Mean Time to Repair (MTTR) has long been a standard metric in Site Reliability Engineering (SRE) and DevOps, representing the average time to restore normal operations after a system incident.
While MTTR offers a straightforward measure of system reliability, its effectiveness in modern, complex systems is increasingly questioned.
Limitations of MTTR:
- Varied Incident Complexity: Not all incidents are alike. For example, a simple server reboot might take minutes, whereas resolving a database corruption could span hours. Averaging these disparate events can lead to misleading conclusions, as incident resolution times often follow a power-law distribution, not a normal one.
- Human Factors: MTTR doesn't account for elements like team coordination, decision-making processes, or external dependencies. Factors such as incidents occurring during off-hours or reliance on third-party services can significantly influence repair times.
- Counterproductive Incentives: A strict focus on reducing MTTR might encourage teams to prioritize speed over quality, leading to rushed fixes or a blame culture, ultimately compromising system resilience.
- Neglect of Impact and Resilience: MTTR measures time to resolution but doesn't consider the incident's impact on users or the system's ability to withstand failures gracefully.
Alternative Metrics:
- Time to Mitigate (TTM): Focuses on how quickly a team can reduce the impact of an incident, even if full resolution takes longer.
- Service Level Indicators (SLIs) & Objectives (SLOs): Emphasize user-centric metrics like latency, availability, and error rates to gauge actual service performance.
- Incident Complexity & Learning Metrics: Evaluate the depth of post-incident analyses and the effectiveness of implemented improvements to prevent recurrence.
- Adaptive Capacity & Resilience Metrics: Assess the system's ability to adapt to unexpected conditions, such as the success rate of automatic failovers or the effectiveness of self-healing mechanisms.
While MTTR can provide some insights, it's essential to recognise its limitations and complement it with metrics that offer a more holistic view of system reliability and user experience.
Shifting the focus from merely repairing failures to enhancing system resilience and reducing user impact aligns better with the goals of modern reliability engineering.