What does the term "five nines" refer to in high availability practices?

The term "five nines" refers to a system's availability level of 99.999%. This metric signifies that a service or system is operational and accessible 99.999% of the time within a given period, typically annually. This level of availability translates to very minimal downtime—specifically, about 5.26 minutes of downtime per year.

In the context of high availability practices, achieving "five nines" is a significant benchmark for critical systems that require maximum reliability and uptime. The concept emphasizes the importance of rigorous design, redundancy, and failover strategies in systems architecture to mitigate any potential disruptions to service.

Other metrics such as 99.9% or 99.99% denote lower levels of availability, which are less stringent and result in considerably more allowable downtime. For instance, 99.99% allows for approximately 52 minutes of downtime per year, while 99.9% permits about 8.76 hours of downtime annually. This illustrates the greater challenges and investments required to move from one level of availability to another, especially as you aim for "five nines."