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Definition Of 99.999% Availability

Five-nines reliability and what it stands for.

Network infrastrutcure providers, Telephone companies and Software as a Service (SaaS), as well as network equipment providers claiming to offer 99.999% up-time or reliability. But how many people actually know what it means and that a 99.999% reliability comes at a premium price.

The five-nines reliability comes originally from the telecom industry.

In a matter of fact, what is claimed as reliability is actuality availability. It means how much of the time a network or a piece of equipment is available for you to use. Availability is measured in how often something breaks and how long it takes to become available again. A server, router or switch that crashes and is up again within a couple of minutes falls under the five-nines reliability.

Should the problem be bigger and the fix takes 15 or 20 minutes it wouldn’t fall under the 99.999% reliability anymore but under the 99.99%.

To give you an idea what the various measurements mean, following a few examples:

99.9999% (Six-Nines) availability does only allow for 32 seconds or less of downtime per year

99.999% (Five-Nines) availability allows for 5 minutes and 15 seconds or less of downtime per year

99.99% (Four-Nines) availability allows for 52 minutes, 36 seconds or less of downtime per year

99.9% (Three-Nines) availability allows for 8 hours, 46 minutes or less of downtime per year

99% (Two-nines) availability allows for 3 days 15 hours and 40 minutes per year

In order to get more availability more reliable and expensive equipment needs to be put in place. It means also, buying not only the best equipment but also the easiest to repair and building redundancy.

In addition make sure that all components have the same rating. How much does it help, if your core server hardware has a 99.999% reliability, but some of the components in the server, like network interface cards (NIC) have only a 99% reliability.

It is also important to ask how much reliability is needed, since higher reliability will come at a premium price/cost.

When offering service, evaluate whether your clients need 99.999% reliability or maybe they can live with a 99% reliability, since you then can chose when to take you network or service offline for maintenance, since that is also a part that needs to be considered in your availability calculation.


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