Veritas Storage Foundation 5.1 SP1 for Oracle RAC Administrator"s Guide (5900-1512, April 2011)

See Figure 1-10 on page 55.
In such a configuration, if the site with two coordinator disks is inaccessible,
the other site does not survive due to a lack of a majority of coordination points.
I/O fencing would require extension of the SAN to the third site which may
not be a suitable solution. An alternative is to place a CP server at a remote
site as the third coordination point.
Note: The CP server provides an alternative arbitration mechanism without having
to depend on SCSI-3 compliant coordinator disks. Data disk fencing in CVM will
still require SCSI-3 I/O fencing.
Figure 1-10
Skewed placement of coordinator disks at Site 1
Coordinator disk #1
Coordinator disk #2
Coordinator disk #3
Site 1 Site 2
Node 1 Node 2
Public
Network
SAN
Defining Coordination Points
Three or more odd number of coordination points are required for I/O fencing. A
coordination point can be either a CP server or a coordinator disk. A CP server
provides the same functionality as a coordinator disk in an I/O fencing scenario.
Therefore, it is possible to mix and match CP servers and coordinator disks for
the purpose of providing arbitration.
Symantec supports the following three coordination point configurations:
■ Vxfen driver based I/O fencing using SCSI-3 coordinator disks
■ Customized fencing using a combination of SCSI-3 disks and CP server(s) as
coordination points
■ Customized fencing using only three CP servers as coordination points
55Overview of Veritas Storage Foundation for Oracle RAC
About preventing data corruption with I/O fencing