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Availability includes non-operational periods associated with reliability, maintenance, and logistics. This is measured in terms of nines. Five-9's (99.999%) means less than 5 minutes when the system is not operating correctly over the span of one year. Availability is only meaningful for supportable systems.
A highly available system would disable the malfunctioning portion and continue operating at a reduced capacity. In contrast, a less capable system might crash and become totally nonoperational. Availability is typically given as a percentage of the time a system is expected to be available, e.g., 99.999 percent ("five nines").
Barlow and Proschan [1975] define availability of a repairable system as "the probability that the system is operating at a specified time t." Blanchard [1998] gives a qualitative definition of availability as "a measure of the degree of a system which is in the operable and committable state at the start of mission when the mission is called ...
High availability (HA) is a characteristic of a system that aims to ensure an agreed level of operational performance, usually uptime, for a higher than normal period. [ 1 ] There is now more dependence on these systems as a result of modernization.
For databases reliability, availability, scalability and recoverability (RASR), is an important concept. Atomicity, consistency, isolation (sometimes integrity), durability is a transaction metric. When dealing with safety-critical systems, the acronym reliability, availability, maintainability and safety is frequently used.
Operational availability is used to evaluate the following performance characteristic. For a system that is expected to be available constantly, the below operational availability figures translate to the system being unavailable for approximately the following lengths of time (when all outages during a year are added together):
The terms high availability, continuous operation, and continuous availability are generally used to express how available a system is. [3] [4] The following is a definition of each of these terms. High availability refers to the ability to avoid unplanned outages by eliminating single points of failure. This is a measure of the reliability of ...
In engineering, reliability, availability, maintainability and safety (RAMS) [1] [2] is used to characterize a product or system: Reliability: Ability to perform a specific function and may be given as design reliability or operational reliability; Availability: Ability to keep a functioning state in the given environment