Symantec Veritas 5 - Manual (Page 91)

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http://entsupport.symantec.com/docs/283161
■ Each of the coordinator disks must use a physically separate disk or LUN.
Symantec recommends using the smallest possible LUNs for coordinator disks.
■ Each of the coordinator disks should exist on a different disk array, if possible.
■ The coordinator disks must support SCSI-3 persistent reservations.
■ Symantec recommends using hardware-based mirroring for coordinator disks.
■ Coordinator disks must not be used to store data or must not be included in
disk groups that store user data.
■ Coordinator disks cannot be the special devices that array vendors use. For
example, you cannot use EMC gatekeeper devices as coordinator disks.
The I/O fencing configuration files include:
You must create this file to include the coordinator disk
group information. /etc/vxfendg
You must set the I/O fencing mode to SCSI-3.
You can configure the vxfen module to use either DMP
devices or the underlying raw character devices. Note that
you must use the same SCSI-3 disk policy on all the nodes.
The SCSI-3 disk policy can either be raw or dmp. The policy
is raw by default. If you use iSCSI devices, you must set the
disk policy as dmp.
/etc/vxfenmode
91 Configuring VCS clusters for data integrity
About setting up disk-based I/O fencing