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More simply, going from 99.9% availability to 99.95% availability is a factor of 2 (0.1% to 0.05% unavailability), but going from 99.95% to 99.99% availability is a factor of 5 (0.05% to 0.01% unavailability), over twice as much. [note 3] A formulation of the class of 9s based on a system's unavailability would be
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
In reliability engineering, the term availability has the following meanings: . The degree to which a system, subsystem or equipment is in a specified operable and committable state at the start of a mission, when the mission is called for at an unknown, i.e. a random, time.
For example, 90% would be described as "one nine"; 99% as "two nines"; 99.9% as "three nines"; and so forth. However, there are different conventions for representing inexact multiples of 9. 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 ...
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.
[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.
A service-level objective (SLO), as per the O'Reilly Site Reliability Engineering book, is a "target value or range of values for a service level that is measured by an SLI." [1] An SLO is a key element of a service-level agreement (SLA) between a service provider and a customer. SLOs are agreed upon as a means of measuring the performance of ...
For outages due to issues with general computer systems, the term computer outage (also IT outage or IT drought) can be used. The term is also commonly applied in industrial environments in relation to failures in industrial production equipment. Some facilities measure the downtime incurred during a work shift, or during a 12- or 24-hour period.