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Availability is the probability that an item will be in an operable and committable state at the start of a mission when the mission is called for at a random time, and is generally defined as uptime divided by total time (uptime plus downtime).
Mean Time Between Failure (MTBF) depends upon the maintenance philosophy. If a system is designed with both redundancy and automatic fault bypass, then MTBF is the anticipated lifespan of the system if these features cover all possible failure modes (infinity for all practical purposes). Such systems will continue without noticeable ...
A system that has been up for 365 days in a non-leap year might have been eclipsed by a network failure that lasted for 9 hours during a peak usage period; the user community will see the system as unavailable, whereas the system administrator will claim 100% uptime. However, given the true definition of availability, the system will be ...
Uptime is a measure of system reliability, expressed as the period of time a machine, typically a computer, has been continuously working and available. Uptime is the opposite of downtime . Htop adds an exclamation mark when uptime is longer than 100 days.
Five nines, commonly taken to mean "99.999%", may refer to: High availability of services, when a service is available for 99.999% of the time, or around 5 minutes of downtime per year; Nine (purity), a 99.999% pure substance; German 15 cm (5.9 in) artillery shells used in World War I
[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 the hardware, operating system, middleware, and database manager software.
For example, a percentage of 99.5% could be expressed as "two nines five" (2N5, or N2.5) [2] or as 2.3 nines, [citation needed] following from the logarithm definition. A percentage of 100% would, in theory, have an infinite number of nines – though, in the context of purity of materials, 100% is virtually unachievable.
While usually sub-second units are represented with SI prefixes on the second (e.g. milliseconds), this system can be extrapolated further, such that a "Third" would mean 1 ⁄ 60 of a second (16.7 milliseconds), and a "Fourth" would mean 1 ⁄ 60 of a third (278 microseconds), etc. These units are occasionally used in astronomy to denote angles.