The nine’s of availability
Availability is often quantified by uptime (or downtime) as a percentage of time the service is available. It is generally measured in the number of 9s. If availability is 99.00% available, it is said to have “2 nines” of availability, and if it is 99.9%, it is called “3 nines”, and so on.| Availability (Percent) | Downtime (Year) | Downtime (Month) | Downtime (Week) |
|---|---|---|---|
| 90% (one nine) | 36.53 days | 72 hours | 16.8 hours |
| 99% (two nines) | 3.65 days | 7.20 hours | 1.68 hours |
| 99.9% (three nines) | 8.77 hours | 43.8 minutes | 10.1 minutes |
| 99.99% (four nines) | 52.6 minutes | 4.32 minutes | 1.01 minutes |
| 99.999% (five nines) | 5.25 minutes | 25.9 seconds | 6.05 seconds |
| 99.9999% (six nines) | 31.56 seconds | 2.59 seconds | 604.8 milliseconds |
| 99.99999% (seven nines) | 3.15 seconds | 263 milliseconds | 60.5 milliseconds |
| 99.999999% (eight nines) | 315.6 milliseconds | 26.3 milliseconds | 6 milliseconds |
| 99.9999999% (nine nines) | 31.6 milliseconds | 2.6 milliseconds | 0.6 milliseconds |
Availability in sequence vs parallel
If a service consists of multiple components prone to failure, the service’s overall availability depends on whether the components are in sequence or in parallel.Sequence
Overall availability decreases when two components are in sequence. For example, if bothFoo and Bar each had 99.9% availability, their total availability in sequence would be 99.8%.
Parallel
Overall availability increases when two components are in parallel. For example, if bothFoo and Bar each had 99.9% availability, their total availability in parallel would be 99.9999%.