Activation modes of shared disk groups


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Activation modes of shared disk groups

A shared disk group must be activated on a node in order for the volumes in the disk group to become accessible for application I/O from that node. The ability of applications to read from or to write to volumes is dictated by the activation mode of a shared disk group. Valid activation modes for a shared disk group are exclusivewrite, readonly, sharedread, sharedwrite, and off (inactive). These activation modes are described in detail in the table Activation modes for shared disk groups.

Note Disk group activation was a new feature in Veritas Volume Manager 3.0. To maintain compatibility with previous releases, the default activation mode for shared disk groups is shared-write.

Special uses of clusters, such as high availability (HA) applications and off-host backup, can use disk group activation to explicitly control volume access from different nodes in the cluster.

Activation modes for shared disk groups

Activation mode	Description
`exclusivewrite (ew)`	The node has exclusive write access to the disk group. No other node can activate the disk group for write access.
`readonly (ro)`	The node has read access to the disk group and denies write access for all other nodes in the cluster. The node has no write access to the disk group. Attempts to activate a disk group for either of the write modes on other nodes fail.
`sharedread (sr)`	The node has read access to the disk group. The node has no write access to the disk group, however other nodes can obtain write access.
`sharedwrite (sw)`	The node has write access to the disk group.
`off`	The node has neither read nor write access to the disk group. Query operations on the disk group are permitted.

The following table summarizes the allowed and conflicting activation modes for shared disk groups:

Allowed and conflicting activation modes

Disk group activated in cluster as...	Attempt to activate disk group on another node as...
Disk group activated in cluster as...	exclusive- write	readonly	sharedread	sharedwrite
exclusivewrite	Fails	Fails	Succeeds	Fails
readonly	Fails	Succeeds	Succeeds	Fails
sharedread	Succeeds	Succeeds	Succeeds	Succeeds
sharedwrite	Fails	Fails	Succeeds	Succeeds

To place activation modes under user control, create a defaults file /etc/default/vxdg containing the following lines:

enable_activation=true

default_activation_mode=activation-mode

The activation-mode is one of exclusivewrite, readonly, sharedread, sharedwrite, or off.

When a shared disk group is created or imported, it is activated in the specified mode. When a node joins the cluster, all shared disk groups accessible from the node are activated in the specified mode.

Note The activation mode of a disk group controls volume I/O from different nodes in the cluster. It is not possible to activate a disk group on a given node if it is activated in a conflicting mode on another node in the cluster. When enabling activation using the defaults file, it is recommended that this file be made identical on all nodes in the cluster. Otherwise, the results of activation are unpredictable.

If the defaults file is edited while the vxconfigd daemon is already running, the vxconfigd process must be restarted for the changes in the defaults file to take effect.

If the default activation mode is anything other than off, an activation following a cluster join, or a disk group creation or import can fail if another node in the cluster has activated the disk group in a conflicting mode.

To display the activation mode for a shared disk group, use the vxdg list diskgroup command as described in Listing shared disk groups.

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.

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 and Setting exclusive open access to a volume by a node.


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