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3.5.2 Availability, a complement to reliability

For many products, availability is an important consideration in setting a failure intensity objective. Availability A is the fraction of time during which a system is functioning acceptably. It is given by


where is the uptime, and is the downtime. We may express the downtime as


where is the downtime per failure and l is the failure intensity.
For software, we usually take the downtime per failure as the time to recover from the failure, not the time required to find and remove the fault that is causing the failure. From Equations (3.20) and (3.21) we obtain


If we specify the availability for a product, then the downtime per failure will determine what the failure intensity objective must be:


For example, if a product must be available 99 percent of the time and downtime is 6 minutes (0.1 hr), then the failure intensity objective will be approximately 0.1 failure / hr or 100 failures / Khr.