2. What is reliability

You say a person is 'reliable' when you are confident that they will do what is expected of them.

Computer systems are much the same, but in this case, reliability can be measured i.e. it has a 'metric'.

One of the targets that a typical system administrator will have is the reliability of the system they are responsible for.

The 'metric' tends to be an average measure of some kind. For example the ones below can be used to assess reliability.

AVAIL

This is the percentage of time that the system is available to the user or customer but taking out any planned downtime perhaps for maintenance.

For example, if, in an uptime planned period of 1000 hours a computer system is available for 950 hours. The system AVAIL figure is 95%. i.e. 950/1000 * 100%.

MTTF

Stands for 'Mean Time To Failure'.

This is the average number of hours that the system runs for before something breaks.

This tends to be a measure for hardware such as a server. For example if a hard disk fails on average after 500 hours, then its MTTF figure is 500 hours.

There are a number of other metrics of reliability as well such as 'probability of failure of demand'.