Coordination points 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 coordination points before it can fence the peer from the data drives. VCS prevents split-brain when vxfen races for control of the coordination points and the winner partition fences the ejected nodes from accessing the data disks.
The coordination points can either be disks or servers or both.
Coordinator disks
Disks that act as coordination points are called coordinator disks. 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.
Dynamic Multi-pathing (DMP) allows coordinator disks to take advantage of the path failover and the dynamic adding and removal capabilities of DMP. On cluster nodes with HP-UX 11i v3, you must use DMP devices or iSCSI devices for I/O fencing. The following changes in HP-UX 11i v3 require you to not use raw devices for I/O fencing:
Provides native multipathing support
Does not provide access to individual paths through the device file entries
The metanode interface that HP-UX provides does not meet the SCSI-3 PR requirements for the I/O fencing feature. You can configure coordinator disks to use Veritas Volume Manager Dynamic Multi-pathing (DMP) feature.
See the Veritas Storage Foundation Administrator's Guide.
Coordination point servers
The coordination point server (CP server) is a software solution which runs on a remote system or cluster. CP server provides arbitration functionality by allowing the VCS cluster nodes to perform the following tasks:
Self-register to become a member of an active VCS cluster (registered with CP server) with access to the data drives
Check which other nodes are registered as members of this active VCS cluster
Self-unregister from this active VCS cluster
Forcefully unregister other nodes (preempt) as members of this active VCS cluster
In short, the CP server functions as another arbitration mechanism that integrates within the existing I/O fencing module.
Multiple VCS clusters running different operating systems can simultaneously access the CP server. TCP/IP based communication is used between the CP server and the VCS clusters.