Quorum devices

A quorum device helps a cluster manager make cluster management decisions when the cluster manager's normal decision process does not produce a clear choice. When a cluster manager has to choose between multiple potential actions, the cluster manager counts how many cluster domain nodes support each of the potential actions; and then cluster manager chooses the action that is supported by the majority of cluster domain nodes. If exactly the same number of cluster domain nodes supports more than one choice, then the cluster manager refers to a quorum device to make the choice.

db2haicu supports the quorum devices listed in the following table.

Table 1. Types of quorum device supported by db2haicu
Quorum device	Description
network	A network quorum device is an IP address to which every cluster domain node can connect at all times.
majority	Cluster must have an odd number of hosts. In the event of an outage, quorum is preserved as long as a majority of nodes in the cluster survives.
disk	A disk that is shared by all nodes in the peer domain. In case of an outage, any node that cannot acquire this lock is assumed to be out of quorum.

Table 2. The following table lists the advantages and disadvantages of each quorum type supported by db2haicu
Quorum Type	Pros	Cons
Network IP Tiebreaker	Simple setup, easy to find a network IP accessible by all nodes in the cluster.	Dependency on all network components (switch, cable, adapters). A failure in these components or a prolonged network blip can lead to total cluster down. It is also possible for both sides to make the tiebreaker reservation if the cluster nodes lose communication with each other, which leads to a cluster split-brain situation.
Disk Tiebreaker	Generally seen as more reliable than Network IP Tiebreaker. No dependency on IP network.	Requires a tie-breaking capable shared disk device to be accessible by all hosts.
Majority Node Set	The most desirable/reliable quorum device. No disk or network dependency.	Requires an extra node in the cluster. Works best if all nodes are all located on either the same site, or all different sites.