Class of Nines

When the Availability of a cluster is expressed as a decimal (or a percentage), the number of initial leading nines in the figure is referred to as the ``Class of Nines''; thus

• 0.99987 is class 3 (or 3 nines)

• 0.999967 is class 4 (or 4 nines)

and so on. Each class corresponds to a maximum allowable amount of down time per year in the cluster:

• class 3 is no more than 8 hours, 45 minutes

• class 4 is no more than 52 minutes

• class 5 is no more than 5 minutes, 12 seconds