How to Measure Systems Performance Reliability

Instructor: Martin Gibbs

Martin has 16 years experience in Human Resources Information Systems and has a PhD in Information Technology Management. He is an adjunct professor of computer science and computer programming.

System performance can have a direct business impact. This lesson will cover the methods for measuring system performance and reliability, providing examples.

Systems Reliability

Even the most advanced system is going to fail. It may not fail for a very long time, or it might fail every so often, but be easily fixed. The worst scenario is a system that fails consistently, takes a long time to fix issues, or both. We want a reliable system, just as drivers want reliable vehicles. As IT professionals, our customers and decision makers will want us to be able to measure reliability.

We definitely will hear when the system is down, but anecdotes are not data. Instead, we can measure reliability as a combination of two important measurements: Mean Time Between Failures (MTBF) and Mean Time to Repair (MTTR).

Mean Time Between Failures (MTBF) is the measurement of time between system breakdowns, but it only includes the time the system is working (and not being worked on). A system that had been running strong for a year may suddenly have two down times in a one-month period; this will drag down MTBF. It should also warrant discussion on what updates/patches may have been missed during those long stretches of uptime.

Mean Time to Repair (MTTR) is the number you use to measure the average time it takes to fix a system. The following chart shows sample data on a given system and the mean time for repairs.

Mean Time to Repair

Scheduled maintenance is not included in these numbers: If you have told users that the system will be down from 3AM to 4AM on the last Tuesday of every month, we cannot count these hours against reliability. However, if the system fails at 5AM and takes three more hours to repair, this time is definitely counted against MTTR.

Downtime and uptime can also be expressed in levels; what software vendors call the nines.

The Nines

One way to measure uptime/reliability is the nines approach. Vendors often sell their systems on a certain percentage of uptime. The nines here are the variations of 99% uptime. 99% is actually two nines, which is just under 4 days of downtime in a year. However, vendors like to sell the coveted five nines (99.999%) uptime (less than seven minutes per year of downtime).

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