Updated: 11 December 1998

Volume Shadowing for OpenVMS

Generally, users and applications access a shadow set through the virtual unit. Occasionally, you may want to change the data on an individual shadow set member and then pass the changed data to other shadow set members.

The following series of commands demonstrates how you can dissolve and recreate the shadow set to perform specialized processes on one shadow set member and transfer the change to the other shadow set members.

The following command mounts a shadow set with three shadow set members:

$ MOUNT DSA9:/SHADOW=($45$DUA2:,$45$DUA4:,$45$DUA8:) LURK1

The following command dissolves the shadow set mounted in the previous command and makes the individual shadow set members available:

$ DISMOUNT/NOUNLOAD DSA9:

The following command mounts one former shadow set member as a disk volume outside of the shadow set:

$ MOUNT/OVERRIDE=SHADOW_MEMBERSHIP $45$DUA2: LURK1

In this command, in order to have write access, you must use the /OVERRIDE=SHADOW_MEMBERSHIP qualifier to erase the shadow set generation number. At this point, the disk is mounted as a nonshadowed volume and can be modified as required.

Before creating a new shadow set, dismount the $45$DUA2 physical disk, as follows:

$ DISMOUNT/NOUNLOAD $45$DUA2 $ MOUNT DSA9:/SHADOW=$45$DUA2: LURK1

The second command recreates the shadow set with $45$DUA2 as the only member.

Note that mounting $45$DUA2 with the /OVERRIDE=SHADOW_MEMBERSHIP qualifier automatically erased the volume shadowing generation number. If you were to mount all the former members of the shadow set at the same time, the MOUNT command would consider $45$DUA2 an unrelated volume and would determine that it requires a copy operation. This would overwrite the modifications performed earlier.

To save the current contents of $45$DUA2, add the other two former shadow set members to the new shadow set with a subsequent MOUNT command:

$ MOUNT DSA9:/SHADOW=($45$DUA4:,$45$DUA8:) LURK1

In this command, $45$DUA4 and $45$DUA8 are added to the shadow set DSA9. This recreates the original shadow set, except that each shadow set member now has the benefit of the changed data that was done to the single shadow set member.

7.3 Performing Backup Operations on a Shadow Set

You should think of a shadow set as a single, highly available disk unit. As such, backup techniques for nonshadowed disks apply to shadow set virtual units. However, to preserve the consistency and integrity of the shadow set, avoid removing a physical member of the shadow set without dismounting the virtual unit. If you leave some disk members of a shadow set active during the backup operation, data integrity is compromised because some disks in the shadow set may have files open. If you dismount the entire shadow set by dismounting the virtual unit, files are properly closed and, after the backup, any member of the shadow set can be used as the source of a valid backup operation.

The following list describes options that are available when backing up shadow sets that are not available with nonshadowed disks.

• To obtain a defragmented backup of a shadowed disk, begin by closing files and stopping application access to the disks. Dismount the virtual unit to dissolve the shadow set. Use the /NOUNLOAD qualifier to avoid spinning down the members of the shadow set. Remount the virtual unit as a private device, and use BACKUP/IMAGE (see Section 7.3.4) with the virtual unit as the source of the backup operation. This is the recommended method of backing up shadow sets.
• To minimize the amount of time that data is unavailable to applications, after dismounting the virtual unit consider remounting the shadow set with one less member. Then back up the dismounted member. Using this technique keeps the shadow set in service at the same time that you perform a backup operation. Once the backup is complete, remount the member into the shadow set. The shadowing software performs a copy operation to make that member consistent with the other members of the shadow set.
  If a spare disk of the type present in the shadow set is available, consider mounting the spare disk into the shadow set to minimize the time that the shadow set runs with reduced membership. Then, the member that served as the source of the backup can become a spare disk.
• To ensure complete integrity of the backup of the system disk, you must shut down the systems that boot from it. For system disk shadow sets, you should also dismount the virtual unit by any other systems that have it mounted. Then remount the virtual unit as a private device on one of the systems that was not shut down, and use it as the source for a BACKUP/IMAGE operations (see Section 7.3.4).
  In addition, to provide system disk shadowing quickly as you perform a backup operation, remount the shadow set minus one member. Back up that member and either remount it into the shadow set or mount a spare disk. You can use standalone BACKUP (VAX) or the menu-driven BACKUP procedure (Alpha) on one of the systems that is down while the other systems are rebooted.
• To do an incremental backup, use the virtual unit, not a single member of the shadow set. This is because incremental backups alter information in file headers. If you perform an incremental backup on a single member of a shadow set, that member needs to be the target of a copy operation.

