What does 99.9% uptime mean?
99.9% allows 8h 45m 36s of downtime a year — 43m 12s a month, or 10m 5s a week.
| Per day | 1m 26s |
|---|---|
| Per week | 10m 5s |
| Per month (30 days) | 43m 12s |
| Per year (365 days) | 8h 45m 36s |
An availability target is a downtime budget, and it is easier to judge as time than as a percentage. At 99.9%, a service may be unavailable for 43m 12s 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 tighter, 99.95%, cuts the yearly budget to 4h 22m 48s. One step looser, 99.5%, raises it to 1d 19h 48m.
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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