Updated: 11 December 1998

Volume Shadowing for OpenVMS

When creating a shadow set virtual unit, you must specify the /SYSTEM and /CLUSTER qualifiers as necessary to provide access by all users on the system.

In Example 4-3, the first command creates a shadow set with one shadow set member; the second command adds two more member units 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.

Example 4-3 Using the /CLUSTER Qualifier

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

In the second MOUNT command, you need only to 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 implies clusterwide access.

4.4 Adding Shadow Set Members

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.

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

Example 4-4 Adding Disk to an Existing Shadow Set

$ MOUNT/CONFIRM/SYSTEM DSA23: /SHADOW = ($4$DUA9,$4$DUA3) VOLUMELABEL

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 a OpenVMS Cluster system, the shadowing software automatically adds the new members on each OpenVMS Cluster node.

When you add a disk to an existing shadow set, a copy operation is necessary. Because volume shadowing automatically performs the copy operation, Digital recommends the use of the /CONFIRM qualifier whenever you mount a disk as a precaution to avoid erasing important data. See Chapter 6 for more information about copy operations.

The following series of examples demonstrate various ways to use the MOUNT command and its qualifiers to mount shadow sets.

Examples

$ 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)
$ 
 
      

This example shows how to use the /CONFIRM qualifier to check the status of potential shadow set members before any data is erased.

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.

$ 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)
 
      

This example shows how to use the /NOCOPY qualifier to check the status of potential shadow set members before any data is erased.

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.

$ 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
 
      

This example shows how to reconstruct a shadow set in which the volume shadowing software determines which disk volumes are former members of 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 software 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. If you want to override this default behavior, include the /OVERRIDE=SHADOW_MEMBERSHIP qualifier on the MOUNT command line 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.

$ MOUNT/SYSTEM DSA4: /SHADOW = ($3$DIA7:, $3$DIA8:)   FORMERSELF
%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
 
%MOUNT-I-SHDWMEMCOPY, _$3$DIA6: (DISK302) added to the shadow set
                      with a copy operation
 
      

This example shows two commands. The first command creates the shadow set with two members; the next adds a third volume to that shadow set.

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.

You can mount shadow sets as constituent members of volume sets. Example 4-5 mounts a volume set in which each volume (DSA23, DSA24, DSA26) is a shadow set. You must include the /BIND=volume-set-name qualifier on the MOUNT command line when you create a volume set or add a new volume to a set.

Example 4-5 Mounting a Shadowed Volume Set

$ MOUNT/BIND=LIBRARY DSA23: (1) /SHADOW = ($7$DUA4:, (2) $7$DUA5:,$7$DUA7:), - _$ DSA24: (1) /SHADOW = ($7$DIA9:, (2) $7$DIA10:, $7$DIA12:), - _$ DSA26: (1) /SHADOW = ($42$DUA20:, (2) $42$DUA21:, $42$DUA22:)- _$ LURKINGVOL1, LURKINGVOL2, LURKINGVOL3 (3) DISK$LURKINGVOL (4)

When mounting volume sets, always list the volume with the largest storage capacity first. You should name the largest volume first because the volume set and directory information goes on the first volume listed in a MOUNT command line. A small-capacity disk may not have adequate storage for the volume and directory information.

1. These are the names of the virtual units that represent shadow sets mounted in the volume set. DSA23 is the root volume of the set because it is the first volume specified in the command line. Although each shadow set must consist of the same type of disk, the example shows that a volume set can bind shadow sets consisting of RA devices with a shadow set consisting of RF devices.
2. $7$DUA4, $7$DUA5, and $7$DUA7 through $42$DUA22 are the physical units that are shadow set members. Note that one of the shadow sets uses disks from a different allocation class.
3. LURKINGVOL1, LURKINGVOL2, and LURKINGVOL3 are the volume labels of the virtual units mounted in a volume set.
4. DISK$LURKINGVOL is the logical name that represents the volume set.

Refer to OpenVMS System Manager's Manual for further information about volume sets.

4.6 Dismounting Shadow Sets

You can dismount shadow set virtual units and member units with the DCL command DISMOUNT. You must have GRPNAM and SYSNAM user privileges to dismount group and system volumes. 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 virtual unit 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 a OpenVMS Cluster system, include the /CLUSTER qualifier with the DISMOUNT command. If you dismount a virtual unit 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 you also specify the /NOUNLOAD qualifier on the DISMOUNT command line, the disk volume or volumes do not physically spin 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 OpenVMS DCL Dictionary for more information about using the DISMOUNT command and its qualifiers.

