sysop@hub:~/tools/uptime-sla-calculator$

uptime-sla-calculator

"Three nines" sounds like always. This turns the percentage into minutes, shows what a number of parts in a row does to it, and what it takes to promise it.

Availability in percent Service hours per week Downtime last month, in minutes Parts in a chain Availability of each part

Simple arithmetic: it assumes failures are independent and counts a month as a twelfth of a year. Read an agreement for what counts as downtime: planned maintenance is usually excluded, and often so is anything the provider did not notice itself. A percentage says nothing about how long one outage may last: 99.9% can be one outage of nine hours a year. Ask for the recovery time as well. Related: backup planner, SNMP and monitoring cheat sheet, RAID calculator.