If there is one thing I learned early in my career, it is “metrics can show you anything you want them to show”. One metric companies often use to measure the availability or uptime of their systems is “number of 9s”. Computer system uptime is often measured by the percentage of uptime/availability in a year. When you hear the term 9s it is referring to the number of 9s in the percentage.

In other words, five 9s equals 99.999% uptime. This is really good availability of a system. This means that this system was available for use all but 5 MINUTES in a single year. Most systems, particularly Microsoft, require regular application of patches, either for security, bug fixes, or new functionality. If you are applying patches to your systems, which usually require a reboot, then there is no way you can achieve 99.999% uptime in a year. If you’re designing a highly-available system, then you need to plan for this and use some type of clustering or distributed computing, where you are able to patch one node or component while the other is still providing service.

The following chart shows the amount of downtime allowed for a system based on the number of 9s.

Number of 9s

Percentage of Uptime

Amount of Downtime
(Year)

Two 9s 99% 3 Days 15 hours
Three 9s 99.9% 8 Hours 45 Minutes
Four 9s 99.99% 52 Minutes 33 Seconds
Five 9s 99.999% 5 Minutes 15 Seconds

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