Service Availability Target

When deciding the level of availability we want for our services, the target that we want to achieve is often described as a percentage of time the service is available.

It's worth noting that 100% availability, while it sounds like a nice thing to have, is immensely expensive to achieve, and also pointless to a big extent - what's the point of having 100% available service which is used on a below-100% available device, through a below-100% available network?

This number is famous as a "number of nines". A system that's 99% available is said to have "two nines" service availability target, while 99.999% available system has "five nines".

The table below represents availability targets, together with how much downtime they allow per year/quarter/month/week/day:

Availability Year Quarter Month Week Day
90% 36.5d 9d 3d 16.8h 2.4h
95% 18.25d 4.5d 1.5d 8.4h 1.2h
99% 3.65d 21.6h 7.2h 1.68h 14.4m
99.5% 1.83d 10.8h 3.6h 50.4m 7.20m
99.9% 8.76h 2.16h 43.2m 10.1m 1.44m
9.95% 4.38h 1.08h 21.6m 5.04m 43.2s
99,99% 52.6m 12.96m 4.32m 60.5s 8.64s
99.999% 5.26m 1.3m 25.9s 6.05s 0.87s

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