Uptime is a metric representing the duration a computer system, server, or network component remains operational and accessible. It is typically tracked from the last reboot or measured as a percentage of total time within a specific reporting window, such as a month or a year.
In Unix-like operating systems, the uptime command provides the current duration of the system session along with system load averages. For service providers, uptime is a critical KPI defined in Service Level Agreements (SLAs). High availability is often categorized by "nines": 99.9% (three nines) allows roughly 43 minutes of downtime per month, while 99.999% (five nines) limits downtime to about 5 seconds per month.
Calculation and Factors
The standard formula is: (Total Time - Downtime) / Total Time * 100%. Maintaining high uptime requires redundant hardware, Uninterruptible Power Supplies (UPS), and proactive monitoring. System administrators use tools like Zabbix or Prometheus to track availability and receive alerts when services go offline.
- Scheduled maintenance: Planned downtime that may be excluded from SLA calculations.
- Unplanned downtime: Service interruptions caused by hardware failure or network issues.
- High Availability (HA): Clusters designed to maintain uptime even during component failure.