VERITAS Volume Manager 3.5 Administrator's Guide (September 2002)

Chapter 10, Administering Cluster Functionality
Overview of Cluster Volume Management
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You can also use the vxdg command to change the activation mode on a shared disk
group as described in “Changing the Activation Mode on a Shared Disk Group” on
page 266.
For a description of how to configure a volume so that it can only be opened by a single
node in a cluster, see “Creating Volumes with Exclusive Open Access by a Node” on
page 266 and “Setting Exclusive Open Access to a Volume by a Node” on page 267.
Connectivity Policy of Shared Disk Groups
The nodes in a cluster must always agree on the status of a disk. In particular, if one node
cannot write to a given disk, all nodes must stop accessing that disk before the results of
the write operation are returned to the caller. Therefore, if a node cannot contact a disk, it
should contact another node to check on the disk’s status. If the disk fails, no node can
access it and the nodes can agree to detach thedisk. If thedisk does notfail, but ratherthe
access paths from some of the nodes fail, the nodes cannot agree on the status of the disk.
Either of the following policies for resolving this type of discrepancy may be applied:
◆ Under the global connectivity policy, the detach occurs cluster-wide (globally) if any
node in the cluster reports a disk failure. This is the default policy.
◆ Under the local connectivity policy, in the event of disks failing, the failures are
confined to the particular nodes that saw the failure. Note that an attempt is made to
communicate with all nodes in the cluster to ascertain the disks’ usability. If all nodes
report a problem with the disks, a cluster-wide detach occurs.
See “Setting theConnectivity Policy on aSharedDisk Group”on page 266 forinformation
on howto use the vxedit command to setthe connectivity policy on a shared disk group.
Limitations of Shared Disk Groups
The cluster functionality of VxVM does not support RAID-5 volumes, or task monitoring
for cluster-shareable disk groups. These features can, however, be used in private disk
groups that are attached to specific nodes of a cluster. Online relayout is supported
provided that it does not involve RAID-5 volumes.
The root disk group (rootdg) cannot be made cluster-shareable. It must be private.
Only raw device access may be performed via the cluster functionality of VxVM. It does
not support shared access to file systems in shared volumes unless the appropriate
software is installed and configured.
If ashared disk group contains unsupportedobjects, deport it andthen re-import thedisk
group as private on one of the cluster nodes. Reorganize the volumes into layouts that are
supported forshared diskgroups, and thendeport and reimport the disk group as shared.