High-availability (HA) clusters¹

High-availability clusters (a.k.a Failover clusters) are implemented primarily for the purpose of improving the availability of services that the cluster provides. They operate by having redundant nodes, which are then used to provide service when system components fail. The most common size for an HA cluster is two nodes, which is the minimum requirement to provide redundancy.

HA cluster implementations attempt to manage the redundancy inherent in a cluster to eliminate single points of failure, including multiple network connections and data storage that is multiply connected via storage area networks.

HA clusters usually use a heartbeat private network connection that is used to monitor the health and status of each node in the cluster.

Availability²

Availability is the ability of an IT Service or component to perform its required function at a stated instant or over a stated period of time. Availability is correlated to the reliability and maintainability of the IT Infrastructure and effectiveness of the IT support organization. Availability depends on the:

• availability of components
• resilience to failure
• quality of maintenance and support
• quality, pattern and extent of deployment of operational process and procedures
• security, integrity and availability of data

Reliability²

The reliability of an IT Service can be qualitatively stated as freedom from operational failure. The reliability of an IT Service is determined by the:

• reliability of each component within the IT Infrastructure delivering the IT Service, i.e. the probability that a component will fail to provide its required functions
• level of resilience designed and built into the IT Infrastructure, i.e. te ability of an IT component failure to be masked to enable normal business operations to continue.

Maintainability²

Maintainability relates to the ability of an IT Infrastructure component to be retained in, or restored to, an operational state. Maintainability of an IT Infrastructure component can be divided into 7 separate stages:

• anticipation of failures
• detection of failures
• diagnosing of failures
• resolving of failures
• recovery from failures
• restoration of the data and IT Service
• levels of preventive maintenance applied to prevent failures occurring

Serviceability²

Serviceability describes the contractual arrangements made with Third Party IT service providers to provided or maintain IT Services or components.

¹ From Wikipedia
² From ITIL Service Delivery