High Availability

The term high availability is used for computer systems that are optimized for non-stop operation with a minimum of unplanned downtime. Usually this is achieved by avoiding single points of failure and adding additional redundancy to the entire system. Common solutions are hot-standby systems that take over the role of a crashed server in a split second. More sophisticated solutions offer both scalability and high availability by using entire clusters of servers that run their applications in a distributed manner but look like one virtual server to their clients.

But how high is HA? Have a look at our table or enter your own percentage to see how much unplanned downtime per year or month has to be tolerated at a certain service level.

Common pitfalls

1. Planned downtime for maintenance (e.g. upgrading the hardware, installing new software) is usually not included in the guaranteed maximum downtime. That's why it's called unplanned downtime.
2. An availability of n% per year is not the same like n% per month, since it can not be guaranteed that the entire downtime per year will be evenly distributed over all twelve months. Example: to get an availability of 97% per month you need approximately 99.75% per year in the worst case.