Five nines mean that systems and services are available 99.999% of the time. It also means that both planned and unplanned downtime is less than 5.26 minutes per year. The chart in the figure provides a comparison of the downtime for various availability percentages.
High availability refers to a system or component that is continuously operational for a given length of time. To help ensure high availability:
Eliminate single points of failure
Design for reliability
Detect failures as they occur
Sustaining high availability at the standard of five-nines can increase costs and utilize many resources. The increased costs are due to the purchase of additional hardware such as servers and components. As an organization adds components, the result is an increase in configuration complexity. Unfortunately, increased configuration complexity increases the risk factors. The more moving parts involved, the higher the likelihood of failed components.
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