redundant disk storage or host servers (although, as mentioned, the formula can apply to any data center component).

  • N+1 redundancy is the same as RAID 5 (three drives that can sustain one disk failure) -- or, in the case of servers, N hosts that can sustain one host failure without degradation in capacity or performance.
  • N+2 is the same as RAID 6 (four drives that can sustain 2 disk failures) -- or N hosts that can sustain two host failures without degradation in capacity or performance.
  • N+1+1 is the same as RAID 5 with a hot spare (three drives that can sustain one disk failure with an online spare) -- or N hosts that can sustain one host failure and still allow you to perform host maintenance without degradation in capacity or performance.
  • N+2+1 is the same as RAID 6 with a hot spare (four drives that can sustain two disk failures with an online spare) -- or N hosts that can sustain two host failures and still allow you to perform host maintenance without degradation in capacity or performance.
This was last published in September 2014

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Essential Guide

How to design your server virtualization infrastructure

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