A quorum node is an additional VM required to host quorum disks. Quorum disks are typically 1 GB in size each and are used for quorum failure groups within ASM disk groups. A quorum failure group is a special type of failure group that contains mirror copies of voting files when voting files are stored in NORMAL or HIGH redundancy disk groups (GRID disk group). Additionally, other disk groups also benefit from having quorum failure groups as they store additional copies of Oracle ASM metadata for better failure handling.
The quorum failure groups can help to determine if the disk group can be mounted in the event of the loss of one or more failure groups as the necessary number of copies of the Partner Status Table (PST) is maintained to accurately determine the most up to date and correct version of the PST. Because disks in quorum failure groups do not contain user data, a quorum failure group is not considered when determining redundancy requirements in respect to storing user data.
Quorum nodes require fewer resources than database nodes. However, to ensure stable operations having a dedicated CPU core is important.
Note: There is no Oracle Database software installed on the quorum nodes. Hence, the quorum nodes do not increase the number of licensed CPUs.
In a typical FlashGrid Cluster with two RAC nodes, all disk groups are configured with NORMAL redundancy. This configuration includes 2 database nodes + 1 quorum node and the cluster can survive loss of any one node.
In a typical FlashGrid Cluster with three RAC nodes, all disks groups are configured with HIGH redundancy. This configuration includes 3 database nodes + 2 quorum nodes and the cluster can survive loss of any two nodes.