5.5 Quorum Resource

The Quorum Resource is an example of a technology used in Cluster Server that Microsoft received from two of its key partners Digital Equipment Corporation and Tandem Computer Corporation, both of which are now part of Compaq Computer Corporation. An easy way to understand what a Quorum Resource does is to think of it as sort of a "tie-breaker" for determining who gets control of the cluster. Microsoft took advantage of standard protocols that are part of the SCSI standard that allow for access control of disks on a SCSI bus that has more than one computer on it. The Quorum device is currently implemented as a SCSI disk that can be "owned" by only one system at a time. This condition is guaranteed by the SCSI control protocol, which supports commands that ensure that a disk is under control of only one SCSI controller at a time. Because of this, the SCSI protocol must be used to control the disk designated as Quorum Resources at this time. The Quorum device must use the SCSI protocol no matter how it is physically connected. That means that even though it might be connected using Fibre Channel, it must still talk the SCSI protocol to work as a Quorum device.

The Quorum Device is important because of a situation that Microsoft calls a "split-brain" cluster. This condition can occur when two or more nodes in a cluster are both up and running. The problem occurs when each one thinks that...