Quorum in Clusters With Four Rooms or More

The following figure illustrates a four-node campus cluster with fully connected storage. Each node is in a separate room. Two rooms also contain the shared storage devices, with data mirrored between them.

Note that the quorum devices are marked optional in the illustration. This cluster does not require a quorum device. With no quorum devices, the cluster can still survive the loss of any single room.

Consider the effect of adding Quorum Device A. Because the cluster contains four nodes, each with a single quorum vote, the quorum device receives three votes. Four votes (one node and the quorum device, or all four nodes) are required to form the cluster. This configuration is not optimal, because the loss of Room 1 brings down the cluster. The cluster is not available after the loss of that single room.

If you then add Quorum Device B, both Room 1 and Room 2 have four votes. Six votes are required to form the cluster. This configuration is clearly better, as the cluster can survive the random loss of any single room.

Figure 7–4 Four-Room, Four-Node Campus Cluster

In Figure 7–4, the cluster interconnect is not shown.

Consider the optional I/O connection between Room 1 and Room 4. Although fully connected storage is preferable for reasons of redundancy and reliability, fully redundant connections might not always be possible in campus clusters. Geography might not accommodate a particular connection, or the project's budget might not cover the additional fiber.

In such a case, you can design a campus cluster with indirect access between some nodes and the storage. In Figure 7–4, if the optional I/O connection is omitted, Node 4 must access the storage indirectly.