“Uptime” is a term that indicates the duration or period during which a computer, server, software application, or other IT resource is operational or available. In contrast, "downtime" represents the time when the resource is unavailable or offline.
The terms “uptime” and “downtime” are often used to evaluate the reliability and availability of systems. The higher the uptime, the more reliable and continuous the operation is. Uptime is usually expressed as a percentage and indicates how long a system has been running without downtime.
The uptime percentages are often presented as follows:
- **99% Uptime:** This would mean that the system is only down about 1% of the time, which is about 3.65 days per year.
- **99.9% Uptime (also referred to as "three nines"):** The system would only be unavailable for approximately 8.76 hours per year.
- **99.99% uptime (also referred to as "four nines"):** This equates to only about 52.56 minutes of downtime per year.
The uptime objective depends on the requirements and expectations of the respective system or service. Mission-critical systems, such as financial services or healthcare, typically strive for very high uptime to minimize downtime and ensure service continuity. Uptime is therefore an important factor in assessing the performance and reliability of IT infrastructures.