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
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