HSC BACKUP and RESTORE techniques are not recommended for saving and restoring the contents of a shadow set member. These HSC utilities are applicable to the disk geometry only, not to the OpenVMS file system. Although HSC BACKUP and RESTORE techniques save and restore the contents of an entire disk volume (including blocks that may not be in use by the file system on that volume), they do not save and restore specific files, groups of files, directories, or subdirectories. In addition, these utilities do not defragment a disk. Moreover, the utilities cannot restore the context of a shadow set virtual unit.

The following sections describe several approaches to shadow set backup operations.

7.3.1 Restrictions on BACKUP Procedures

On VAX systems, Digital does not support accessing of shadow sets from standalone BACKUP. The command procedures supplied by Digital for building standalone BACKUP kits make every effort to prevent standalone BACKUP from using volume shadowing improperly. However, these checks can easily be overridden by a well-informed and sufficiently privileged user.

Note the following restrictions for standalone BACKUP on VAX systems that use volume shadowing:

• Do not boot standalone BACKUP from an alternative root on a shadowed system disk while other nodes are booting from the same shadowed system disk. If you do this, the boot attempt will fail.
• Standalone BACKUP does not mount virtual units. This makes access to virtual units impossible from standalone BACKUP.
• Do not assume that standalone BACKUP prevents you from accessing a shadow set member unit. You must prevent standalone BACKUP from sending output to a disk mounted on any other OpenVMS Cluster member, either as a directly accessible disk or as the member of a shadow set.

On Alpha computers, the same restrictions apply. You cannot use the standalone, menu-driven procedure included on the OpenVMS Alpha operating system distribution compact disc to perform BACKUP operations on shadow sets.

7.3.2 Using Copy Operations to Create a Backup

This example shows how to use volume shadowing copy operations to create an offline identical disk volume that you can then use as a backup of your shadow set. The following command creates a shadow set with one shadow set member:

$ MOUNT/SYSTEM DSA0:/SHADOW=$1$DUA10: SHADOWFACTS %MOUNT-I-MOUNTED, SHADOWFACTS mounted on _DSA0: %MOUNT-I-SHDWMEMSUCC, _$1$DUA10: (DISK01) is now a valid member of the shadow set

The following command adds a second member, $1$DUA11, to the shadow set:

$ MOUNT/SYSTEM DSA0:/SHADOW=$1$DUA11: SHADOWFACTS %MOUNT-I-SHDWMEMCOPY, _$1$DUA11: (DISK02) added to the shadow set with a copy operation

At this point you must wait for the copy operation to complete before dismounting the shadow set. When the copy operation is complete, messages are sent to the system console and to any operators enabled to receive them.

The following command dismounts the shadow set, leaving $1$DUA10 and $1$DUA11 with logically identical volumes:

$ DISMOUNT DSA0:

At this point you can re-create the shadow set with one of the volumes and keep the other as a backup, or use it as a source for the backup operation.

7.3.3 Using the OpenVMS Backup Utility

Generally you can use the OpenVMS Backup utility (BACKUP) with shadow sets as you do with regular volumes. (See the OpenVMS System Manager's Manual: Essentials for a description of how to back up volumes.) You can create BACKUP save sets or copies from shadow sets by using the shadow set virtual unit name instead of a physical device name as the input specifier. However, you cannot always restore to a shadow set by listing the virtual unit name as an output specifier. The main restriction to any backup restoration is that you cannot mount the target volume with the /FOREIGN qualifier. The proper procedure for a BACKUP/IMAGE restoration is described in Section 7.3.4.

The format for a BACKUP command is as follows:

BACKUP input-specifier output-specifier

The format is the same as for any BACKUP operation. The following command, for example, designates a virtual unit for the input specifier:

$ BACKUP/RECORD DSA2:[*...]/SINCE=BACKUP MTA0:23DEC.BCK

$ BACKUP/RECORD DSA2:[*...]/SINCE=BACKUP MTA0:23DEC.BCK

This command saves all files on the shadow set DSA2 that have been created or modified since the last backup and records the current time as their new backup date.

