Nodes are deployed in pairs known as input/output (I/O) groups, and multiple I/O groups comprise a clustered system. For the system 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 volumes 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, which leaves half of the nodes remaining in the system. This type of problem causes a loss of communication between the node canisters that remain in the system and those that do not remain. The nodes are split into groups where the remaining nodes in each group can communicate with each other but not with the other group of nodes that were formerly part of the system.
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 system, the nodes in that group stop operating and processing I/O requests from hosts.