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Five nines, commonly taken to mean "99.999%", may refer to: High availability of services, when a service is available for 99.999% of the time, or around 5 minutes of downtime per year; Nine (purity), a 99.999% pure substance; German 15 cm (5.9 in) artillery shells used in World War I
Uptime is the opposite of downtime. Htop adds an exclamation mark when uptime is longer than 100 days. It is often used as a measure of computer operating system reliability or stability, in that this time represents the time a computer can be left unattended without crashing or needing to be rebooted for administrative or maintenance purposes.
It can be formally defined as (1 – (down time/ total time))*100%. Although the minimum required availability varies by task, systems typically attempt to achieve 99.999% (5-nines) availability. This characteristic is weaker than fault tolerance , which typically seeks to provide 100% availability, albeit with significant price and performance ...
Recovery time (or estimated time of repair (ETR), also known as recovery time objective (RTO) is closely related to availability, that is the total time required for a planned outage or the time required to fully recover from an unplanned outage. Another metric is mean time to recovery (MTTR). Recovery time could be infinite with certain system ...
If we are using equipment which has a mean time to failure (MTTF) of 81.5 years and mean time to repair (MTTR) of 1 hour: MTTF in hours = 81.5 × 365 × 24 = 713940 (This is a reliability parameter and often has a high level of uncertainty!) Inherent availability (Ai) = 713940 / (713940+1) = 713940 / 713941 = 99.999860%
Monitoring can cover many things that an application needs to function, like network connectivity, Domain Name System records, database connectivity, bandwidth, and computer resources like free RAM, CPU load, disk space, events, etc. Commonly measured metrics are response time and availability (or uptime), but consistency and reliability ...
Continuous availability is an approach to computer system and application design that protects users against downtime, whatever the cause and ensures that users remain connected to their documents, data files and business applications.
This is known as N-model redundancy, where faults cause automatic fail-safes and a warning to the operator, and it is still the most common form of level one fault-tolerant design in use today. Voting was another initial method, as discussed above, with multiple redundant backups operating constantly and checking each other's results.