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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
The subject of the terms is thus important here: whether the focus of a discussion is the server hardware, server OS, functional service, software service/process, or similar, it is only if there is a single, consistent subject of the discussion that the words uptime and availability can be used synonymously.
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.
If we are using equipment which has a mean time to failure (MTTF) of 81.5 years and mean time to repair (MTTR) of 1 hour: MTTF in hours = 81.5 × 365 × 24 = 713940 (This is a reliability parameter and often has a high level of uncertainty!) Inherent availability (Ai) = 713940 / (713940+1) = 713940 / 713941 = 99.999860%
Mean Time To Recover (MTTR) is the length of time required to restore operation to specification. This includes three values. Mean Time To Discover; Mean Time To Isolate; Mean Time To Repair; Mean Time To Discover is the length of time that transpires between when a failure occurs and the system users become aware of the failure.
[10]: 241 In the article Stiglitz spoke of the damaging impact of economic inequality involving 1% of the U.S. population owning a large portion of economic wealth in the country, while 99% of the population hold much less economic wealth than the richest 1%: [I]n our democracy, 1% of the people take nearly a quarter of the nation's income …
[1] [citation needed] In early days of computing, availability was not considered business critical. With the increasing use of mobile computing, global access to online business transactions and business-to-business communication, continuous availability is increasingly important based on the need to support customer access to information ...
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.