Reliability refers to the probability that a system will perform its intended function without failure over a specified period of time. It is a measure of the system's dependability and is often expressed as the mean time between failures (MTBF) or as a probability.
Availability, on the other hand, measures the proportion of time that a system is operational and available for use. It takes into account not only the reliability of the system but also the time it takes to repair or restore the system to its operational state when a failure occurs. Availability is often expressed as a percentage or as the mean time between failures (MTBF) divided by the sum of the mean time between failures and the mean time to repair (MTTR).
The relationship between reliability and availability can be expressed through the following formula:
Availability = MTBF / (MTBF + MTTR)
In this formula:
MTBF (Mean Time Between Failures) represents the average time between system failures and is a measure of reliability.
MTTR (Mean Time To Repair) represents the average time it takes to repair the system and restore it to its operational state after a failure occurs.
The higher the reliability (MTBF) and the shorter the repair time (MTTR), the greater the system's availability. In other words, to maximize availability, one should aim to increase reliability and reduce repair time. This can be achieved by designing robust systems, implementing regular maintenance, using redundancy, and employing efficient repair strategies.