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

to the device. A single preempt and abort command ejects a node from all paths
to the storage device.
About I/O fencing operations
I/O fencing, provided by the kernel-based fencing module (vxfen), performs
identically on node failures and communications failures. When the fencing
module on a node is informed of a change in cluster membership by the GAB
module, it immediately begins the fencing operation. The node tries to eject the
key for departed nodes from the coordinator disks using the preempt and abort
command. When the node successfully ejects the departed nodes from the
coordinator disks, it also ejects the departed nodes from the data disks. In a
split-brain scenario, both sides of the split would race for control of the coordinator
disks. The side winning the majority of the coordinator disks wins the race and
fences the loser. The loser then panics and restarts the system.
About I/O fencing communication
The vxfen driver connects to GAB port b to intercept cluster membership changes
(reconfiguration messages). During a membership change, the fencing driver
determines which systems are members of the cluster to allow access to shared
disks.
After completing fencing operations, the driver passes reconfiguration messages
to higher modules. CVM handles fencing of data drives for shared disk groups.
After a node successfully joins the GAB cluster and the driver determines that a
preexisting split-brain does not exist, CVM can import all shared disk groups. The
CVM master coordinates the order of import and the key for each disk group. As
each slave joins the cluster, it accepts the CVM list of disk groups and keys, and
adds its proper digit to the first byte of the key. Each slave then registers the keys
with all drives in the disk groups.
About I/O fencing components
The shared storage for SF Oracle RAC must support SCSI-3 persistent reservations
to enable I/O fencing. SF Oracle RAC involves two types of shared storage:
■ Data disks—Store shared data
See “About data disks” on page 46.
■ Coordination points—Act as a global lock during membership changes
See “About coordination points” on page 46.
45Overview of Veritas Storage Foundation for Oracle RAC
About preventing data corruption with I/O fencing