Mistake: treating maximum availability as automatically worthwhile. Reaching five nines may require costly local and geographically dispersed redundant infrastructure plus redundant private links, so the target must match the cost of downtime. section 13.2
A sales application outage
An unavailable sales application loses revenue when customers cannot place orders. Longer-term effects may include lost customers, lower morale, reputational damage, or regulatory noncompliance; compare these costs with the infrastructure investment. section 13.2
Table 13.1 Levels of availability (view table figure)
| Level of availability |
Downtime per week |
Downtime per month |
|---|---|---|
| 99% |
1 hour, 40 minutes, 48 seconds |
7 hours, 18 minutes, 17 seconds |
| 99.9% |
10 minutes, 4 seconds |
43 minutes, 49 seconds |
| 99.99% |
1 minute |
4 minutes, 23 seconds |
| 99.999% |
6 seconds |
26 seconds |
Use the availability table to translate targets into downtime: 99.9% permits 43 minutes 49 seconds monthly, while 99.999% permits 26 seconds. section 13.2
