Veritas Storage Foundation 5.1 SP1 Cluster File System Administrator"s Guide (5900-1738, April 2011)

With SCSI-3 PR technology, blocking write access is as easy as removing a
registration from a device. Only registered members can "eject" the registration
of another member. A member wishing to eject another member issues a "preempt
and abort" command. Ejecting a node is final and atomic; an ejected node cannot
eject another node. In SFCFS, a node registers the same key for all paths 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 components
The shared storage for SFCFS must support SCSI-3 persistent reservations to
enable I/O fencing. SFCFS involves two types of shared storage:
■ Data disks—Store shared data
See “About data disks” on page 33.
■ Coordination points—Act as a global lock during membership changes
See “About coordination points” on page 34.
About data disks
Data disks are standard disk devices for data storage and are either physical disks
or RAID Logical Units (LUNs).
These disks must support SCSI-3 PR and must be part of standard VxVM or CVM
disk groups. CVM is responsible for fencing data disks on a disk group basis. Disks
that are added to a disk group and new paths that are discovered for a device are
automatically fenced.
33Storage Foundation Cluster File System architecture
About I/O fencing