Measuring availability

The idea behind availability is that the service provider tries to guarantee a certain level of it, and clients can then expect that or more. In some cases (depending on service contracts), a penalty fee or a decreased subscription fee is the consequence of an unexpected downtime.

The quality of availability is measured in fraction of percentages, for example, 99.99 percent or 99.999 percent, which are referred to as "four nines" and "five nines" respectively. These values are considered pretty good availability values, but there is a small trick in computing this value. If the provider has a planned downtime that is announced in advance; for example, the annual or bi-annual maintenance for water pipes in a town doesn't make the availability number worse, then availability is only measured outside the planned maintenance window.

However, let's consider the 0.001 percent downtime for the "five nines" case. Users experience denied or delayed service only 5 minutes and...