1. Installing and Configuring an Oracle Solaris Cluster 4.4 Environment
  2. Planning the Oracle Solaris Cluster Configuration
  3. Planning the Oracle Solaris Cluster Environment
  4. Oracle Solaris Cluster Configurable Components
  5. Global Fencing

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 prefer3. With the prefer3 setting, the SCSI-3 protocol is used.

If any device is unable to use the SCSI-3 protocol, the pathcount setting should be used instead, where the fencing protocol for the shared disk is chosen based on the number of DID paths that are attached to the disk. Non-SCSI-3 capable devices are limited to two DID device paths within the cluster. Fencing can be turned off for devices which do not support either SCSI-3 or SCSI-2 fencing. However, data integrity for such devices cannot be guaranteed during split-brain situations.

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 in certain situations.

Caution:

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

The situations in which you can disable global fencing are as follows:

  • 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 Oracle Solaris 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 Concepts for Oracle Solaris Cluster 4.4. For more information about setting the fencing protocol of individual shared disks, see the cldevice(8CL) man page. For more information about the global fencing setting, see the cluster(8CL) man page.