In addition to the two types of highly available systems described in this chapter, a third type which is a hybrid of the two is also possible. This is a multi-node asymmetric high availability system. In this type, “N” disk arrays and “N” nodes all use the same backup node which is held in reserve and is not active normally. This backup node is capable of running Calendar Server for any of the “N” nodes. It shares each of the “N” node's disk array, as shown in the preceding graphic. If multiple nodes fail at the same time, the backup node must be capable of running up to “N” instances of Calendar Server concurrently. Each of the “N” nodes has its own disk array.
The advantages of the N+1 model are that Calendar Server load can be distributed to multiple nodes, and that only one backup node is necessary to sustain all the possible node failures.
The disadvantage of this type of high availability is the same as any asymmetric system; the backup node is idle most of the time. In addition, the N+1 high availability system backup node must have excess capacity in the event it must host multiple instances of Calendar Server. This means a higher cost machine is sitting idle. However, the machine idle ratio is 1:N as opposed to 1:1, as is the case in a single asymmetric system.
To configure this type of system, use the instructions for the asymmetric high availability system for each of the “N” nodes and the backup. Use the same backup node each time, but with a different primary node.