Veritas Volume Manager 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Chapter 13, Administering Cluster Functionality
Overview of Cluster Volume Management
355
Local Detach Policy
Caution Do not use the local detach policy if you use the VCS agents that monitor the
cluster functionality of VERITAS Volume Manager, and which are provided
with VERITAS Storage Foundation
TM
for Cluster File System HA and VERITAS
Storage Foundation for databases HA. These agents do not notify VCS about
local failures.
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 a slave node, the master node performs the usual
I/O recovery operations to repair the failure, and additionally contacts all the nodes to see
if the disk is still acceptable to them. If the write failure is not seen by all the nodes, I/O is
stopped for the node that first saw the failure, and the application using the volume is also
notified about the failure.
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.
See “Setting the DIsk Detach Policy on a Shared Disk Group” on page 377 for information
on how to use the vxdg command to set the disk detach policy on a shared disk group.
The table, “Cluster Behavior Under I/O Failure to a Mirrored Volume for Different Disk
Detach Policies,” summarizes the effect on a cluster of I/O failure to the disks in a
mirrored volume:
Cluster Behavior Under I/O Failure to a Mirrored Volume for Different Disk Detach Policies
Type of I/O Failure Local (diskdetpolicy=local) Global (diskdetpolicy=global)
Failure of path to one disk
in a volume for a single
node
Reads fail only if no plexes remain
available to the affected node.
Writes to the volume fail.
The plex is detached, and I/O
from/to the volume continues. An
I/O error is generated if no plexes
remain.
Failure of paths to all
disks in a volume for a
single node
I/O fails for the affected node. The plex is detached, and I/O
from/to the volume continues. An
I/O error is generated if no plexes
remain.