Veritas Volume Manager 5.1 SP1 Administrator"s Guide (5900-1506, April 2011)

Global detach policy
The global detach policy is the traditional and default policy for all nodes on the
configuration. If there is a read or write I/O failure on a slave node, the master
node performs the usual I/O recovery operations to repair the failure, and, if
required, the plex is detached cluster-wide. All nodes remain in the cluster and
continue to perform I/O, but the redundancy of the mirrors is reduced. When the
problem that caused the I/O failure has been corrected, the disks should be
re-attached and the mirrors that were detached must be recovered before the
redundancy of the data can be restored.
Local detach policy
The local detach policy is designed to support failover applications in large clusters
where the redundancy of the volume is more important than the number of nodes
that can access the volume. If there is a write failure on any node, the usual I/O
recovery operations are performed to repair the failure, and additionally all the
nodes are contacted to see if the disk is still accessible to them. If the write failure
is local, and only seen by a single node, I/O is stopped for the node that first saw
the failure, and an error is returned to the application using the volume. The write
failure is global if more than one node sees the failure. The volume is not disabled.
If required, configure the cluster management software to move the application
to a different node, and/or remove the node that saw the failure from the cluster.
The volume continues to return write errors, as long as one mirror of the volume
has an error. The volume continues to satisfy read requests as long as one good
plex is available.
If the reason for the I/O error is corrected and the node is still a member of the
cluster, it can resume performing I/O from/to the volume without affecting the
redundancy of the data.
The vxdg command can be used to set the disk detach policy on a shared disk
group.
See “Setting the disk detach policy on a shared disk group” on page 458.
Table 13-3 summarizes the effect on a cluster of I/O failure to the disks in a
mirrored volume.
433Administering cluster functionality (CVM)
Overview of clustering