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Number of service breaks



MTRS should be used to avoid the ambiguity of the more common industry term Mean Time To Repair (MTTR), which in some definitions includes only repair time, but in others includes recovery time. The downtime in MTRS covers all the contributory factors that make the service, component or CI unavailable:

  • Time to record
  • Time to respond
  • Time to resolve
  • Time to physically repair or replace
  • Time to recover.

Example: A situation where a 24 x 7 service has been running for a period of 5,020 hours with only two breaks, one of six hours and one of 14 hours, would give the following figures:

Availability = (5,020–(6+14)) / 5,020 x 100 = 99.60%

Reliability (MTBSI) = 5,020 / 2 = 2,510 hours

Reliability (MTBF) = 5,020–(6+14) / 2 = 2,500 hours

Maintainability (MTRS) = (6+14) / 2 = 10 hours

Serviceability: the ability of a 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.

These aspects and their inter-relationships are illustrated in Figure 4.14.

Figure 4.14 Availability terms and measurements

Although the principal service target contained within SLAs for the customers and business is availability, as illustrated in Figure 4.14, some customers also require reliability and maintainability targets to be included as well. Where these are included they should relate to service reliability and maintainability targets, whereas the reliability and maintainability targets contained in OLAs and contracts relate to component and supporting service targets and can often include availability targets relating to the relevant components or supporting services.

The term Vital Business Function (VBF) is used to reflect the business critical elements of the business process supported by an IT service. An IT service may support a number of business functions that are less critical. For example, an automated teller machine (ATM) or cash dispenser service VBF would be the dispensing of cash. However, the ability to obtain a statement from an ATM may not be considered as vital. This distinction is important and should influence availability design and associated costs. The more vital the business function generally, the greater the level of resilience and availability that needs to be incorporated into the design required in the supporting IT services. For all services, whether VBFs or not, the availability requirements should be determined by the business and not by IT. The initial availability targets are often set at too high a level, and this leads to either over-priced services or an iterative discussion between the service provider and the business to agree an appropriate compromise between the service availability and the cost of the service and its supporting technology.



Certain VBFs may need special designs, which are now being used as a matter of course within Service Design plans, incorporating:

  • High availability: a characteristic of the IT service that minimizes or masks the effects of IT component failure to the users of a service.
  • Fault tolerance: the ability of an IT service, component or CI to continue to operate correctly after failure of a component part.
  • Continuous operation: an approach or design to eliminate planned downtime of an IT service. Note that individual components or CIs may be down even though the IT service remains available.
  • Continuous availability: an approach or design to achieve 100% availability. A continuously available IT service has no planned or unplanned downtime.

Many suppliers commit to high availability or continuous availability solutions only if stringent environmental standards and resilient processes are used. They often agree to such contracts only after a site survey has been completed and additional, sometimes costly, improvements have been made.

Availability Management commences as soon as the availability requirements for an IT service are clear enough to be articulated. It is an ongoing process, finishing only when the IT service is decommissioned or retired. The key activities of the Availability Management process are:





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