17.7 Quorum and Voting Strategies

Some system administrators have questioned the wisdom of needing quorum to be able to form a cluster, especially in large cluster configurations due to the number of nodes that would need to be up simultaneously. We do not see this as a problem because one of the purposes of a cluster is high availability – the point is to keep as many members participating in the cluster as possible. Furthermore, requiring a majority of votes (quorum) in order to form and maintain a cluster, more than one cluster will not be able to exist at any point in time, thereby preserving data integrity.

The questions that inevitably follow:

"Well, what if I want to take half of my systems down for maintenance?"
"What if I need to take several systems out of the cluster for a while to use on a development project?"
"What if half my systems crash?"

While we wonder why someone would break apart their production cluster, extenuating and unforeseen circumstances do occur which often require creative solutions, so…

You should plan your cluster for maximum availability at all times even when some members will be unavailable. Here are a few tips:

If you have a cluster with an even number of voting members, configure a quorum disk. See section 17.8.3.1.
If you have a cluster with an odd number of voting members, do not configure a quorum disk (or configure the quorum disk with zero votes – see section 17.8.3.1.1).
Only members with a direct, physical connection to the cluster_root, cluster_usr, and cluster_var file systems should be voting members.
If a member will be down for a planned but extended period of time, set the member's votes to zero (i.e., a non-voting member). See section 17.8.2.1.
If you lose several members unexpectedly such that one more failure will cause the cluster to lose quorum, adjust the cluster's expected votes until the members are once again added to the cluster. See section 17.8.2.2.

Both expected votes and member votes can be administratively modified, so it is no big deal to modify the votes within the cluster so that it requires fewer votes to form or continue. In the following sections we will discuss managing quorum in a cluster.