Fencing in VCS involves coordinator disks and data disks. Each component has a unique purpose and uses different physical disk devices. The fencing driver is vxfen.
Data disks are standard disk devices for data storage and are either physical disks or RAID Logical Units (LUNs). These disks must support SCSI-3 PR and are part of standard VxVM or CVM disk groups.
CVM is responsible for fencing data disks on a disk group basis. Disks added to a disk group are automatically fenced, as are new paths discovered to a device.
Coordinator disks are three standard disks or LUNs set aside for I/O fencing during cluster reconfiguration. Coordinator disks do not serve any other storage purpose in the VCS configuration. Users cannot store data on these disks or include the disks in a disk group for user data. The coordinator disks can be any three disks that support SCSI-3 PR. Coordinator disks cannot be special devices that array vendors use. For example, you cannot use EMC gatekeeper devices as coordinator disks.
Symantec recommends using the smallest possible LUNs for coordinator disks. Because coordinator disks do not store any data, cluster nodes need only register with them and do not need to reserve them.
These disks provide a lock mechanism to determine which nodes get to fence off data drives from other nodes. A node must eject a peer from the coordinator disks before it can fence the peer from the data drives. This concept of racing for control of the coordinator disks to gain the ability to fence data disks is key to understanding prevention of split brain through fencing.
Dynamic Multipathing devices with I/O fencing
DMP allows coordinator disks to take advantage of the path failover and the dynamic adding and removal capabilities of DMP. You can configure coordinator disks to use Veritas Volume Manager Dynamic Multipathing (DMP) feature.
For more information on using DMP, see the Veritas Volume Manager Administrator's Guide.
See Updating /etc/vxfenmode file