4.6.1 Removing Member Units 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:

$ DISMOUNT $5$DUA7:

When you dismount an individual shadow set member, all outstanding I/O operations are completed and the member is removed from the set. However, note that files that are open on the shadow set virtual unit remain open because access to the shadow set continues. Because of this, file integrity on the removed physical unit cannot be guaranteed. Therefore, the disk is marked as being improperly dismounted so that a rebuild is initiated the next time the disk is mounted. To guarantee file integrity, you must dismount a virtual unit to dissolve the entire shadow set (described in Section 4.6.2). See Section 6.1 for important information about the data consistency issues that result if you dismount an individual shadow set member.

You dismount a virtual unit by specifying the virtual unit name with the DISMOUNT command. Specifying the virtual unit name causes all members in the shadow set to be dismounted, thereby dissolving the shadow set on the node issuing the DISMOUNT command. The command in Example 4-6 includes the /CLUSTER qualifier to dissolve the DSA36 shadow set across an OpenVMS Cluster system.

Example 4-6 Dissolving a Shadow Set

$ DISMOUNT /CLUSTER DSA36:

File integrity is ensured when you dismount a shadow set virtual unit. Dismounting the virtual unit can be done only after all files are closed, thereby ensuring that the dismounted disks are fully consistent. 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.

You can use the DCL command SHOW DEVICES or the F$GETDVI lexical function to get information about a shadow set virtual unit and the physical volumes that make up the member units. You can also use the System Dump Analyzer (SDA) to get more information about shadow sets.

The following sections describe how to use these tools to examine volume shadowing virtual units and shadow set members. See also the OpenVMS DCL Dictionary for a full description of how to use the SHOW DEVICES command and the F$GETDVI lexical function. See the System Dump Analyzer Utility Manual for more information about how to use SDA.

You can use any of the SHOW DEVICES qualifiers when you examine virtual units or shadow set members.

Use SHOW DEVICES in the following format to display information about virtual units:

SHOW DEVICES [virtual-unit-name[:]]

The variable virtual-unit-name replaces device-name as the SHOW DEVICES command parameter for virtual units. Use the virtual unit naming format DSAn:.

As with any SHOW DEVICES command, the colon is optional. Note also that you can specify a complete virtual unit name (or a portion of a virtual unit name) just as you can with device names. If you omit the virtual unit number, SHOW DEVICES lists all the shadow set virtual units that represent shadow set member disks of the type specified. If you truncate a device name (for example, if you specify D), SHOW DEVICES lists all the devices and all the virtual units that begin with the letters you entered (in this case, D).

When you specify the virtual unit number, SHOW DEVICES displays the names of the shadow set members it represents. If you use the /FULL qualifier, SHOW DEVICES displays full information about the virtual unit and all the associated shadow set members.

Because individual shadow set members that are mounted for systemwide or clusterwide access are not allocated or mounted in the traditional sense, a SHOW DEVICES command with the /ALLOCATED or /MOUNTED qualifiers displays only virtual units.

4.7.2 Listing Shadow Set Members

Use the same format for the SHOW DEVICES command with shadow set members as you use with other physical devices. The command lists all shadow set members of the device name you specify.

Because shadow set members are not mounted in a traditional sense and they all have the same device characteristics, SHOW DEVICES displays most of the relevant data with the associated virtual unit. Listings of shadow set members include information about current membership status.

If a shadow set is undergoing a copy or a merge operation, the display resulting from the SHOW DEVICES command includes the percentage of the disk that has been copied or merged. The SHOW DEVICES information is available on all nodes that have the shadow set mounted.

Note that if a copy and a merge operation are occurring at the same time in the same shadow set, the number indicating the percentage merged remains static until the copy completes. Then the merge operation proceeds to completion.

4.7.3 Examples of the SHOW DEVICES Command

The following examples of output from the SHOW DEVICES command illustrate various types of shadow set information.

