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High Availability

Measuring Availability
Availability is the fraction of time that the system is actually capable of performing its mission.

On the product side, availability is easiest to define at the component level, whether the component is a board, a power supply, an operating system, or even an entire computer system. Uptime/year, downtime/year, and % available are metrics. [IEEE, 1988] The component (or system) is either available or unavailable. Two different measurements contribute to this definition:

- MTTF Mean Time To Failure; the average time from an initial failure-free state to a failure.

- MTTR Mean Time To Repair; the average time after a failure has occurred until the failure is diagnosed and the component is back in service.

Here, (MTTF + MTTR) equals total time. So the above equation translates to Uptime divided by Total Time. This availability measurement can be applied to components, subsystems, or an entire system.

Measuring availability becomes more complex and difficult in a distributed system. Whenever processing is distributed, transaction information, or "state" can also be distributed. Failures on one system may require the other systems to do special recovery in order to return to a known good state. This issue arises when the applications run on separate computers from the database as well as in more extensive client-server environments.

In spite of this complexity, often the only methodology possible for calculating system availability is careful use of the component availability formula, and it will likely produce optimistic figures.

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