'Nines' are shorthand for describing the actual or (usually) required reliability of a system. Each 'nine' represents a decimal place in the probability that the system will be available/online/functioning/whatever. So, for example, a system that is four nines reliable (or four nines available) will be working with a probability of 0.9999, or 99.99% of the time.

Reliability nines are using in planning and architecting systems of all sorts, ranging from computer networks to spacecraft. They are a good tool for understanding how complex systems that perform a simple task can become so expensive in terms of resources; recall that a system's aggregate reliability is the product of all its subsystems' reliabilities. Therefore, if you have a system that is made up of four subsystems, and each subsystem is four nines reliable, the system as a whole is only 0.9992, or three nines reliable! In systems such as the Space Shuttle, where literally tens of thousands of subsystems are present, it is nearly a physical impossibility to approach reliability numbers that are acceptable without using massive redundancy and overengineering - both of which are extremely expensive both in resources and in system 'payload' performance.

Since each nine represents a geometric increase in the required performance, the cost of adding nines to the reliability of a system is usually roughly equivalent to a tenfold increase in cost per nine.

“Nines” can be turned into time, for the purposes of discussions:

            Uptime        Monthly Downtime    Annual Downtime
Five Nines   99.999%             0:00:26              0:05:11
Four Nines   99.99%              0:04:19              0:51:50
Three Nines  99.9%               0:43.12              8:38:24
Two Nines    99%                 7:12:00             86:24:00

This assumes a thirty-day month, at twenty-four hours a day (720 Hours/Month). The annual number is simply twelve times the monthly (there may be some approximation). These figures should provide an idea what a given “nine” provides, though should be validated before writing it into a service level agreement.

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