A simple symmetric high availability system has two active physical nodes, each with its own disk array with two storage volumes, one volume for the local calendar store, and the other a mirror image of the other node's calendar store. Each node acts as the backup node for the other. When one node fails over to its backup, two instances of Calendar Server run concurrently on the backup node, each running from its own installation directory and accessing its own calendar store. The only thing shared is the computing power of the back up node.
The advantage of this type of high availability system is that both nodes are active simultaneously, thus fully utilizing machine resources. However, during a failure, the backup node will have more resource contention as it runs services for Calendar Server from both nodes.
Symmetric high availability also provides a backup storage array. In the event of a disk array failure, its redundant image can be picked up by the service on its backup node.
To configure a symmetric high availability system, you install the Calendar Server binaries on your shared disk. Doing so might prevent you from performing rolling upgrades, a feature planned for future releases of Calendar Server that enables you to update your system with a Calendar Server patch release with minimal or no down time.