uptime

Uptime refers to the period during which a system, service, or device is operational and accessible to users. In information technology, uptime is usually measured as a fraction of a defined time window and commonly expressed as a percentage, such as 99.9% or 99.99%. It can also be described as the total time a system has been running since a last reboot or failure.

Calculation and related concepts: Uptime percentage is calculated by dividing the time the system was functioning by the total measured time. Availability is a closely related metric, often defined as uptime divided by total time and sometimes adjusted for planned maintenance. In reliability engineering, availability can be expressed as MTBF / (MTBF + MTTR), linking uptime to mean time between failures and mean time to repair.

Downtime and maintenance: Downtime is the complement of uptime and represents periods when the system is not usable. Downtime can be planned (maintenance, upgrades) or unplanned (hardware failure, software crash). The goal of most operations teams is to minimize unplanned downtime and manage planned downtime to limit user impact.

Causes and strategies: Common causes include hardware failures, software defects, power outages, network connectivity problems, and human error. Strategies to improve uptime include redundancy (duplicate components and failover), distributed architectures, monitoring and automated recovery, regular maintenance, and robust change management.

Measurement and usage: Uptime is a key metric in service level agreements and performance dashboards. It is often complemented by related metrics such as monitoring coverage, alert response times, and incident duration to gauge operational reliability. The term has historical roots in the early days of computing and telecommunications, where continuous operation was a critical objective.