Availability (ITILv3): [Service Design] Ability of a Configuration Item or IT Service to perform its agreed Function when required. Availability is determined by Reliability, Maintainability, Serviceability, Performance and Security. Availability is usually calculated as a percentage. This calculation is often based on Agreed Service Time and Downtime. It is Best Practice to calculate Availability using measurements of the Business output of the IT Service.
See also: [ITILv3] Access Management, [ITILv3] Agreed Service Time, [ITILv3] Design
Availability (ITILv2): An umbrella term that includes reliability (including resilience), maintainability, serviceability, and security. A common definition of availability is "the ability of a component or IT service (under combined aspects of its reliability, maintainability and security) to perform its required function at a stated instant or over a stated period of time". Service availability is sometimes expressed as an availability percentage, i.e., the proportion of time that the service is actually available for use by the customers within the agreed service time: (see above formula)
However, this definition of service availability is generally considered to be archaic and immeasurable to any party's real satisfaction in a modern IT environment. Current best practice suggests that availability should be expressed in business centric terms, focusing on the impact of unavailability on business processes.
See also: Availability Management
Availability (ITILv1): An umbrella term to also include serviceability, resilience, reliability and maintainability. The ability of a component or IT service (under combined aspects of its reliability, maintainability and maintenance support) to perform its required function at a stated instant or over a stated period of time. It is usually expressed as the availability ratio, i.e., the proportion of time that the service is actually available for use by the customers within the Agreed Service Time and Downtime. This is calculated as follows: (see above formula)
See also: Availability Management
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There are many views of measuring availability. Some people have simplistic views of five 9s thrust upon them, even for systems of little consequence to the production chain. Yet, the heavy linkage of systems and services also requires that we consider availability as an end-to-end topic, i.e., can work be completed, are all of the applications, databases, systems and networks up in the chain. Finally, the concept of availability under reduced performance emerges as perhaps the most difficult to measure.
The "Classical Availability" formula often seems to be the most practical.
The formula is easy to understand because any Service Level is really nothing more than an expression of assurance, the promise. The level of assurance becomes a function of contract, yet, also of qualitative perception. Need for the promise finds its derivation in user requirements. Our level of assurance in the promise, is a function of trust because it the level of effort increases with the number of factors considered. Setting the level of availability to a number less than 100% represents an acceptance by the customer that there are risks beyond the control of the provider.
The concepts of component availability are equally important and not to be ignored. The late Dr. Juran classified this form of availability under "Parameters of Fitness for Use". As he stated in his text: Juran's Quality Control Handbook, "A product is said to be available when it is in an operative state. The total time in the operative state (also called uptime) is the sum of the time spent (1) in active use and (2) in standby state. The total time in the non-operative state (also called downtime) is the sum of the time spent (3) under active repair (i.e., diagnosis and remedy), and (4) waiting for spare parts, paperwork, etc." Therefore, availability can be expressed as a mathematical ratio:
As most often this deals with components, we reference this as the component availability formula, again described as a ratio:
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