N-Modular Redundancy Explained: N, N+1, N+2, 2N, 2N+1, 2N+2, 3N/2 (2024)

No matter if it’s software or hardware, any IT system should have built-in measures to ensure smooth operations—especially when you encounter unexpected issues. This means that any IT system should be dependable enough to handle unexpected situations which allow users to use the IT system confidently.

One way to achieve this dependability? Introducing redundancy into the system. Introducing redundancy helps to:

  • Minimize service interruptions due to failures
  • Create a fault tolerance architecture

In simple terms, redundancy refers to introducing multiple or duplicates of exciting components that can be used to carry out the required tasks in case of failure. There are different redundancy principles that can be applied to systems.

In this article, we will have a look at N-Modular redundancy and its corresponding redundancy levels.

What is N-modular redundancy?

Redundancy is a critical component in any good IT system. There are different kinds of redundancy levels with redundancy, and we’ll look at these below:

  • N
  • N+1, N+2
  • 2N, 2N+1, 2N+2
  • 3N/2

N redundancy

N redundancy refers to the bare minimum of required components for an IT system to operate. This level is characterized by two factors:

  • No redundancy solution is available for the system.
  • The system will be non-functional and inaccessible in case of a failure until the issue is diagnosed and resolved.

No system should operate at this redundancy level. This level does not provide any redundancy to the system. Instead, users should take this level as the base level and build on top of it to introduce proper redundancy to an IT system.

N+1, N+2 redundancy

As the name suggests, N+1 refers to the base level of resources required for the system functionality—plus a single backup. This is the minimum requirement for introducing redundancy to an IT system.

At this stage, the system can function while providing a single redundancy solution. This redundancy level is appropriate for a small IT system, but it’s not suited for medium- to large-scale systems.

N+2 refers to the next step up, and it consists of the resources needed for the system functionality plus two separate backups. This further increases the redundancy of an IT system as well as the confidence of end-users about the system since there are two separate backups. In this case, the system functionality can be restored through the other backup even if a single backup is corrupted.

In addition to N+1 and N+2, there may be instances where even more backups are maintained. These are referred to as N+X, where X stands for any number of backups to ensure the functionality of the system. This can be +3,+4,+5… Still, most command levels will be N+1 and N+2 unless there is a specific or unique requirement to keep multiple copies, such as a compliance policy.

2N, 2N+1, 2N+2 redundancy

N refers to the minimum number of resources (amount) required to operate an IT system. 2N simply means that there is twice the amount of required resources/capacity available in the system.

For a simple example, let’s consider a server in a data center that has ten servers with an additional ten servers that act as a dedicated capacity. This reserves a combination of 20 servers in total, providing 2N redundancy. This way, 2N always provides an excess capacity to the IT system.

2N+1 correlates to a system with twice the required resources/capacity to function normally plus a backup as an additional redundancy step. These additional backup systems can provide redundancy even if there is an issue with all of the additional capacity.

2N+2 refers to a system with additional capacity plus two backups to provide one of the highest levels of redundancy. In this method, if there are ten servers in a data center, it will have another ten identical servers as reserved capacity while having two more servers that will act as backups in case of an emergency.

2N+2 is considered the highest level of redundancy methodology that is commonly used in the IT industry.

3N/2, 4N/3 redundancy

3N/2, 4N/3, or more specifically AN/B, refers to a redundancy methodology where additional capacity is based on the load of the system.

For example, consider 3N/2 redundancy applied to power delivery infrastructure in a data center environment. In this instance, there will be separate power delivery systems powering two workloads (two servers). This results in each power delivery system only using 67% of the available capacity at a specific time (or the inverse of the 3N/2 ratio).

If we consider a 4N/3 scenario, four power delivery systems will power three servers, resulting in each power delivery system utilizing 75% of the available capacity.

Redundancies can degrade performance

All the methodologies mentioned above provide some kind of redundancy to an IT system.

However, it’s crucial to understand that using backups (spare components/resources) may lead to performance degradations. That’s because these spare resources may not be identical to the original system resources in terms of capacity. This only affects backups or (+X) scenarios when dealing with capacity reservation (XN), as the additional capacity is identical to the operating system without any impact on the performance of the system.

Redundancy configurations

Active, passive, and load sharing (standby) are redundancy configurations available when implementing a redundancy methodology.

  • Active. In an active configuration, the redundant component is operated simultaneously with the original component. However, in case the original fails, the redundant component will be used.
  • Passive. In a passive config, redundant component is available yet not operational while the original component is active. It will be activated to provide functionality in the event of a failure.
  • Load sharing (standby). This fulfills the availability gap until the original or active component is completely available. Additionally, load sharing can be used here as a partial or a temporary redundancy method to provide additional capacity.

The configuration and the methodology you choose will depend on facts such as:

  • User and system requirements
  • Cost
  • Resource availability
  • Compliance requirements

The higher the level of redundancy requirements in a data center, a more complex redundancy methodology will be required to handle it. While this complexity will lead to higher costs and resource requirements to implement and manage properly, it will provide the best redundancy for the underlying system.

