High availability is a characteristic of a system, which aims to ensure an agreed level of operational performance, usually uptime, for a higher than normal period.
Modernization has resulted in an increased reliance on these systems. For example, hospitals and data centers require high availability of their systems to perform routine daily activities. Availability refers to the ability of the user community to obtain a service or good, access the system, whether to submit new work, update or alter existing work, or collect the results of previous work. If a user cannot access the system, it is - from the users point of view - unavailable. Generally, the term downtime is used to refer to periods when a system is unavailable.
- Elimination of single points of failure. This means adding redundancy to the system so that failure of a component does not mean failure of the entire system.
- Reliable crossover. In redundant systems, the crossover point itself tends to become a single point of failure. Reliable systems must provide for reliable crossover.
- Detection of failures as they occur. If the two principles above are observed, then a user may never see a failure. But the maintenance activity must.
Availability is usually expressed as a percentage of uptime in a given year. The following table shows the downtime that will be allowed for a particular percentage of availability, presuming that the system is required to operate continuously. Service level agreements often refer to monthly downtime or availability in order to calculate service credits to match monthly billing cycles. The following table shows the translation from a given availability percentage to the corresponding amount of time a system would be unavailable.
|Availability %||Downtime per year||Downtime per month||Downtime per week||Downtime per day|
|90% ("one nine")||36.5 days||72 hours||16.8 hours||2.4 hours|
|95%||18.25 days||36 hours||8.4 hours||1.2 hours|
|97%||10.96 days||21.6 hours||5.04 hours||43.2 minutes|
|98%||7.30 days||14.4 hours||3.36 hours||28.8 minutes|
|99% ("two nines")||3.65 days||7.20 hours||1.68 hours||14.4 minutes|
|99.5%||1.83 days||3.60 hours||50.4 minutes||7.2 minutes|
|99.8%||17.52 hours||86.23 minutes||20.16 minutes||2.88 minutes|
|99.9% ("three nines")||8.76 hours||43.8 minutes||10.1 minutes||1.44 minutes|
|99.95%||4.38 hours||21.56 minutes||5.04 minutes||43.2 seconds|
|99.99% ("four nines")||52.56 minutes||4.38 minutes||1.01 minutes||8.66 seconds|
|99.995%||26.28 minutes||2.16 minutes||30.24 seconds||4.32 seconds|
|99.999% ("five nines")||5.26 minutes||25.9 seconds||6.05 seconds||864.3 milliseconds|
|99.9999% ("six nines")||31.5 seconds||2.59 seconds||604.8 milliseconds||86.4 milliseconds|
|99.99999% ("seven nines")||3.15 seconds||262.97 milliseconds||60.48 milliseconds||8.64 milliseconds|
|99.999999% ("eight nines")||315.569 milliseconds||26.297 milliseconds||6.048 milliseconds||0.864 milliseconds|
|99.9999999% ("nine nines")||31.5569 milliseconds||2.6297 milliseconds||0.6048 milliseconds||0.0864 milliseconds|
Uptime and availability can be used synonymously, as long as the items being discussed are kept consistent. That is, a system can be up, but its services are not available, as in the case of a network outage. This can also be viewed as, a system can be available to work on, but its services are not up from a functional perspective (as opposed to software service/process perspective). The perspective is important here, whether the item being discussed is the server hardware, server OS, functional service, software service/process...etc. Keep the perspective consistent throughout a discussion, then uptime and availability can be used synonymously.