To attack both of these problems, high availability systems invariably
contain redundant components (High Availability and More, 2001; Standards for a Service
Availability, 2002).
??? Redundancy:.Redundancy augments the reparability of individual components by
establishing a backup or standby for various components in the system. When there
are multiple resources providing the same functionality, they can take over for one
another very quickly in case of failure, driving the system MTTR toward zero. The
effectiveness of redundancy is a function of how much redundancy a system contains,
and how quickly backup components can be brought into service when failures occur.
Costs.of.Service.Downtime..
Uptime is usually the measurement of availability that refers to the time at which users can
access the application. Downtime is the opposite of uptime; it is the amount of time that a
database system or service becomes unavailable to users.
There are two ways to categorize system downtime. Some downtimes are results from a
system failure and others are results from scheduled outages. Scheduled outages, such as
those for repair and upgrades that have minimal impact on the business, are considered
maintenance.
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