Redundancy increases reliability & availability

The dependability of an IT system will differ based on its reliability and availability. Redundancy is one of the best ways to increase the reliability and availability of a system using different redundancy methodologies.

One factor that affects the complexity and requirements of a redundancy method is the failure rate of individual components of a system:

  • If individual components have low failure rates, a simple redundancy policy can be applied to provide high availability.
  • Components with higher failure rates require complex redundancy policies to mitigate any issues and provide guaranteed availability.

Another factor to consider when implementing a redundancy methodology is the workload of the system, as higher workloads cause system components to be constantly stressed, leading to faster component degradation.

(Understand how redundancy affects availability.)

Redundancy is critical

Redundancy is an essential component in any IT system. It ensures that an IT system can function properly in the event of an unforeseen issue within this rapidly evolving technological landscape. Thus, redundancy allows users to use the IT system reliably, leading to a more satisfying end-user experience.

Related reading

  • BMC IT Operations Blog
  • Reliability vs Availability: What’s The Difference?

These postings are my own and do not necessarily represent BMC's position, strategies, or opinion.

See an error or have a suggestion? Please let us know by emailing blogs@bmc.com.

N-Modular Redundancy Explained: N, N+1, N+2, 2N, 2N+1, 2N+2, 3N/2 (2024)


What is the N modular redundancy? ›

N Modular Redundancy, also known as Parallel Redundancy, refers to the approach of having multiply units running in parallel. All units are highly synchronized and receive the same input information at the same time. Their output values are then compared and a voter decides which output values should be used.

What is n-1 and n-2 redundancy? ›

While N+1 introduces some redundancy, it still presents a risk in the event of multiple simultaneous failures. To minimize this risk, some data centers utilize an N+2 redundancy design to provide two extra components. In our example, this would provide six UPS units instead of five.

What is 3N-2 redundancy? ›

The three-to-make-two or 3N/2 redundant model refers to a redundancy methodology where additional capacity is based on the load of the system.

What is the N 1 level of redundancy? ›

With an N+1 system, the 'N' still stands for the minimum number of independent modules required for a system to operate. The "1" in this equation refers to the number of additional components that act as an independent backup that allows the system to operate as intended if one of the modules within 'N' fails.

What is the formula for redundancy? ›

Working backwards from the relevant date, you must add all of the following that apply: 1 and a half week's pay for each full year they were aged 41 years or older. 1 week's pay for each full year they were aged 22 or older, but aged under 41. half a week's pay for each full year they were aged under 22.

What does 2N mean in electrical? ›

2N definition

This means that even if one power source has an interruption or loss of power, the other should still supply power and accommodate full load, thereby eliminating any potential downtime from the loss of one side or leg of the system.

What is 4 to make 3 redundancy? ›

The “four-to-make-three” (4M3) design automatically switches power distribution to the remaining systems — whether due to unplanned downtime or for cases of maintenance and repair — until full power distribution can be restored.

What is meant by n-1 redundancy? ›

N+1 redundancy is a system design where there is one additional component available to support a failure or required maintenance on a component. This means that if the component has an interruption or an outage, another source can supply that component to some capacity.

What are the three R's of redundancy? ›

The 3 R's - Restructure, Reorganisation and Redundancy.

What is n/m redundancy? ›

The definition of N+M redundancy requires to consider M failures in arbitrary order, thus considering super-exponentially many cases for large M. As, however, balancing moves instances anyway, the redundancy level mainly depends on the amount of node resources available to the instances in a node group.

What is the formula for redundancy in math? ›

The formula for redundancy is R = 1 - H/Hmax (Note R is always less than 1).

What is an example of 2N redundancy? ›

For a simple example, let's consider a server in a data center that has ten servers with an additional ten servers that act as a dedicated capacity. This reserves a combination of 20 servers in total, providing 2N redundancy. This way, 2N always provides an excess capacity to the IT system.

How do you determine redundancy? ›

How to evaluate potentially redundant roles
  1. Identify essential roles within your company. ...
  2. Review key personell and their ability to take on additional roles within your company. ...
  3. Consider which roles or departments currently present limited benefits. ...
  4. Evaluate the employment history of each employee in those roles.

What is redundancy calculator? ›

If you're being made redundant, or think you might be at risk, there are some steps you need to take. Our redundancy pay calculator can help with all of that. You should understand your rights, work out what you are owed, see how long your money will last, check your insurance and see what benefits you're entitled to.

What is module redundancy? ›

A redundancy module is used to decouple two power supplies and ensures the high availability and productivity of your system. A redundant power supply is particularly necessary in applications with stringent demands regarding operational safety.

What is modular ratio N? ›

Modular ratio (n) is the ratio of the elastic modulus of a particular material in a cross-section to the elastic modulus of the “base” or the reference material.

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