HP OpenVMS Systems Documentation

Volume Shadowing for OpenVMS

This section briefly describes the MOUNT command qualifiers that are useful for shadow set management. Refer also to the OpenVMS System Management Utilities Reference Manual for complete information about these and other DCL commands.

You must use the /SHADOW qualifier when you create a new shadow set or when you add a member to an existing shadow set. You can also use the optional qualifiers described in Table 4-1 and in Table 4-2. These qualifiers require the VOLPRO and OPER privileges, or your user identification code (UIC) must match the owner UIC of the volume being mounted. To mount a shadow set throughout the system, you must also have the SYSNAM privilege.

Detailed examples and descriptions of how to use these qualifiers are included in Section 4.5. In addition to the shadowing-specific qualifiers described in Table 4-1, the /NOASSIST, /SYSTEM, /GROUP, and /CLUSTER qualifiers are also frequently used when mounting shadow sets, as described in Table 4-2 and in Section 4.4.2.

4.4.1 MOUNT Command Qualifiers Specific to Shadowing

The MOUNT command qualifiers described in Table 4-1 are specific to shadowing.

The MOUNT command qualifiers described in this section are not specific to shadowing but can be very useful when creating shadow sets. These additional qualifiers are described in Table 4-2 and in the examples that follow.

You may occasionally find it useful to specify the /NOASSIST qualifier on the MOUNT command. For example, you can use the MOUNT/NOASSIST command in startup files to avoid failure of a MOUNT command when a device you specify in the command is not available. The /NOASSIST qualifier can be used in startup files because operator intervention is impossible during startup.

The MOUNT/NOASSIST qualifier can successfully mount the shadow set as long as at least one of the devices included in the MOUNT command is available for mounting. Example 4-2 shows an example of the /NOASSIST qualifier and the resulting messages when one of the members included in the command is not available for mounting.

$ MOUNT/SYS DSA65:/SHADOW=($4$DIA6,$4$DIA5) GALEXY/NOASSIST
%MOUNT-I-MOUNTED, GALEXY mounted on _DSA65:
%MOUNT-I-SHDWMEMSUCC, _$4$DIA6: (READY) is now a valid member of the shadowset
%MOUNT-I-SHDWMEMFAIL, $4$DIA5 failed as a member of the shadow set
-SYSTEM-F-VOLINV, volume is not software enabled

Even though device $4$DIA5 is not available for mounting, the MOUNT command continues to create the shadow set with $4$DIA6 as its only member. If the command did not include the /NOASSIST qualifier, the MOUNT command would not mount the shadow set.

4.4.4 Creating a Shadow Set With /SYSTEM and With /CLUSTER

When you create a shadow set, you must specify either the /SYSTEM qualifier or the /CLUSTER qualifier, or both (see Table 4-2) to provide access for all users on a single system or on a cluster.

In Example 4-3, if the shadow set (identified by its virtual unit name DSA2 ) is not currently mounted, the first command creates a shadow set with one shadow set member; the second command adds two more members to the same shadow set. An automatic copy operation causes any data on the second and third volumes to be overwritten as the shadow set members are added.

In the second MOUNT command, you need only specify the /SYSTEM when you add the $6$DIA5 and $6$DIA6 devices to the shadow set. Do not use /CLUSTER. These disks are added with the same status that the shadow set currently has, which in this case is clusterwide access.

$ MOUNT DSA2: /CLUSTER /SHADOW=$6$DIA4: PEAKSISLAND DISK$PEAKSISLAND
$ MOUNT DSA2: /SYSTEM/SHADOW=($6$DIA5:,$6$DIA6:) PEAKSISLAND DISK$PEAKSISLAND

Once a shadow set is created, you can add and remove individual members by mounting or dismounting physical disk devices. The shadowing software allows you to add and remove shadow set members at any time, transparently to user processes or applications running on the system.

4.5.1 Adding a Disk to an Existing Shadow Set

Example 4-4 shows how to add the disk $4$DUA3 to the DSA23 shadow set.

$  MOUNT/CONFIRM/SYSTEM DSA23: /SHADOW=($4$DUA9,$4$DUA3) volume-label

The command in Example 4-4 specifies both the currently active shadow set member ($4$DUA9) and the new member ($4$DUA3). Although it is not necessary to include them when mounting additional physical devices, you can specify current shadow set members without affecting their membership state.

