Achieving very high levels of availability for
a system with varied and interdependent components is not usually simple or
inexpensive.
To provide some perspective, consider Table 11-1, which translates the percentage of
system availability into days, minutes, and hours of annual downtime based on a
365-day year.
What Is High Availability? | 255
Large-scale systems that achieve over 99 percent availability can cost millions of dollars
to design and implement and can have correspondingly high ongoing operational
costs. Marginal increases in availability can require large incremental investments in
system components. Moving from 95 to 99 percent availability is likely to be costly,
while moving from 99 to 99.99 percent will probably be costlier still.
Another key aspect of measuring availability is the definition of when the system
must be available. A required availability of 99 percent of the time during normal
working hours (e.g., from 8 a.m. to 5 p.m.) is very different from 99 percent availability
based on a 24-hour day.
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