Nodes are deployed in pairs known as input/output (I/O) groups, and one to four I/O groups comprise a cluster. For the cluster to be functional, at least one node in each I/O group must be operational. If both of the nodes in an I/O group are not operational, access is lost to the virtual disks (VDisks) that are managed by the I/O group.
Quorum disks are used when there is a problem in the SAN fabric or when nodes are shut down, leaving half of the nodes remaining in the cluster. This type of problem causes a loss of communication between the nodes that remain in the cluster and those that do not. The nodes are split into groups where the nodes in each group can communicate with each other, but not with the other group of nodes that were formerly part of the cluster.
In this situation, some nodes must stop operating and processing I/O requests from hosts to preserve data integrity while maintaining data access. If a group contains less than half the nodes that were active in the cluster, the nodes in that group stop operating and processing I/O requests from hosts.
It is possible for a cluster to split into two groups with each group containing half the original number of nodes in the cluster. A quorum disk determines which group of nodes stops operating and processing I/O requests. In this tie-break situation, the first group of nodes that accesses the quorum disk marks their ownership of the quorum disk and as a result continues to operate as the cluster, handling all I/O requests. If the other group of nodes cannot access the quorum disk or finds it owned by another group of nodes, it stops operating as the cluster and does not handle I/O requests.