I/O Fencing

I/O fencing allows write access to members of the active cluster and blocks access to non-members. The physical components of I/O fencing are data disks and coordinator disks. Each has a unique purpose and uses different physical disk devices.

Data Disks

Data disks are standard disk devices used for data storage. These can be physical disks or RAID Logical Units (LUNs). These disks must support SCSI-3 PGR. Data disks are incorporated in standard VxVM/CVM disk groups. CVM is responsible for fencing data disks on a disk-group basis. Because VxVM enables I/O fencing, several other features are also provided. Disks added to a group are automatically fenced, as are new paths to a device.

Coordinator Disks

Coordinator disks are special-purpose disks. They comprise three (or an odd number greater than three) standard disks, or LUNs, set aside for use by I/O fencing during cluster reconfiguration.

The coordinator disks act as a global lock device during a cluster reconfiguration. This lock mechanism determines which node is allowed to fence off data drives from other nodes. From a high level, a system 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 coordinator disks is the key to understanding how fencing helps prevent split-brain.

Coordinator disks cannot be used for any other purpose. You cannot store data on them, or include them in a disk group for user data. They can be any three disks that support SCSI-3 PGR. VERITAS recommends the coordinator disks use the smallest LUNs. Because coordinator disks do not store data, cluster nodes need only register with them, not reserve them.