Using $DISMOU to Dismount Shadow Sets

You can use the $DISMOU system service to perform the following three shadow set operations:

Remove a member from a shadow set
Remove a member from a shadow set for a minicopy operation (as described in “Guidelines for Using a Shadow Set Member for Backup”)
Dismount a shadow set across a cluster from a single node
Dismount and dissolve a shadow set

The call to the $DISMOU system service has the following format:

SYS$DISMOU devnam, flags

The action that $DISMOU takes depends in part on whether you specify a shadow set virtual unit or a shadow set member in the devnam argument.

For a complete description of the $DISMOU service and its arguments, see the HP OpenVMS System Services Reference Manual.

Removing Members from Shadow Sets

If you want to remove a single member from a shadow set, you must make a call to $DISMOU. In the devnam argument, you should specify the name of the shadow set member you want to remove. The specified member is spun down unless you specify the DMT$M_NOUNLOAD option in the flags argument.

The MACRO-32 code in Example 5-4 demonstrates a call to $DISMOU that removes the member $2$DUA9 from a shadow set.

Example 5-4 Removing a Member from a Shadow Set

$DMTDEF
FLAGS:  .LONG DMT$M_NOUNLOAD
MEMBER001: .ASCID /$2$DUA9:/
	.
	.
	.

$DISMOU_S -
	devnam = MEMBER001, -
	flags = FLAGS
	.
	.
	.
.END

Dismounting and Dissolving Shadow Sets

If you want to dismount a shadow set on a single node, you must make a call to $DISMOU. In the devnam argument, you should specify the name of the virtual unit that represents the shadow set you want to dismount. If you want to dismount the shadow set clusterwide, specify the DMT$M_CLUSTER option in the flags argument of the call.

When you dismount a shadow set on a single node in an OpenVMS Cluster system, and other nodes in the OpenVMS Cluster still have the shadow set mounted, none of the shadow set members contained in the shadow set are spun down, even if you have not specified the DMT$M_NOUNLOAD flag. After this call completes, the shadow set is unavailable on the node from which the call was made. The shadow set is still available to other nodes in the cluster that have the shadow set mounted.

If the node on which the shadow set is being dismounted is the only node that has the shadow set mounted, the shadow set dissolves. The shadow set member devices are spun down unless you specify the DMT$M_NOUNLOAD flag.

The MACRO-32 code in Example 5-5 demonstrates how to use the $DISMOU system service to dismount the shadow set represented by the virtual unit DSA23.

Example 5-5 Dismounting and Dissolving a Shadow Set Locally

$DMTDEF
FLAGS:    .LONG 0
DSA23:  .ASCID /DSA23:/
	.
	.
	.

$DISMOU_S -
	devnam = DSA23, -
	flags = FLAGS
	.
	.
	.
.END

When a shadow set is dissolved:

Each of the former shadow set members can be mounted as a single disk for other purposes.
Each volume, however, continues to be marked as having been part of a shadow set. After you dissolve a shadow set, each volume retains the volume shadowing generation number that identifies it as being a former shadow set member (unless you remount the volume outside of the shadow set). Volumes marked as having been part of a shadow set are automatically software write-locked to prevent accidental deletion of data. You cannot mount these volumes for writing outside of a shadow set unless you use the MNT$M_OVR_SHAMEM option with the system service MNT$_FLAGS item code.
The virtual unit changes to an offline state.

The MACRO-32 code in Example 5-6 demonstrates a call to the $DISMOU system service to perform a dismount across the cluster. When the shadow set is dismounted from the last node, the shadow set is dissolved.

Example 5-6 Dismounting and Dissolving a Shadow Set Across the Cluster

$DMTDEF
FLAGS:   .LONG  DMT$M_CLUSTER
DSA23:  .ASCID /DSA23:/
	.
	.
	.

$DISMOU_S -
	devnam = DSA23, -
	flags = FLAGS
	.
	.
	.
.END

You must specify the DMT$M_CLUSTER option with the flags argument if you want the shadow set dismounted from every node in the cluster. When each node in the cluster has dismounted the shadow set (the number of hosts having the shadow set mounted reaches zero), the volume shadowing software dissolves the shadow set.

Setting $DISMOU Flags for Shadow Set Operations

Table 5-1 lists the options for the $DISMOU flags argument and describes the shadow set operations that use these options. For a full description of each of these flag options, see the description of the $DISMOU service in the HP OpenVMS System Services Reference Manual.

Table 5-1 $DISMOU Flag Options

Option	Description
DMT$M_MINICOPY_REQUIRED	$DISMOU fails if minicopy has not been enabled on the disk.
DMT$M_MINICOPY_OPTIONAL	$DISMOU takes place, regardless of whether minicopy is enabled on the disk.
DMT$M_FORCE	If connectivity to a device has been lost and the shadow set is in mount verification, this flag causes a named shadow set member to be immediately expelled from the shadow set.
DMT$M_UNLOAD	Valid for all shadowing-related requests.
DMT$M_CLUSTER	Valid for all shadowing-related requests.
DMT$M_ABORT	Honored for virtual units, ignored for members.
DMT$M_UNIT	Ignored for virtual units and their members.