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Downtime ( engl. For downtime, lost time, shut-down) is the common name of the time in which a system esp. A computer system is not available or not working. A distinction is made between planned and unplanned downtime.

Planned downtime

Planned downtime is a targeted and scheduled shutdown of a computer system. Often, a planned downtime is placed in the non-working night hours and announced to the users. Alternative systems are usually available. A planned downtime is e.g. B. necessary for hardware upgrades or relocations.

Unplanned downtime

Unplanned downtime is the term used to describe suddenly occurring faults, such as B. as a result of head crashes in hard drives or overheating. Unplanned downtime can lead to severe financial losses, since the business processes supported by the IT system concerned cannot run or can only run with additional effort.

In the case of a web shop , for example, this would result in a lack of orders. The failure of an ERP system could paralyze an entire factory after a short time because just-in-time production is no longer possible. As a countermeasure, a redundant backup system is available that can take over the functions of the primary system in an emergency. One speaks of failover .

In computer systems is availability in time of uptime in the total time measured and reported in percent. A 24/7 system with 99% availability may fail for 3.6 days per year, a 99.99% available system 52 minutes and a 99.999% ( five nines ) available system only five minutes. Computer systems can be made highly available by using internal redundancy , e.g. B. several power supplies or RAID , or by operating them as distributed systems .

The opposite of downtime is uptime .