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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.
Availability means the probability that a system is operational at a given time, i.e. the amount of time a device is actually operating as the percentage of total time it should be operating. High-availability systems may report availability in terms of minutes or hours of downtime per year.
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):
Systems that exhibit truly continuous availability are comparatively rare and higher priced, and most have carefully implemented specialty designs that eliminate any single point of failure and allow online hardware, network, operating system, middleware, and application upgrades, patches, and replacements. For certain systems, scheduled ...
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 ...
The operating system may choose to adjust the scheduling of each transition (high-low or low-high) based on an internal clock. The latency is the delay between the process instruction commanding the transition and the hardware actually transitioning the voltage from high to low or low to high.
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 ...
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.