Part 2 ยท Reliability math
Every extra nine costs more

Why the nines matter

Each additional nine cuts allowed failure by ten, which is why reliability targets must be deliberate.

30-day month downtime allowance

99%
7.2 h
99.9%
43.2 m
99.99%
4.32 m
99.999%
26 s

What this means operationally

  • Tighter targets reduce room for failed releases.
  • Higher targets require more automation, redundancy, and process control.
  • Zero-risk language without cost framing is not an SRE answer.
Reliability is a feature with a cost.

The target should come from user tolerance and business impact, not from ambition alone.

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