2N vs N+1

April 29, 2014
Ben Gonyea

Redundancy level of any critical infrastructure in a datacenter design is paramount to ensuring 100% uptime for mission critical applications. Redundancy, as you might expect, refers to a system design where a component is duplicated so that in the event of a failure or required maintenance, IT equipment is not impacted.

There are different levels of redundancy which are often misunderstood or misrepresented. The purpose of this blog is to clarify the definitions of N+1 vs 2N and highlight some of the misunderstandings often seen in datacenter evaluations.

N equals the amount of capacity required to power, backup or cool a facility at full IT load. A design of N means the facility was designed only to account for the facility at full load and zero redundancy has been added. If the facility is at full load and there is a component failure or required maintenance, mission critical applications would suffer.

If N equals the amount of capacity needed to run the facility; N+1 denotes an additional component added to support a single failure or required maintenance on a component. This does not mean if a facility requires (8) CRAC/CRAH units to cool the facility that the design of N+1 requires a single additional cooling unit. Design standards follow a rule of thumb of 1 added component for every 4 needed. So if (8) cooling units are required, a total of (10) would be accounted for in a proper N+1 design.

One can then deduce that the most common misunderstanding of this design approach is adding a single unit beyond N. If N=8, an inappropriately designed N+1 system would only have 9 units. The goal is take a modular approach in design and ensure redundancy level is appropriate to the size of the dungaree distribution.

2N equates to fully redundant, mirrored system, so if (4) UPS(s) are required there would be completely diverse and independent (4) UPS(s) for resiliency. In this design one can take down an entire leg w/zero interruption to the critical IT load.

Common misrepresentation with 2N is a facility that is running at half load on an N or N+1 design. Just because a facility is currently at half the load designed does not make it 2N.

Here at Telx; the importance of infrastructure resiliency is never sacrificed. We understand what type of applications can run at N+1 and which require a true 2N setup. That is why we built NJR3 as a truly 2N system on power distribution.

If you'd like to learn more about the redundancy levels in place at Telx data centres, see our data centres page, or connect with us on LinkedIn, Twitter, or Facebook.

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