A vocabulary collection for SREs (mostly influenced by Google SREs)
"Fundamentally, it’s what happens when you ask a software engineer to design an operations function" -- Ben Treynor, VP of Engineering @ Google[1].
| Availability % | Downtime per year | Downtime per month | Downtime per Week |
|---|---|---|---|
| 90% | 36.5 days | 72 hours | 16.8 hours |
| 95% | 18.25 days | 36 hours | 8.4 hours |
| 98% | 7.30 days | 14.4 hours | 3.36 hours |
| 99% | 3.65 days | 7.20 hours | 1.68 hours |
| 99.5% | 1.83 days | 3.60 hours | 50.4 minutes |
| 99.8% | 17.52 hours | 86.23 minutes | 20.16 minutes |
| 99.9% | 8.76 hours | 43.2 minutes | 10.1 minutes |
| 99.95% | 4.38 hours | 21.56 minutes | 5.04 minutes |
| 99.99% | 52.6 minutes | 4.32 minutes | 1.01 minutes |
| 99.999% | 5.26 minutes | 25.9 seconds | 6.05 seconds |
| 99.9999% | 31.5 seconds | 2.59 seconds | 0.605 seconds |
Downtime per month is calculated at 30 days[2].
Mean Time To Recover. MTTR is the average time that a device will take to recover from any failure[4].
Mean Time Between Failures. MTBF is the predicted elapsed time between inherent failures of a mechanical or electronic system, during normal system operation. MTBF can be calculated as the arithmetic mean (average) time between failures of a system. The term is used for repairable systems, while mean time to failure (MTTF) denotes the expected time to failure for a non-repairable system[5].
Mean Time To Failure. MTTF denotes the expected time to failure for a non-repairable system[5].
Service Level Agreement. A SLA is a (legal) agreement with repercussions for failure to meet[3].
Service Level Identicator. A SLI is a carefully defined quantitative measure of some aspect of the level of service that is provided[3].
Service Level Objective. A SLO is a target value or range of values for a service level that is measured by an SLI[3].
[1] Google SRE Interview, Niall Murphy and Ben Treynor, "What is 'Site Reliability Engineering', 2018-09-26, https://landing.google.com/sre/interview/ben-treynor.html [2] https://interworks.com/blog/rclapp/2010/05/06/what-does-availabilityuptime-mean-real-world/ [3] Google Cloud Next 2018: Nori and Dan, "Best Practices from Google SRE", 2018-07-26, https://www.youtube.com/watch?v=XPtoEjqJexs [4] https://en.wikipedia.org/wiki/Mean_time_to_recovery [5] https://en.wikipedia.org/wiki/Mean_time_between_failures