Availability is the proportion of time a system is in a functioning condition. This is often described as a mission capable rate. Mathematically, this is expressed as 1 minus unavailability. Availability combines MTBF with a second metric called mean time to repair (MTTR) that measures the time required to acknowledge a problem, respond to it and complete a repair. Availability is typically expressed as a number of “nines” representing the percentage of time over a year’s worth of use that a given system is operational. For example, a device with an MTBF of 500,000 hours and a MTTR of four hours would have an availability of .999992, or 99.9992 percent (500,000 ÷ 500,004). That translates to an expected downtime of 4.2 minutes per year.
If an outage occurs what is the last line of defense in most cases? A UPS system, battery-backup and a diesel generator. You are now relying on equipment that sits in standby mode and is usually never activated unless a planned failure test is orchestrated - it is only then do you get to see the failure plan operate. Typical companies put this plan into action 2 times per year for mission-critical environments and once in normal data centers.
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