Uptime describes how long or how reliably a system has been running. It may be defined as an absolute value (e.g., 64 days) or a percentage (e.g., 99.5%). Uptime percentage is a common metric used to determine the reliability of a web server.
Absolute uptime measures the length of time a system has been running since it was booted. For example, if a system was rebooted at 1:00 PM and is still running at 3:30 PM the next day, its uptime will be 26.5 hours. When a system is restarted, the uptime number resets.
Uptime percentage is determined by dividing the total time has been available (or online) by the time it has been active. For example, if a web server has been active for one year, but was not available for a total of 32 hours during the year, its uptime percentage would be calculated as follows:
365 days x 24 hours = 8,760 active hours
8,760 active hours - 32 hours unavailable = 8,728 available hours
8,728 available hours ÷ 8,760 active hours = 99.6347% uptime
An uptime of 99.9% is a reasonable goal for a web server. That percentage may sound high, but even a few hours of downtime can have a significant impact on a business. For this reason, a high-traffic enterprise server may have an uptime goal of 99.999%. This level of uptime can be achieved by using multiple servers for redundancy and load balancing.