High availability (HA) is a complex variable, because a deep understanding of how every component interacts with each other must be in place. Metrics come to the rescue; they provide a quantitative perspective of the system operations discipline, but in some cases, not all of the factors are correlated, and a qualitative perspective can help. This database of indicators can be used to build a model to represent the availability qualities of a system.
A system's availability can be calculated by the number of nines in the digits, representing the percentage of time a system is operating, without downtime. This is shown in the following table:
|
Number of nines |
Uptime % |
Downtime per day |
|
1 |
90% |
2.4 hours |
|
2 |
99% |
14 minutes |
|
3 |
99.9% |
86 seconds |
|
4 |
99.99% |
8.6 seconds |
|
5 |
99.999% |
.86 seconds |