High availability
lastmod 2026-05-28created 2020-06-232 edits
High availability (HA) is the property of an information system, such as servers, networks, and programs, being able to operate normally and continuously for a considerably long period of time. High availability means "availability is high", and it signifies "never failing". High availability is commonly expressed as the percentage of available time, such as 99% or 99.9%. Five nines (5 nines), that is 99.999%, which means allowing no more than 5 minutes and 15 seconds of downtime per year excluding planned downtime, is a very high level and is known to be the target of high-quality data centers. If high availability is required of an information system, all parts and components of that system must be well designed in advance and fully tested before actual use.
With a high availability solution (HACMP), systems can be grouped around a shared disk and tied together into a cluster. Many systems can be connected into a cluster at once, but connecting 2 servers is the most common approach. If one of the 2 clustered servers fails, the other server immediately takes over its work, so a system failure can be recovered in just a few seconds. RAID and SAN are widely used as high availability storage.