Redundancy: N+1, N+2 vs. 2N vs. 2N+1

A typical definition of redundancy in relation to engineering is:  “the duplication of critical components or functions of a system with the intention of increasing reliability of the system, usually in the case of a backup or fail-safe.”   When it comes to datacenters,  the need for redundancy focuses on how much extra or spare power the data center can offer its customers as a back up during a power outage.  Unexpected power outages are the overwhelming usual cause for datacenter downtime.*

 

Photo Courtesy of the Ponemon Institute

Photo Courtesy of the Ponemon Institute

According to the industry-leading Ponemon Institute’s 2013 Study on Data Center Outages (or “downtime” – a four-letter word in the data center industry) that surveyed 584 individuals in U.S. organizations who have responsibility for data center operations in some capacity, from the “rank and file” to C-Level, 85% participants report their organizations experienced a loss of primary utility power in the past 24 months.  And of that 85% – 91% reported their organizations had an unplanned outage.  That means that most data centers experienced downtime in the last 24 months.  During these outages respondents averaged two complete data center shutdowns, with an average downtime of 91 minutes per failure.

The entire study also speaks of the implementation and the impact of DCIM (Data Center Infrastructure Management) – and how it was used to fix or correct the root cause of the outages.*

The most common are due to the weather, but they can also occur from simple equipment failure or even an accidental cutting of a power line due to a backhoe.  No matter what the reasons are, an unplanned outage can cost a company a lot of money, especially if there revenues are dependent upon internet sales.

For example, if you’re Amazon and you go down, you lose a mind-blowing amount of money: an estimated $1,104 in sales for every second of downtime. The “average” U.S. data center loses $138,000 for one hour of data center downtime per year.  So, if one compares Ponemon’s 91-minute average downtime per year – that’s an approximate loss of $207,000 for each organization accessing the data center.

What’s this all mean?  Downtown matters, and downtime prevention matters, so Redundancy matters.

Perferably, large businesses and corporations  have their servers set up at either Tier 3 or Tier4 data centers because they offer a sufficient amount of redundancy in case of a unforeseen power outage. With this in mind, not all data center’s redundancy power systems are created equal.  Some offer N+1, 2N, and 2N +1 redundancy systems.

What’s the Difference Between N+1, 2N and 2N+1?

The simple way to look at N+1 is to think of it in terms of throwing a birthday party for your child or yourself, because who doesn’t love cupcakes?.  Say you have ten guests and need ten cupcakes, but just in case you have that  “unexpected” guest show up, you order eleven cupcakes.  “N” represents the exact amount of cupcakes you need, and the extra cupcake represents the +1.  Therefore you have N+1 cupcakes for the party.  In the world of datacenters, an N+1system, also called parallel redundancy, and is a safeguard to ensure that an uninterruptible power supply (UPS) system is always available. N+1 stands for the number of UPS modules that are required to handle an adequate supply of power for essential connected systems, plus one more, so 11 cupcakes for 10 people, and less chance of downtime.

Although an N+1 system contains redundant equipment, it is not, however, a fully redundant system and can still fail because the system is run on  common circuitry or feeds at one or more points rather than two completely separate feeds.

Back at the birthday party!  If you plan a birthday party with a 2N redundancy system in place, then you would have the ten cupcakes you need for the ten guests, plus an additional ten cupcakes, so 20 cupcakes.  2N is simply two times, or double the amount of cupcakes you need.   At a data center, a 2N system contains double the amount of equipment needed that run separately with no single points of failure.  These 2N systems are far more reliable than an N+1 system because they offer a fully redundant system that can be easily maintained on a regular basis without losing any power to subsequent systems.  In the event of an extended power outage, a 2N system will still keep things up and running.  Some data centers offer 2N+1, which is actually double the amount needed plus an extra piece of equipment as well, so back at the party you’ll have 21 cupcakes, 2 per guest and 3 for you!

 For more information on Redundancy, N+1, 2N, 2N+1, and the difference between them, as well as, the different Tier levels offered by datacenters around the world visit www.datacenters.com or call us at (877) 406-2248.

*Sources: Ponemon Institute 2013 Study on Data Center Outages: Sponsored by Emerson Network Power – This link will take you to the entire study, which is also an interesting read about the how data center employees view their data center’s structure and superiors.

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