HP OpenVMS Systems Documentation

Content starts here Removing Members and Dissolving Shadow Sets
HP Volume Shadowing for OpenVMS: OpenVMS Version 8.4 > Chapter 4 Creating and Managing Shadow Sets Using DCL Commands

Removing Members and Dissolving Shadow Sets

You can remove shadow set members and dissolve shadow sets with the DCL command DISMOUNT. You must have GRPNAM and SYSNAM user privileges to dismount group and system volumes. You must also have the LOG_IO user privilege to use the /POLICY=[NO]MINICOPY [=OPTIONAL] qualifier).

The DISMOUNT command has the following format:

DISMOUNT {device-name[:] virtual-unit-name}

The action taken differs depending on whether you specify an individual shadow set member or the shadow set (by its virtual unit name) on the DISMOUNT command:

  • If you specify the device name of a shadow set member, only that member is dismounted, and the remaining shadow set members continue servicing I/O requests.

  • If you specify a shadow set virtual unit, all shadow set members are dismounted and the shadow set is dissolved.

To dismount a shadow set that is mounted across an OpenVMS Cluster system, include the /CLUSTER qualifier with the DISMOUNT command. If you dismount a shadow set without including the /CLUSTER qualifier, only the node from which you issued the command dismounts the shadow set. The shadow set remains operational on the other OpenVMS Cluster nodes that have the shadow set mounted.

If the disks on your system are neither SCSI nor Fibre Channel disks, you can use the /NOUNLOAD qualifier on the DISMOUNT command to prevent the disk volume or volumes from spinning down. The devices remain in a ready state. If you specify the /UNLOAD qualifier when dismounting a virtual unit, the disk volumes are physically spun down after the shadow set is dissolved. See the HP OpenVMS DCL Dictionary for more information about using the DISMOUNT command and its qualifiers.

Removing Members from Shadow Sets

To remove an individual member from a shadow set, specify the name of the physical device with the DISMOUNT command. For example:


When you dismount an individual shadow set member, all outstanding I/O operations are completed and the member is removed from the set.

Starting with OpenVMS Alpha Version 7.3, the /FORCE_REMOVAL ddcu: qualifier is available. If connectivity to a device has been lost and the shadow set is in mount verification, /FORCE_REMOVAL ddcu: can be used to immediately expel a named shadow set member (ddcu: ) from the shadow set. If you omit this qualifier, the device is not dismounted until mount verification completes. Note that this qualifier cannot be used in conjunction with the /POLICY=[NO]MINICOPY [=OPTIONAL] qualifier.

The device specified must be a member of a shadow set that is mounted on the node where the command is issued.

The /FORCE_REMOVAL qualifier gives system managers greater control of shadow sets whose members are located at different sites in an OpenVMS Cluster configuration. SET SHADOW command qualifiers are also available for specifying management attributes for shadow set members located at the same or different sites, as described in “Managing Shadow Sets With SET SHADOW (Integrity servers and Alpha)” and in “Managing Copy and Merge Operations (Integrity servers and Alpha)”.

NOTE: You cannot dismount a device if it is the only source member in a shadow set. All shadow sets must have at least one valid source member. If you try to dismount the only source member device, the DISMOUNT command fails and returns the message:
%DISM-F-SRCMEM, Only source member of shadow set cannot be dismounted

The only way to dismount the last source member of a shadow set is to dissolve the shadow set by specifying the virtual unit name on the DISMOUNT command.

Dissolving Shadow Sets

The way you dissolve a shadow set depends on whether it is mounted on a single system or on two or more systems in an OpenVMS Cluster system. In both cases, you use the DISMOUNT command. If the shadow set is mounted on a single system, you can dissolve the shadow set by specifying its virtual unit name with the DISMOUNT command. If the shadow set is mounted in a cluster, you must include the /CLUSTER qualifier to dissolve the DSA36 shadow set across the cluster. For example:


Dismounting the shadow set can be done only after all files are closed, thereby ensuring that the dismounted disks are fully consistent from a file system perspective. The dismount operation marks the shadow set members as being properly dismounted so that a rebuild is not required the next time the disks are mounted. However, if a merge operation was either pending or in progress, then the dismount operation marks the shadow set members as being improperly dismounted and requires a merge operation.

NOTE: If you dismount a virtual unit while a copy operation is in progress for the shadow set, the copy operation aborts and the shadow set is dissolved. You receive OPCOM messages similar to those in the following example:

%%%%%%%%%%%  OPCOM  24-MAR-1990 20:29:57.52  %%%%%%%%%%%
$7$DUA6:  (WRKDSK) has been removed from shadow set.
%%%%%%%%%%%  OPCOM  24-MAR-1990 20:29:57.68  %%%%%%%%%%%
$7$DUA56: (PLADSK)  has been removed from shadow set.
%%%%%%%%%%%  OPCOM  24-MAR-1990 20:29:57.88  %%%%%%%%%%%
Message from user SYSTEM on SYSTMX

Dismounting Shadow Sets in Site-Specific Shutdown Procedures

Site-specific shutdown command procedures can be created for each system in your cluster, as described in the OpenVMS System Manager’s Manual. The default SHUTDOWN.COM procedure that ships with the operating system performs a DISMOUNT/ABORT/OVERRIDE=CHECKS operation on all mounted volumes. If files are left open on any mounted shadow sets, a merge operation is required for these shadow sets when the system is rebooted.

To prevent such unnecessary merge operations, HP recommends that you modify each site-specific SYSHUTDWN.COM command procedure to dismount the shadow sets without using the DISMOUNT/ABORT/OVERRIDE=CHECKS qualifiers. If open files are found, they should be closed.

Dismounting and Remounting With One Less Member for Backup

As discussed in “Dissolving Shadow Sets ”, the virtual unit can be dismounted on the system or across an OpenVMS Cluster system. To ensure that the virtual unit has been dismounted correctly, the following steps are recommended:

  1. Issue the MOUNT/NOWRITE command, followed by the SHOW DEVICE command, for example:

    $ MOUNT/NOWRITE DSA42: /SHADOW=($4$DUA3,$4$DUA4,$4$DUA5) volume-label 
  2. Observe that the virtual unit is in a steady state; that is, all members are consistent and no copy or merge operation is in progress. If a copy or merge operation is in progress, you must wait for the operation to complete.

  3. When the virtual unit is in a steady state, remove a member from the shadow set with the DISMOUNT command, as shown in the following example:

    $ DISMOUNT $4$DUA5
  4. Dismount the virtual unit and then remount it with one less member, as shown by the following command:

    $ MOUNT/SYS DSA42: /SHADOW=($4$DUA3,$4$DUA4) volume-label

    The shadow set member that was removed can now be used for a backup operation of the virtual unit.

NOTE: If your application must run continuously (that is, you cannot dismount the virtual unit without disrupting your business), you can still remove a shadow set member that you plan to return later to the shadow set. Your application and recovery procedures must be designed to ensure data consistency, as described in “Guidelines for Using a Shadow Set Member for Backup”.