How To Calculate High Availability Uptime Percentage?

Availability is measured by how much time a specific system stays fully operational during a particular period, usually a year.

It is expressed as a percentage. Note that uptime does not necessarily have to mean the same as availability. A system may be up and running, but not available to the users. The reasons for this may be network or load balancing issues.

The uptime is usually expressed by using the grading with five 9’s of availability.

If you decide to go for a hosted solution, this will be defined in the Service Level Agreement (SLA). A grade of “one nine” means that the guaranteed availability is 90%. Today, most organizations and businesses require having at least “three nines,” i.e., 99.9% of availability.

Businesses have different availability needs. Those that need to remain operational around the clock throughout the year will aim for “five nines,” 99.999% of uptime. It may seem like 0.1% does not make that much of a difference. However, when you convert this to hours and minutes, the numbers are significant.

Refer to the table of nines to see the maximum downtime per year every grade involves:

As the table shows, the difference between 99% and 99.9% is substantial.

Note that it is measured in days per year, not hours or minutes. The higher you go on the scale of availability, the cost of the service will increase as well.

How to calculate downtime? It is essential to measure downtime for every component that may affect the proper functioning of a part of the system, or the entire system. Scheduled system maintenance must be a part of the availability measurements. Such planned downtimes also cause a halt to your business, so you should pay attention to that as well when setting up your IT environment.

As you can tell, 100% availability level does not appear in the table.

Simply put, no system is entirely failsafe. Additionally, the switch to backup components will take some period, be that milliseconds, minutes, or hours.