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405Administering cluster functionality
Overview of cluster volume management
policy. However, in some cases, it is not desirable to have all nodes react in this
way to I/O failure. To address this, an alternate way of responding to I/O
failures, known as the local detach policy, was introduced in release 3.2 of VxVM.
The local detach policy is intended for use with shared mirrored volumes in a
cluster. This policy prevents I/O failure on a single slave node from causing a
plex to be detached. This would require the plex to be resynchronized when it is
subsequently reattached. The local detach policy is available for disk groups
that have a version number of 70 or greater.
Note: For small mirrored volumes, non-mirrored volumes, volumes that use
hardware mirrors, and volumes in private disk groups, there is no benefit in
configuring the local detach policy. In most cases, it is recommended that you
use the default global detach policy.
The detach policies have no effect if the master node loses access to all copies of
the configuration database and logs in a disk group. If this happened in releases
prior to 4.1, the master node always disabled the disk group. Release 4.1
introduces the disk group failure policy, which allows you to change this
behavior for critical disk groups. This policy is only available for disk groups
that have a version number of 120 or greater.
The following sections describe the detach and failure policies in greater detail.
Global detach policy
Caution: The global detach policy must be selected when Dynamic MultiPathing
(DMP) is used to manage multipathing on Active/Passive arrays, This ensures
that all nodes correctly coordinate their use of the active path.
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 mirrors that
were detached must be recovered before the redundancy of the data can be
restored.