Examples

$ SHOW DEVICES D
Device               Device        Error    Volume         Free  Trans Mnt
 Name                Status        Count     Label        Blocks Count Cnt
DSA0:                Mounted           0  SHADOWDISK        8694   151   1
DSA9999:             Mounted           0  APPARITION      292971     1   1
$4$DUA0:   (SYSTMX)  Online            0
$4$DUA8:     (FUSS)  ShadowSetMember   0  (member of DSA0:)
$4$DUA10:  (SYSTMX)  ShadowSetMember   0  (member of DSA9999:)
$4$DUA11:  (SYSTMX)  ShadowSetMember   0  (member of DSA9999:)
$4$DUA12:  (SYSTMX)  ShadowSetMember   0  (member of DSA9999:)
$4$DUA89:    (FUSS)  ShadowSetMember   0  (member of DSA0:)
      

By truncating the device name, you cause the SHOW DEVICES command to list all the devices and all the virtual units that begin with the letters you entered (in this case, D). This example shows that two virtual units, DSA0 and DSA9999, are active on the local node. Both shadow sets are in a steady state.

$ SHOW DEVICES DSA8
Device               Device        Error    Volume         Free  Trans Mnt
 Name                Status        Count     Label        Blocks Count Cnt
DSA8:                Mounted           0  APPARITION      890937     1   1
$11$DUA8:   (FUSS)   ShadowSetMember   0  (member of DSA8:)
$11$DUA89:  (MUSS)   ShadowSetMember   0  (member of DSA8:)
      

This example shows the membership and status of the shadow set represented by the DSA8 virtual unit. The SHOW DEVICES display provides information not only about the virtual unit DSA8, but also about the physical devices $11$DUA8 and $11$DUA89 that are members of the shadow set. The device status "ShadowSetMember" indicates that the shadow set is in a steady state---the shadow set members are consistent with each other. The shadow set is distributed across the OpenVMS Cluster nodes FUSS and MUSS.

$ SHOW DEVICES DSA
Device               Device        Error    Volume         Free  Trans Mnt
 Name                Status        Count     Label        Blocks Count Cnt
DSA7:      (BGFUSS)  Mounted           0  PHANTOM          27060    35   7
DSA8:        (FUSS)  Mounted           0  APPARITION      890937     4   6
      

You might specify DSA on the SHOW DEVICES command to request information about all the virtual units on the local node. Entering a generic virtual unit name, such as DSA, as a parameter produces a display of all virtual units representing shadow sets mounted on the local system. This example shows that two shadow sets are mounted on the local node, represented by the virtual units DSA7 and DSA8.

$ SHOW DEVICES $11$DUA8:
Device               Device        Error    Volume         Free  Trans Mnt
 Name                Status        Count     Label        Blocks Count Cnt
DSA8:                Mounted           0  APPARITION      890937     1   1
$11$DUA8:    (FUSS)  ShadowSetMember   0  (member of DSA8:)
$11$DUA89:   (FUSS)  ShadowSetMember   0  (member of DSA8:)
      

Although the SHOW DEVICES command specifies the name of a single device, the resulting display includes information about the membership and status of the shadow set represented by the DSA8 virtual unit to which the $11$DUA8 device belongs. The device status "ShadowSetMember" indicates that the shadow set is in a steady state---the shadow set members are consistent with each other. The shadow set members are accessed through the controller named FUSS.

$ SHOW DEVICES $11$DUA8:
Device               Device        Error    Volume         Free  Trans Mnt
 Name                Status        Count     Label        Blocks Count Cnt
DSA8:                Mounted           0  APPARITION      890937     1   1
$11$DUA8:    (FUSS)  ShadowSetMember   0  (member of DSA8:)
 
$11$DUA89:   (FUSS)  ShadowCopying     0  (copy trgt DSA8:  48% copied)
 
      

The output from this SHOW DEVICES command shows a shadow set that is in a transient state. The device status "ShadowCopying" indicates that the physical device $11$DUA89 is the target of a copy operation, and 48% of the disk has been copied. The device $11$DUA8 is the source member for the copy operation.

$ SHOW DEVICES DSA8
Device               Device        Error    Volume         Free  Trans Mnt
 Name                Status        Count     Label        Blocks Count Cnt
DSA8:                Mounted           0  APPARITION      890937     1  12
 
$11$DUA8:    (FUSS)  ShadowCopying     0  (copy trgt DSA8: 5% copied)
$11$DUA89:   (MUSS)  ShadowMergeMbr    0  (merging DSA8:   0% merged)
 
      

This example shows how the SHOW DEVICES command displays a shadow set during a copy operation after a node in an OpenVMS Cluster fails. In this example, the shadow set members are located on different nodes, and a node in a OpenVMS Cluster system with the shadow set mounted fails. At the time of the failure, the shadow set was in a transient state, with the $11$DUA8 device undergoing a copy operation. The SHOW DEVICES command shows the state of the shadow set during the copy operation, before the merge operation occurs.

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