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Availability

Availability is often measured by the fraction of time the system is up and running or (put another way) by the amount of time the system is down. In other words, how much of the time is the system available for use? The term five 9s is sometimes used to describe systems that are up 99.999% of the time. Table 19-1 compares some uptime percentages with the actual amounts of downtime per year. As you can see, achieving five 9s is quite respectable, as well as quite challenging.

One of the ugly truths about availability metrics is that they do not always count software bugs that cause failures for users. Most of the time, the availability that is being measured is whether or not the system or application is up and running and at least performing basic operations. In this context, a bug that causes the software to miscalculate one out of every ten transactions, for example, does not count as an availability problem.


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