About I/O Fencing Components; About Data Disks; About Coordination Points - Symantec Veritas Cluster Server Installation Manual

88 Configuring VCS clusters for data integrity

About I/O fencing components

About I/O fencing components

About data disks

About coordination points

If a system is so busy that it appears to stop responding, the other nodes could
declare it as dead. This declaration may also occur for the nodes that use the
hardware that supports a "break" and "resume" function. When a node drops
to PROM level with a break and subsequently resumes operations, the other
nodes may declare the system dead. They can declare it dead even if the system
later returns and begins write operations.
I/O fencing is a feature that prevents data corruption in the event of a
communication breakdown in a cluster. VCS uses I/O fencing to remove the risk
that is associated with split brain. I/O fencing allows write access for members of
the active cluster. It blocks access to storage from non-members so that even a
node that is alive is unable to cause damage.
After you install and configure VCS, you must configure I/O fencing in VCS to
ensure data integrity.
The shared storage for VCS must support SCSI-3 persistent reservations to enable
I/O fencing. VCS involves two types of shared storage:
Data disks—Store shared data
See "About data disks"
Coordination points—Act as a global lock during membership changes
See "About coordination points"
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 are 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.
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. Racing for
control of the coordination points to fence data disks is the key to understand
how fencing prevents split brain.
on page 88.
on page 88.