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Availability, Reliability and Serviceability

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.

Availability is a measure of the amount of time a system or component performs its specified function. Availability is related to, but different than reliability. Reliability measures how frequently the system fails; availability measures the percentage of time the system is in its operational state.

To calculate availability, both the Mean Time To Failure (MTTF) and the Mean Time To Recovery (MTTR) need to be known. The MTTR is a measure of how long, on average, it takes to restore the system to its operational state after a failure. If both the MTTF and the MTTR are known, availability can be calculated using the following formula:

* Availability = MTTF / (MTTF + MTTR)

For example, if the data center takes an average of six months to fail (MTTF = six months) and it takes 20 minutes, on average, to return the data center to its operational state (MTTR = 20 minutes), then the data center availability is:

* Availability = 6 months / (6 months + 20 minutes) = 99.992 percent.

Therefore, there are two ways to improve the availability of the system; increase MTTF or reduce MTTR. Having realized that system failures do occur or are unavoidable, system and database administrators need to focus on designing a reliable system with redundant components as well as setting up reliable recovery methodology for when system failures happen.

Availability is actually the probability that an application service is available for use. Availability is usually expressed as a percentage of hours per week, month, or year during which an application service can be used for normal business.

To a large extent, reliability and serviceability influence availability. The more failure reduction built into a system, the more available it is likely to be. Features and technologies that reduce failure recovery time, or that speed up diagnosis and repair, increase a system?s overall availability. The acronym RAS, which stands for reliability, availability, and serviceability, is often used in describing availability features of a system.


Reliability is the starting point for building increasingly available systems, since a measure of system reliability is how long it has been up and/or how long it typically stays up between failures. The nature of the failure is not important ? any failure affects the system?s overall availability. As presented in the previous section, mean time between failures (MTBF) is often considered an important metric with respect to measuring system reliability.

There are two primary means of achieving greater reliability:

* Building high MTBF components into the system.

* Adding MTBF components in redundant (N+1) configurations.

Including technology that maintains data integrity, reducing the probability that bad data will flow undetected through the system.


Serviceability defines the time it takes to isolate and repair a fault or, more succinctly, the time it takes to restore a system to service following a failure. Mean Time to Repair, or MTTR, is considered an important metric when discussing the serviceability of a system or some component of the system. MTTR, however, is a unit of time and does not factor into the cost of service.


This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


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