Availability Fundamentals
Steady-state availability is a commonly quoted availability metric for most computer systems. Availability is often measured by the uptime ratio—this is a close approximation of the steady-state availability value and represents the percentage of time a computer system is available throughout its useful life. Another frequently used availability metric is downtime per year, which is calculated using the following formula:
A system is often rated by the number of 9s displayed in its availability classification. Table 1-1 presents the different availability classes and corresponding annual downtime. For example, a system ...
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