HA and DR Terminologies

The following terms are important in the world of HA and DR so that you can correctly choose the best possible HA and DR solutions and for the better understanding of HA and DR concepts.

Availability

Availability or uptime is defined as the percentage that a system or an application should be available for in a given year. Availability is expressed as Number of Nines.

For example, a 90%, one nine, availability means that a system can tolerate a downtime of 36.5 hours in a year, and a 99.999%, five nines, availability means that a system can tolerate a downtime of 5.26 minutes per year.

The following table, taken from https://en.wikipedia.org/wiki/High_availability, describes the availability percentages and the downtime for each percentage:

Note

This link also talks about how this is calculated. You can look at it, but a discussion on calculation is out of the scope of this book.

Figure 1.4: Availability table

In the preceding table, you can see that as...