Understanding high availability

High availability (HA) refers to a characteristic of a computing system that aims to provide an agreed level of performance. This normally refers to the proportion of time that the system is available and is given in terms of a percentage. You can convert this percentage into the number of days in a year that the system is guaranteed to be available, or conversely, the maximum number of days in a year during which the system can be down or unavailable.

For example, if the availability of a system is given as 95%, then the system is guaranteed to be available for approximately 347 days in a year and could be unavailable for up to 18 days in a year. Does that sound good?

The answer to that question, like everything, is “It depends.” A continuous 18-day outage could lose you customers, but when you look at it from a monthly perspective, that would translate to an average outage of 1.5 days every month. Does that sound acceptable? That...