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(MTTR) The average time that a device will take to recover from a non-terminal failure. Examples of such devices range from self-resetting fuses (where the MTTR would be very short, probably seconds), up to whole systems which have to be replaced.
The MTTR would usually be part of a maintenance contract, where the user would pay more for a system whose MTTR was 24 hours, than for one of, say, 7 days. This means the supplier is guaranteeing to have the system up and running again within 24 hours (or 7 days) of being notified of the failure.
Some devices have a MTTR of zero, which means that they have redundant components which can take over the instant the primary one fails, see RAID for example.
See also Mean Time Between Failures.