Note that when you add volumes to an existing shadow set mounted across an OpenVMS Cluster system, the shadowing software automatically adds the new members on each OpenVMS Cluster node.

4.5.2 Creating a Two-Member Shadow Set and Adding a Third Member

Example 4-5 shows two commands. The first command creates the shadow set with two members; the next adds a third member to that shadow set.

$ MOUNT/SYSTEM DSA4: /SHADOW = ($3$DIA7:, $3$DIA8:)
FORMERSELF(1)
%MOUNT-I-MOUNTED, FORMERSELF   mounted on DSA4:
%MOUNT-I-SHDWMEMSUCC, _$3$DIA7: (DISK300) is now a valid member of
                      the shadow set
%MOUNT-I-SHDWMEMSUCC, _$3$DIA8: (DISK301) is now a valid member of
                      the shadow set

$ MOUNT/SYSTEM DSA4: /SHADOW = $3$DIA6:  FORMERSELF(2)

%MOUNT-I-SHDWMEMCOPY, _$3$DIA6: (DISK302) added to the shadow set
                      with a copy operation

1. A shadow set is created whose virtual unit name is DSA4. The member disks are $3$DIA7 and $3DIA8.
2. The disk $3$DIA6 is mounted systemwide in the shadow set identified by virtual unit DSA4. The shadow set now includes three shadow set members: $3$DIA6, $3$DIA7, and $3$DIA8. In this example, when you add $3$DIA6 after the shadow set already exists, the added volume becomes the target of a copy operation.

When you add a disk to an existing shadow set, a copy operation is necessary. Volume shadowing automatically performs the copy operation, unless you use the /CONFIRM qualifier or the /NOCOPY qualifier. When you specify the /CONFIRM qualifier, as shown in Example 4-6, the MOUNT command displays the targets of copy operations and requests permission before the operations are performed. This precaution prevents the erasing important data. For more information about copy operations, see Chapter 6.

$ MOUNT/CONFIRM DSA23: /SHADOW=($1$DUA4:,$1$DUA6:) SHADOWVOL (1)
%MOUNT-F-SHDWCOPYREQ, shadow copy required
Virtual Unit - DSA23 Volume Label - SHADOWVOL (2)
     Member                    Volume Label Owner UIC (3)
     $1$DUA6: (LOVE)           SCRATCH      [100,100]
Allow FULL shadow copy on the above member(s)? [N]: NO (4)
$

1. This command instructs MOUNT to build a shadow set with the specified devices and to prompt for permission to perform any copy operations.
2. Because a copy operation is necessary, the virtual unit name and the volume label are displayed.
3. The display also includes the physical device name, the volume label, and the volume owner of the potential shadow set member that requires the copy operation.
4. A response of No causes MOUNT to quit without mounting or copying.

When you specify more than one disk, the shadowing software automatically determines the correct copy operation to perform in order to make shadow set members consistent with each other (see Section 6.2 for details). The Mount utility interprets information recorded on each member to determine whether a member requires a copy operation, a merge operation, or no copy operation. If you are not sure which disks might be targets of copy operations, you can specify the /CONFIRM qualifier or the /NOCOPY qualifier as a precaution against overwriting important data when you mount a disk. With the /NOCOPY qualifier, you disable the copy operation.

Example 4-7 shows how to use the /NOCOPY qualifier to check the status of potential shadow set members before any data is erased.

$ MOUNT/NOCOPY DSA2: /SHADOW=($1$DUA4:,$1$DUA6:,$1$DUA7:) -
_$ SHADOWVOL DISK$SHADOWVOL (1)
%MOUNT-F-SHDWCOPYREQ, shadow copy required
%MOUNT-I-SHDWMEMFAIL, DUA7: failed as a member of the shadow set
%MOUNT-F-SHDWCOPYREQ, shadow copy required (2)
$  MOUNT/COPY(3) DSA2: /SHADOW=($1$DUA4:,$1$DUA6:,$1$DUA7:) -
_$ SHADOWVOL DISK$SHADOWVOL
%MOUNT-I-MOUNTED, SHADOWVOL    mounted on _DSA2:
%MOUNT-I-SHDWMEMSUCC, _$1$DUA4: (VOLUME001) is now a valid member of
                      the shadow set
%MOUNT-I-SHDWMEMSUCC, _$1$DUA6: (VOLUME002) is now a valid member of
                      the shadow set
%MOUNT-I-SHDWMEMCOPY, _$1$DUA7: (VOLUME003) added to the shadow set
                      with a copy operation (4)

