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The following concepts are fundamental to the understanding and application of Availability Management.


1. Availability
2. Service Availability
3. Component Availability
4. Reliability
5. Maintainability
The ability of a service, component or CI to perform its agreed function when required. It is typically measured an reported as a percentage using the following formula:

Availability (%) = Agreed Service Time – Downtime x 100 %
Agreed Service Time

This means that if a service is only partly functional, or the performance is degraded to a point outside of normal service operation, then the service should be classed as unavailable.

Involves all aspects of service availability and unavailability and the impact of component availability.
Involves all aspects of component availability and unavailability

A measure of how long a service, component or CI can perform its agreed function without interruption. This metric provides an understanding of the frequency of disruption and is often reported as Mean Time Between Service Incidents (MTBSI) or Mean Time Between Failures (MTBF). It is typically calculated with the formulas:

Reliability (MTBSI) = Available time in hours
Number of service disruptions

Reliability (MTBF) = Available time in hours – Total Downtime in hours
Number of service disruptions

A measure of how quickly and effectively a service, component or CI can be restored to normal operation after a failure. This metric is typically measured and reported as the Mean Time to Restore Service (MTRS), which includes the entire time from the start of the disruption until the full recovery. The following formula is normally used:

Maintainability (MTRS) = Total downtime in hours
Number of service disruptions
EXAMPLE: For a service that is provided 24 x 7 and running for a reporting period of 5020 hours with only two disruptions (one of 6 hours and one of 14 hours), the following metrics would result:

Availability (%) = 5020 – 20 x 100 % = 99.60%
Reliability (MTBSI) = 5020 = 2510 hours

Reliability (MTBF) = 5000 = 2500 hours

Maintainability (MTRS) = 20 = 10 hours
6. Serviceability
The ability of an external (third-party) supplier to meet the terms of their contract. Often this contract will include agreed levels of availability, reliability and/or maintainability for a supporting service or component.

7. Vital Business Function (VBF)

8.High Availability

9. Fault tolerance

10.Continuous operation

11. Continuous availability
Defined business critical elements of a business process that are supported by an IT service. While many functions are supported by IT, we typically prioritize our efforts and resources around supporting the critical elements, including the use of redundant and highly resilient components.

Certain VBFs may need special designs which are now used commonly in key infrastructure components (such as servers), which include the following four concepts.

A characteristic of the IT service that minimizes or masks the effects of component failure to the users of a service.

The ability of a service, component or CI to continue to operate correctly after failure of a component part.

An approach or design to eliminate planned downtime of an IT service. This may mean that individual components are disrupted during maintenance, but the IT service as a whole remains available.

An approach or design to achieve theoretical 100% level of service availability. Multiple design factors will support this to occur, but more stringent requirements will also be assessed (e.g. environment).

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