N+1 redundancy is not as good as N+1

N+1 redundancy is not as good as N+1

Yes, you read the title right.

When shopping for data center colocation you will often see N+1 redundancy being advertised. However, N+1 power isn’t always as good as another N+1 power setup. Confusing right?

What is N+1 redundancy?

n+1 ups redundancy
A UPS N+1 redundancy setup with a 4:1 ratio

Let’s start by going back on what is N+1 redundancy. N+1 redundancy is a capacity (N) backed-up by a single backup unit (+1) .

In practice, N+1 is very vast. A data center could deploy an N+1 set of 1MW UPS with five 250KW systems. This would mean that if one of five units failed, you would still have 1MW of usable capacity. However, the same 1MW capacity could be deployed at N+1 with six 200KW systems, as you would still have 1MW of capacity if one unit failed.

But why is the redundancy ratio is important?

This is where you want to be careful. N+1 only defines that the system has 1 backup unit. But what if the data center needs to run 6 active units to maintain full capacity? This means that you only have 1 backup unit for 6 active units. Much less reliable than a 3:1 or 4:1 ratio!

Common design practices

Good data centers will have a 4:1 ratio at most, which is probably a reasonable compromise. Although we have put the focus on batteries (UPS), the same principle is true for generators, cooling units, and other critical components.

4:1 is an acceptable ratio that is tolerable for most SLA’s. N+2 and 2N are still more reliable for many reasons but cost much more.

Read about N+1 vs N+2 vs 2N vs 2N+1 (soon)

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