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Why might someone use an N+1 approach for a cluster?

By Justin Stoltzfus | Last updated: April 27, 2017
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N+1 or N+1 redundancy is a popular concept in network virtualization and the design of IT architectures. Companies generally use this design to provide effective backup or ensure smooth system operation with a single point of failure.

The name “N+1” signifies a process by which engineers include a range of functioning nodes in a cluster, and then add one extra, so that if there is a single point of failure, that one extra unit can stand in the gap. This process can also be called “active/passive” or “standby” redundancy.

Companies use an N+1 design to make sure that if one server or virtual machine fails, the system is not impacted. However, a greater discussion has emerged about whether N+1 redundancy is sufficient for a given system. There is the recommendation against trying to provide a one-size-fits-all approach when providing redundancy for high availability. IT pros also understand that the stricter a client is with high availability requirements, the more redundancy is needed.

In response to this philosophy, engineers have provided things like N+X+Y, in which many more resources are added to the system to make sure that even a multipoint failure doesn't impact operations. Another particular consideration is the size of each virtual machine or node in the cluster – for example, if a single VM is 100 GB and the others are under 50 GB, an N+1 approach would not ensure functionality if that larger VM is compromised.

In general, N+1 is simply a tool and an approach to managing resources like CPU and memory across any shared environment such as a network cluster. It is evaluated for its effectiveness and efficacy in a particular IT system depending on resource allocation and the overall setup.

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Written by Justin Stoltzfus | Contributor, Reviewer

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Justin Stoltzfus is a freelance writer for various Web and print publications. His work has appeared in online magazines including Preservation Online, a project of the National Historic Trust, and many other venues.

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