Calculating Availability
Availability is measured in so-called nines notation, that corresponds to the percentage of time that a service is available. For example, if a service is available 99.9% of the time, we can say that has “three-nines availability”. This notation is used to gives us the specific amount of downtime that a service is allowed to have. Next are the availability and downtime calculations for 99% availability to 99.999% availability : 99% availability (two-nines): * 3.65 days/year (of allowed downtime) * 7.20 hours/month * 1.68 hours/week * 14.4 minutes/day 99.9% availability (three-nines): * 8.76 hours/year * 43.8 minutes/month * 10.1 minutes/week * 1.44 minutes/day 99.99% availability (four-nines): * 52.56 minutes/year * 4.38 minutes/month * 1.01 minutes/week * 8.66 seconds/day 99.999% availability (five-nines): * 5.26 minutes/year * 25.9 seconds/month * 6.05 seconds/week * 864.3 mimiseconds/day