7.3.4 Using BACKUP/IMAGE on a Shadow Set

You must take special precautions when you restore a shadow set from a BACKUP/IMAGE save set. (See the OpenVMS System Manager's Manual: Essentials and OpenVMS System Management Utilities Reference Manual: A--L for a description of BACKUP/IMAGE operations with physical volumes.) A BACKUP/IMAGE operation marks the target volume as more current than the other shadow set members. This designates it as the source of copy operations if you re-create the shadow set with it.

Although you can create BACKUP save sets or copies from shadow set virtual units, you cannot mount your shadow set with the /FOREIGN qualifier to allow a BACKUP/IMAGE restoration.

You should either restore to a physical disk and then re-create the shadow set with the restored disk as a shadow set member (Example 2) or, if the save operation was a copy to a compatible disk, re-create the shadow set with that disk as a member (Example 3). The target of the BACKUP/IMAGE operation becomes the source of copy operations if you re-create the shadow set with it.

Example 1

This example shows how to perform a backup on a former shadow set member after you rebuild the shadow set.

$ MOUNT DSA0:/SHADOW=($1$DUA10:, $1$DUA11:) GHOSTVOL %MOUNT-I-MOUNTED, GHOSTVOL mounted on _DSA0: %MOUNT-I-SHDWMEMSUCC, _$1$DUA10: (DISK01) is now a valid member of the shadow set %MOUNT-I-SHDWMEMSUCC, _$1$DUA11: (DISK02) is now a valid member of the shadow set

The previous command mounts the shadow set DSA0. Make sure all copy operations are finished before you dismount the shadow set by using the following command:

$ DISMOUNT DSA0:

This command dismounts the shadow set.

$ MOUNT/SYSTEM DSA0/SHADOW=$1$DUA10: GHOSTVOL %MOUNT-I-MOUNTED, GHOSTVOL mounted on _DSA0: %MOUNT-I-SHDWMEMSUCC, _$1$DUA10: (DISK01) is now a valid member of the shadow set

This command puts the shadow set back on line without $1$DUA11. You can now perform the backup to tape while the shadow set is on line.

$ MOUNT $1$DUA11: GHOSTVOL %MOUNT-W-VOLSHDWMEM, mounting a shadow set member volume volume write locked %MOUNT-I-MOUNTED, GHOSTVOL mounted on _$1$DUA11: $ MOUNT/FOREIGN MTA0: %MOUNT-I-MOUNTED, ...

These two commands mount the former shadow set member and a magnetic tape in preparation for a BACKUP command.

$ BACKUP/IMAGE $1$DUA11: MTA0:SAVESET.BCK

This command produces a BACKUP/IMAGE save set from $1$DUA11 while the shadow set is on line with $1$DUA10.

Example 2

This example shows how to restore a shadow set from an image save set. Digital does not support restoring an image save set directly to a shadow set because the BACKUP output medium (the shadow set) must be mounted as a foreign volume.

$ DISMOUNT DSA0: $ MOUNT/FOREIGN MTA0: %MOUNT-I-MOUNTED, ... $ MOUNT/FOREIGN/OVERRIDE=SHADOW_MEMBERSHIP $1$DUA10: %MOUNT-I-MOUNTED, ...

These two commands mount the save-set magnetic tape as the input specifier and the former shadow set member as the output specifier for the restore operation.

$ BACKUP/IMAGE MTA0:SAVESET.BCK $1$DUA10:

This command restores $1$DUA10 from the save set.

$ DISMOUNT/NOUNLOAD $1$DUA10:

This command dismounts the restored volume in preparation for mounting into a shadow set.

$ MOUNT/SYSTEM DSA0/SHADOW=($1$DUA10:, $1$DUA11:) GHOSTVOL %MOUNT-I-MOUNTED, GHOSTVOL mounted on _DSA0: %MOUNT-I-SHDWMEMSUCC, _$1$DUA10: (DISK01) is now a valid member of the shadow set %MOUNT-I-SHDWMEMCOPY, _$1$DUA11: (DISK02) added to the shadow set with a copy operation

This command mounts the shadow set with the restored shadow set member. The output of the image backup operation has a newer generation number than other previous members of the shadow set. Therefore, $1$DUA10 (the restored volume) is the source of a copy operation when you form the shadow set.

