In the same way that you must carefully define your
required levels of availability, you must also consider the hours during which availability
is measured. For example, a lot of companies take orders during ???normal???
business hours. The cost of an unavailable order-entry system is very high during the
business day, but drops significantly after hours. Thus, scheduled downtime can make
sense after hours that will, in turn, help reduce unplanned failures during business
hours. Of course, in some multinational companies and the world of the Internet, a
global reach implies that the business day never ends.
That initial requirement that a system be available 24/7 must be put in the context of
the cost in deploying and maintaining such a system. An examination of the
complexity and cost of very high availability will sometimes lead to compromises
reducing goals and budgets for system availability.
The costs of achieving high availability are certainly justified in some cases. It might
cost a brokerage house millions of dollars for each hour that key systems are down.
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