https://en.wikipedia.org/wiki/Mean_time_between_failures
https://en.wikipedia.org/wiki/Mean_time_to_recovery
Maybe better to talk in terms of the reverse: the ability of the system to make soft- or hard-realtime guarantees. “Small downtime” is any failure in a soft/hard-realtime SLA.
https://en.wikipedia.org/wiki/Mean_time_between_failures
https://en.wikipedia.org/wiki/Mean_time_to_recovery