Global Fencing

Fencing is a mechanism that is used by the cluster to protect the data integrity of a shared disk during split-brain situations. By default, the scinstall utility in Typical Mode leaves global fencing enabled, and each shared disk in the configuration uses the default global fencing setting of pathcount. With the pathcount setting, the fencing protocol for each shared disk is chosen based on the number of DID paths that are attached to the disk.

In Custom Mode, the scinstall utility prompts you whether to disable global fencing. For most situations, respond No to keep global fencing enabled. However, you can disable global fencing to support the following situations:

If you disable fencing under other situations than the following, your data might be vulnerable to corruption during application failover. Examine this data corruption possibility carefully when you consider turning off fencing.

• The shared storage does not support SCSI reservations.

  If you turn off fencing for a shared disk that you then configure as a quorum device, the device uses the software quorum protocol. This is true regardless of whether the disk supports SCSI-2 or SCSI-3 protocols. Software quorum is a protocol in Sun Cluster software that emulates a form of SCSI Persistent Group Reservations (PGR).

• You want to enable systems that are outside the cluster to gain access to storage that is attached to the cluster.

If you disable global fencing during cluster configuration, fencing is turned off for all shared disks in the cluster. After the cluster is configured, you can change the global fencing protocol or override the fencing protocol of individual shared disks. However, to change the fencing protocol of a quorum device, you must first unconfigure the quorum device. Then set the new fencing protocol of the disk and reconfigure it as a quorum device.

For more information about fencing behavior, see Failfast Mechanism in Sun Cluster Concepts Guide for Solaris OS. For more information about setting the fencing protocol of individual shared disks, see the cldevice(1CL) man page. For more information about the global fencing setting, see the cluster(1CL) man page.