qdiskd, that provides supplemental heuristics to determine node fitness. With heuristics you can determine factors that are important to the operation of the node in the event of a network partition. For example, in a four-node cluster with a 3:1 split, ordinarily, the three nodes automatically "win" because of the three-to-one majority. Under those circumstances, the one node is fenced. With qdiskd however, you can set up heuristics that allow the one node to win based on access to a critical resource (for example, a critical network path). If your cluster requires additional methods of determining node health, then you should configure qdiskd to meet those needs.
qdiskd is not required unless you have special requirements for node health. An example of a special requirement is an "all-but-one" configuration. In an all-but-one configuration, qdiskd is configured to provide enough quorum votes to maintain quorum even though only one node is working.
qdiskd parameters for your deployment depend on the site environment and special requirements needed. To understand the use of heuristics and other qdiskd parameters, refer to the qdisk(5) man page. If you require assistance understanding and using qdiskd for your site, contact an authorized Red Hat support representative.
qdiskd, you should take into account the following considerations:
qdiskd membership timeout value. The reason is because the quorum daemon must detect failed nodes on its own, and can take much longer to do so than CMAN. The default value for CMAN membership timeout is 10 seconds. Other site-specific conditions may affect the relationship between the membership timeout values of CMAN and qdiskd. For assistance with adjusting the CMAN membership timeout value, contact an authorized Red Hat support representative.
qdiskd, use power fencing. While other types of fencing can be reliable for clusters not configured with qdiskd, they are not reliable for a cluster configured with qdiskd.
qdiskd supports a maximum of 16 nodes. The reason for the limit is because of scalability; increasing the node count increases the amount of synchronous I/O contention on the shared quorum disk device.
qdiskd are a multi-port SCSI RAID array, a Fibre Channel RAID SAN, or a RAID-configured iSCSI target. You can create a quorum disk device with mkqdisk, the Cluster Quorum Disk Utility. For information about using the utility refer to the mkqdisk(8) man page.