Quorum disks

A quorum disk is an MDisk or a managed drive that contains a reserved area that is used exclusively for system management. A system automatically assigns quorum disk candidates.

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 nodes 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.

It is possible for a system to split into two groups where each group contains half the original number of nodes in the system. 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 is marked as the owner of the quorum disk and as a result continues to operate as the system, handling all I/O requests. If the other group of nodes cannot access the quorum disk or finds the quorum disk is owned by another group of nodes , it stops operating as the system and does not handle I/O requests.

A system can have only one active quorum disk that is used for a tie-break situation. However, the system uses three quorum disks to record a backup of system configuration data to be used in the event of a disaster. The system automatically selects one active quorum disk from these three disks. The other quorum disk candidates provide redundancy if the active quorum disk fails before a system is partitioned. To avoid the possibility of losing all the quorum disk candidates with a single failure, assign quorum disk candidates on multiple storage systems.