Downtime

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For other uses, see Downtime (disambiguation).

Downtime refers to a period of time or a percentage of a timespan that a machine or system (usually a computer server) is offline or not functioning, usually as a result of either system failure (such as a crash) or routine maintenance. The opposite is uptime.

Maintenance downtimes have to be carefully scheduled in industries that rely on nearly 24-hour service, such as e-commerce, medical informatics, news reporting, and persistent online games. In many cases, system-wide downtimes can be averted using rolling upgrades.
Downtimes caused by system failures can have serious economic impact. To minimize these effects network monitoring can be used.

In Service Level Agreements, it is common to mention a percentage value (per month or per year) that is calculated by dividing the sum of all downtimes timespans by the total time of a reference time span (e.g. a month). 0% downtime means that the server was available all the time.

For Internet servers downtimes above 1% per year or worse can be regarded as unacceptable as this means a downtime of more than 3 days per year. For e-commerce and other industrial use any value above 0.1% is usually considered unacceptable.