When adding quorum devices to your campus cluster, your goal should be to balance the number of quorum votes in each room. No single room should have a much larger number of votes than the other rooms, because loss of that room can bring the entire cluster down.
For campus clusters with more than three rooms and three nodes, quorum devices are optional. Whether you use quorum devices in such a cluster, and where you place them, depends on your assessment of the following:
Your particular cluster topology
The specific characteristics of the rooms involved
Resiliency requirements for your cluster
As with two-room clusters, locate the quorum device in a room you determine is more likely to survive any failure scenario. Alternatively, you can locate the quorum device in a room that you want to form a cluster, in the event of a failure. Use your understanding of your particular cluster requirements to balance these two criteria.
Refer to your Sun Cluster concepts documentation for general information about quorum devices and how they affect clusters that experience failures. If you decide to use one or more quorum devices, consider the following recommended approach:
For each room, total the quorum votes (nodes) for that room.
Define a quorum device in the room that contains the lowest number of votes and that contains a fully connected shared storage device.
When your campus cluster contains more than two nodes, do not define a quorum device if each room contains the same number of nodes.
The following sections discuss quorum devices in various sizes of campus clusters.
Figure 7–4 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.
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.
In three-room, two-node campus clusters, you should use the third room for the quorum device (Figure 7–1) or a server (Figure 7–3). Isolating the quorum device gives your cluster a better chance to maintain availability after the loss of one room. If at least one node and the quorum device remain operational, the cluster can continue to operate.
In two-room configurations, the quorum device occupies the same room as one or more nodes. Place the quorum device in the room that is more likely to survive a failure scenario if all cluster transport and disk connectivity are lost between rooms. If only cluster transport is lost, the node that shares a room with the quorum device is not necessarily the node that reserves the quorum device first. For more information about quorum and quorum devices, see the Sun Cluster concepts documentation.