1. This command instructs MOUNT to build a shadow set, with the specified devices, but only if a copy or merge operation is not required.
2. MOUNT did not build the shadow set because the specified disk, loaded on device $1$DUA7, required a copy operation. At this point you can verify that the volume in device $1$DUA7 does not contain any useful data.
3. If the device does not contain valuable data, you can reenter the MOUNT command and include the /COPY qualifier. This command instructs MOUNT to mount a shadow set and to proceed with the necessary copy or merge operation.
4. The shadow set is successfully mounted. The $1$DUA7 device is currently the target of a copy operation; it will attain full shadow set membership when the copy operation completes.

If a shadow set is already mounted on one or more nodes in an OpenVMS Cluster system, the /SHADOW qualifier is not required when you mount the same shadow set on other nodes in the cluster. For example, if DSA42 is already mounted in the cluster when a new node is brought into the cluster, you can use the following command to mount DSA42 on the new node:

$ MOUNT/SYS DSA42:  volume-label logical-name

Upon receiving this command, the volume shadowing software creates the virtual unit on the new node with the same members that currently exist in the cluster.

4.6.1 Reconstructing a Shadow Set With /INCLUDE

$ MOUNT /SYSTEM DSA4/SHAD=($4$DIA1,$4$DIA2,$4$DIA3) NEWDISK(1)
%MOUNT-I-MOUNTED, NEWDISK   mounted on _DSA4:
%MOUNT-I-SHDWMEMSUCC, _$4$DIA1: (DISK01) is now a valid member
                          of the shadow set
%MOUNT-I-SHDWMEMCOPY, _$4$DIA2: (DISK02) added to the shadow set
                          with a copy operation
%MOUNT-I-SHDWMEMCOPY, _$4$DIA3: (DISK03) added to the shadow set
                          with a copy operation
$ DISMOUNT DSA4(2)
$
$ MOUNT DSA4:/SYSTEM/SHAD=$4$DIA1 NEWDISK/INCLUDE(3)
%MOUNT-I-MOUNTED, NEWDISK   mounted on _DSA4:
%MOUNT-I-SHDWMEMSUCC, _$4$DIA1: (DISK01) is now a valid member (4)
                          of the shadow set
%MOUNT-I-AUTOMEMCOPY, _$4$DIA2: (DISK02) automatically added (4)
                          to the shadow set
%MOUNT-I-AUTOMEMCOPY, _$4$DIA3: (DISK03) automatically added (4)
                          to the shadow set

1. This is the original MOUNT command that created the shadow set represented by DSA4. The shadow set consists of three shadow set members: $4$DIA1, $4$DIA2, and $4$DIA3.
2. After all copy operations have completed, the DISMOUNT command dissolves the shadow set.
3. The /INCLUDE qualifier triggers the MOUNT command to reconstruct the shadow set back to the way it was before the shadow set was dissolved. The MOUNT command must specify the original virtual unit name (DSA4) and at least one of the original shadow set members ($4$DIA1). The Mount utility reads the membership list on $4$DIA1 (specified in the MOUNT command) to determine that $4$DIA2 and $4$DIA3 are also members of the shadow set.
4. Because the shadow set was properly dismounted, the shadow set members are in a consistent state. The MOUNT status messages indicate that the shadow set devices are added back into the shadow set without the need for copy operations.

Occasionally, you will need to mount a physical shadow set member as a nonshadowed disk. By default, when a shadow set member is mounted outside a shadow set, the Mount utility automatically write-locks the disk. This provides a safeguard against accidental modification, thereby allowing the disk to be remounted into a shadow set at a later time.

To override this default behavior, include the /OVERRIDE=SHADOW_MEMBERSHIP qualifier on the MOUNT command as shown in the following example:

$ MOUNT/OVERRIDE=SHADOW_MEMBERSHIP $4$DUA20: WORKDISK

This command ignores shadow set membership status and mounts a former shadow set member on $4$DUA20 as a nonshadowed disk with write access.