systemdesign systemdesign/concepts
Availability is the probability that a system will be operational and accessible when needed.
The Nine’s of availability
Availability = Uptime / (Uptime + Downtime)
| Availability (Percent) | Downtime (Year) | Downtime (Month) | Downtime (Week) | Downtime (Day) |
|---|---|---|---|---|
| 90% (one nine) | 36.53 days | 72 hours | 16.8 hours | 2.40 hours |
| 99% (two nines) | 3.65 days | 7.20 hours | 1.68 hours | 14.40 mins |
| 99.9% (three nines) | 8.77 hours | 43.8 minutes | 10.1 minutes | 1.44 mins |
| 99.99% (four nines) | 52.6 minutes | 4.32 minutes | 1.01 minutes | 8.64 sec |
| 99.999% (five nines) | 5.25 minutes | 25.9 seconds | 6.05 seconds | 884 ms |
Transclude of ds12.1---fault-tolerance#reliability-and-availability