Example 3

This example illustrates a BACKUP/IMAGE copy operation on a shadow set. The image backup operation stores output files contiguously, eliminating disk fragmentation. Because you must mount the output device of such operations with the /FOREIGN qualifier, you must take special steps as shown with the following commands:

$ MOUNT DSA0:/SHADOW=($1$DUA10:,$1$DUA11:) MEANDMY %MOUNT-I-MOUNTED, MEANDMY mounted on _DSA0: %MOUNT-I-SHDWMEMSUCC, _$1$DUA10: (DISK03) is now a valid member of the shadow set %MOUNT-I-SHDWMEMSUCC, _$1$DUA11: (DISK04) is now a valid member of the shadow set $ MOUNT/FOREIGN $1$DUA20: %MOUNT-I-MOUNTED, ...

The first command mounts the shadow set DSA0. The second command mounts, on $1$DUA20, the volume to be the output of the BACKUP/IMAGE operation. The /FOREIGN qualifier is required.

$ BACKUP/IMAGE/IGNORE=INTERLOCK DSA0: $1$DUA20:

This command performs the image backup using the virtual unit name as the input specifier. The image backup copy of a shadow set has a newer backup revision number than the existing members in the shadow set.

$ DISMOUNT $1$DUA20: $ DISMOUNT DSA0:

These commands dismount the target of the image backup and the shadow set, in preparation for re-creating the shadow set. Note that when you re-create the shadow set, you can lose some of the data written to the shadow set between the start of the backup operation and dismounting.

$ MOUNT/SYSTEM DSA0/SHADOW=($1$DUA10:,$1$DUA11:,$1$DUA20:) MEANDMY %MOUNT-I-MOUNTED, MEANDMY mounted on _DSA0: %MOUNT-I-SHDWMEMSUCC, _$1$DUA20: (DISK05) is now a valid member of the shadow set %MOUNT-I-SHDWMEMCOPY, _$1$DUA10: (DISK03) added to the shadow set with a copy operation %MOUNT-I-SHDWMEMCOPY, _$1$DUA11: (DISK04) added to the shadow set with a copy operation

This command rebuilds the shadow set with the image backup disk as one of the shadow set members. The other former shadow set members receive copy operations.

7.4 Crash Dumping to a Shadowed Disk

If a multiple-member system disk shadow set is mounted and the system crashes, the resulting crash dump information is initially written to the dump file on only one of the shadow set members. Once the dump operation has successfully completed, the unit number of the member with the written dump file is printed on the console device. Error messages display if the dump cannot be written (for example, because the path to the dump unit is unavailable or is unsuitable).

Once the system is rebooted, the shadowing software automatically begins a merge operation. This merge operation automatically propagates the dump file to all of the other members and also corrects any other inconsistencies caused by the system crash.

You can reboot the system from either the original boot device or the current master member device. At boot time, if the paths to all of the members of the shadow set are on the same type of adapter, the shadowing software correctly designates the current master and dump device on all of the booting nodes. At boot time, several OPCOM messages display information about the status of the dump device and the reboot condition of the system.

You cannot reboot the system unless the boot device is a current member of the shadow set. When a multiple member system disk shadow set loses its boot device, a warning is printed on the console device, and an OPCOM message is displayed.

On some systems, you can stipulate that multiple devices be members of the same system disk shadow set. Please refer to the system-specific manual for further details.

If you use the System Dump Analyzer (SDA) to access the dump file on the virtual unit during the merge operation, you can enter the SDA command ANALYZE/CRASH to examine the dump while the shadow set is undergoing a merge operation. If SDA accesses portions of the dump file that have not yet been merged, SDA resolves inconsistent data across the shadow set.

You can also use the Crash Log Utility Extractor (CLUE) commands to access the dump file on the virtual unit during a merge operation. CLUE commands can automatically create a footprint of the crash to a .LIS file and store it for future reference.

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