Uptime is how long a system has been running without an outage. It can be expressed as a number of days, or as a percentage and it’s often used to show how reliable a Web server is.
Absolute uptime tells you how long a system has been up since it was booted. So, if it was switched on at 12:30 PM and is still running at the same time the next day, the uptime is 24 hours. This number is reset every time there’s a reboot.
On a Unix-based system (including macOS), you can type the uptime command in the terminal to find out how long the system has been running.
If you want to work out the uptime percentage, all you need to do is divide the total time that the system has been going for, by how long it’s been active. So, if a system has been running for a year, but had 10 hours downtime during that year, your calculation would look like this:
365 days x 24 hours = 8,760 active hours
8,760 active hours – 10 hours unavailable = 8,750 available hours
8,750 available hours ÷ 8,760 active hours = 99.8858% uptime
99.9% is not uncommon for a web server. It might sound like an impossibly high percentage, but many businesses can’t afford even a few hours of downtime. That’s why high-traffic enterprise servers will often aim to hit 99.999% uptime. They manage to attain this high number by using multiple servers for load balancing and redundancy.