Comprehensive and Detailed Explanation From Exact Extract:
An error budget accounts for all downtime, including both planned and unplanned outages. This is a critical SRE principle: the user does not distinguish between maintenance downtime and accidental downtime — therefore, neither should the SLO nor the error budget.
The SRE Book, Chapter “Service Level Objectives,” states:
“From the user’s perspective, availability is simply whether the service is working or not, regardless of whether the outage was planned or unplanned.”
This means all downtime counts toward the error budget.
Additionally, the SRE Workbook reinforces this point:
“Error budgets must include every form of unavailability — maintenance events, configuration changes, emergency work, and unexpected incidents.”
This confirms that planned outages (maintenance windows) and unplanned outages (incidents) both consume error budget.
Why the other options are incorrect:
A Only includes unplanned incidents; SRE requires counting planned outages as well.
B Defect fixes may contribute to downtime, but “defect fixes” alone are not a downtime category.
D CAB approval has no bearing on whether outages count toward error budgets.
Thus, C is correct: any planned or unplanned outage must be included.
[References:, Site Reliability Engineering Book, “Service Level Objectives”, SRE Workbook, “Implementing SLOs”, , ]
