The SLA for a service specifies a monthly availability of at least 99.95%. A month is taken as 30 days, and planned downtime is included in the calculation. This month the service has already been down for a total of 15 minutes due to failures. How much more downtime is allowed for the rest of this month while still meeting the SLA?

Answer: A. 6.6 minutes

A month is 30 × 24 × 60 = 43,200 minutes, and at 99.95% availability the allowed downtime is 43,200 × 0.0005 = 21.6 minutes. Since 15 minutes have already been used, 21.6 − 15 = 6.6 minutes remain. 21.6 minutes forgets to subtract the downtime that has already occurred, and 36.6 minutes adds it instead of subtracting it. An SLA should specify, as agreed between the service provider and the user, matters such as whether planned downtime counts toward availability and the measurement period.

• A. Correct. Up to 21.6 minutes of downtime is allowed per month, and 15 have been used, leaving 6.6 minutes.
• B. This is the downtime allowed for the whole month, without subtracting the 15 minutes already used.
• C. This adds the 15 minutes already used instead of subtracting them.