Definition: Availability

Availability = uptime / (uptime + downtime)

Availability from a technical perspective is mostly about being fault tolerant. Because the probability of a failure occurring increases with the number of components, the system should be able to compensate so as to not become less reliable as the number of components increases.

For example, availability rate for a given service over an entire year mean the following:

Availability % How much downtime is allowed per year?
90% (“one nine”) More than a month
99% (“two nines”) Less than 4 days
99.9% (“three nines”) Less than 9 hours
99.99% (“four nines”) Less than an hour
99.999% (“five nines”) ~ 5 minutes
99.9999% (“six nines”) ~ 31 seconds

Comments are closed.