SLA: Service level agreement (SLA for short, full name: service level agreement). It is a mutually agreed agreement between service providers and users to ensure the performance and availability of services at a certain cost. Usually, this overhead is the main factor driving the quality of service provided.

In the system's high reliability (also known as availability, which is described as HA, High Available in English), there is a standard to measure its reliability - X 9, and this X represents the numbers 3 to 5. X 9 represents the ratio of the system's normal use time to the total time (1 year) during the system's 1-year use. We can feel the reliability difference of X 9 at different levels through the following calculation.

The following shows the allowable interrupt schedules for these 9's:

 Three 9s: (1-99.9%) * 365 * 24=8.76 hours, which means that the maximum possible service interruption time of the system in one year of continuous operation is 8.76 hours. Four 9s: (1-99.99%) * 365 * 24=0.876 hours=52.6 minutes, which means that the maximum possible service interruption time of the system in one year of continuous operation is 52.6 minutes. Five 9s: (1-99.999%) * 365 * 24 * 60=5.26 minutes, which means that the maximum possible service interruption time of the system in one year of continuous operation is 5.26 minutes.

Expand:

 One 9: (1-90%) * 365=36.5 days, which means that the maximum possible service interruption time of the system in one year of continuous operation is 36.5 days. Two 9s: (1-99%) * 365=3.65 days, which means that the maximum possible service interruption time of the system in one year of continuous operation is 3.65 days. Six 9s: (1-99.9999%) * 365 * 24 * 60 * 60=31 seconds, which means that the maximum possible service interruption time of the system in one year of continuous operation is 31 seconds.

Now you know what we usually mean by 99.9% availability when buying a host product?
 What is the reliability of the five nines, four nines, and three nines that are often mentioned in operation and maintenance?

 What is the reliability of the five nines, four nines, and three nines that are often mentioned in operation and maintenance?