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CAS-002 · Question #760
CAS-002 Question #760: Real Exam Question with Answer & Explanation
The correct answer is C: 98.34 percent. Uptime percentage is calculated by excluding scheduled maintenance from unplanned downtime, then dividing available hours by total hours in the period.
Question
There have been some failures of the company's internal facing website. A security engineer has found the WAF to be the root cause of the failures. System logs show that the WAF has been unavailable for 14 hours over the past month, in four separate situations. One of these situations was a two hour scheduled maintenance time, aimed at improving the stability of the WAF. Using the MTTR based on the last month's performance figures, which of the following calculations is the percentage of uptime assuming there were 722 hours in the month?
Options
- A92.24 percent
- B98.06 percent
- C98.34 percent
- D99.72 percent
Explanation
Uptime percentage is calculated by excluding scheduled maintenance from unplanned downtime, then dividing available hours by total hours in the period.
Common mistakes.
- A. 92.24 percent would require roughly 56 hours of downtime, far exceeding the stated 14 hours and does not correspond to any valid interpretation of the given figures.
- B. 98.06 percent results from dividing all 14 hours (including scheduled maintenance) by 722 without excluding planned downtime: (722 - 14) / 722, which misapplies MTTR methodology.
- D. 99.72 percent would imply only about 2 hours of downtime, which matches only the scheduled maintenance window and ignores the 12 hours of unplanned outages entirely.
Concept tested. MTTR-based availability and uptime percentage calculation
Reference. https://learn.microsoft.com/en-us/azure/well-architected/reliability/metrics
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