VERITAS Storage Foundation 4.1 Cluster File System HP Serviceguard Storage Management Suite Extracts, December 2005

Chapter 6, SFCFS Architecture
About CFS
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File System Tuneables
Tuneable parameters are updated at the time of mount using the tunefstab file or
vxtunefs command. The file system tunefs parameters are set to be identical on all
nodes by propagating the parameters to each cluster node. When the file system is
mounted on the node, the tunefs parameters of the primary node are used. The
tunefstab file on the node is used if this is the first node to mount the file system.
VERITAS recommends that this file be identical on each node.
Split-Brain and Jeopardy Handling
A split-brain occurs when the cluster membership view differs among the cluster nodes,
increasing the chance of data corruption. Membership change also occurs when all
private-link cluster interconnects fail simultaneously, or when a node is unable to respond
to heartbeat messages.With I/O fencing, the potential for data corruption is eliminated.
Jeopardy State
In the absence of I/O fencing, SFCFS installation requires two heartbeat links. When a
node is down to a single heartbeat connection, SFCFS can no longer discriminate between
loss of a system and loss of the final network connection. This state is defined as jeopardy.
SFCFS employs jeopardy to prevent data corruption following a split-brain. Note that in
certain scenarios, the possibility of data corruption remains. For example:
◆ All links go down simultaneously.
◆ A node hangs and is unable to respond to heartbeat messages.
To eliminate the chance of data corruption in these scenarios, I/O fencing is required.
With I/O fencing, the jeopardy state does not require special handling by the SFCFS stack.
Jeopardy Handling
Potential split-brain conditions are
safeguarded by jeopardy handling. If any cluster node fails following a jeopardy state
notification, the cluster file system mounted on the failed nodes is disabled. If a node fails
after the jeopardy state notification, all cluster nodes also leave the shared disk group
membership.