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Uptime is a computer industry term for the time during which a computer or IT system is operational. Uptime can also be a metric that represents the percentage of time that hardware, a computer network, or a device is successfully operational. Uptime is often measured in percentiles, such as “five 9s,” meaning a system that is operational 99.999 percent of the time. Downtime, the opposite of uptime, is the period of time when a system is not operational.

The terms uptime and downtime are used to define the level of success provided by real-time services. A service level agreement (SLA) or other real-time service contract may include uptime/downtime ratios that show how much time a service is expected to remain operational. IT professionals may use uptime to refer to a total consecutive amount of operational time. For example, a computer system that has been running for three weeks has a “three-week uptime.” High availability uses uptime to define an agreed level of operational performance measured against a 100 percent operational standard.

Uptime is often used as a sign of operating system or network reliability, representing the length of time a system can be left unattended without crashing or needing maintenance. Hardware reliability, availability and serviceability (RAS) are important factors in data center uptime. Redundant servers for backup and failover help maintain data center uptime in case of server failure. Server clustering is another uptime strategy that delivers high availability of IT services and workloads. A server cluster is a group of linked servers that work together to improve system performance, load balancing and service availability. If a server fails, other servers in the cluster can take over the functions and workloads of the failed server. SUSE Linux Enterprise Server can help maximize uptime by providing server clustering, by exploiting hardware RAS features, and by enabling live kernel patching without rebooting.