What does 99.999% uptime mean?
99.999% allows 5m 15s of downtime a year — 26s a month, or 6s a week.
| Per day | 0.9s |
|---|---|
| Per week | 6s |
| Per month (30 days) | 26s |
| Per year (365 days) | 5m 15s |
An availability target is a downtime budget, and it is easier to judge as time than as a percentage. At 99.999%, a service may be unavailable for 26s in a month and still have met the target.
Whether that is generous or tight depends on what the target excludes. Most SLAs do not count maintenance announced in advance, and many exclude faults in networks the provider does not run — so two providers quoting the same number can be promising quite different things.
One step looser, 99.99%, raises it to 52m 34s.
How it is measured matters as much as the number. A check every five minutes cannot see a two-minute outage, and a single probe cannot tell an outage apart from its own bad network.
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