99.999 (Five nines or Five 9s)

In computers, 99.999 (often called "five 9s") refers to a desired percentage of availability of a given computer system. Such a system would probably have what some refer to as high availability. As Evan Marcus, Principal Engineer at Veritas Software, observes, 99.999 availability works out to 5.39 minutes of total downtime - planned or unplanned - in a given year.

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In one view, there are three approaches to 99.999 (or even 100) percent availability:

• Special systems that are designed for high availability, such as those from Tandem (Compaq) and Stratus, in which components are duplicated so that a backup component is always available. These tend to be expensive, requiring redundancy of components in which the redundant component is seldom used.
• Shared component systems that are built so that one active system can back up another active system if one of them fails.
• Clustering, a variation of the second approach, in which matched components do not necessarily have to be duplicates of each other