HP OpenVMS Systems Documentation
Volume Shadowing for OpenVMS
Once you have successfully upgraded the system or systems and you have completed other postupgrade work (such as layered product installations), perform the following steps:
8.2 Modifying Data on Individual Shadow Set Members
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:
The following command dissolves the shadow set mounted in the previous command and makes the individual shadow set members available:
The following command mounts one former shadow set member as a disk volume outside of the shadow set:
In this command, in order to have write access, you must use the /OVERRIDE=SHADOW_MEMBERSHIP qualifier to zero 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:
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 zeroed the volume shadowing generation number. If you were to specify all the former members of the shadow set in the same command line, the MOUNT command would consider $45$DUA2 an unrelated volume and would determine that it requires a copy operation. This would overwrite the earlier modifications.
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:
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.
You should think of a shadow set as a single, highly available disk. 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 unless you have scrupulously followed the guidelines in Section 7.12. 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. Refer to Section 4.8.3 for information about obtaining a member of a shadow set for the source of a backup operation.
The following list describes options that are available when backing up shadow sets that are not available with nonshadowed disks.
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
On VAX systems, accessing shadow sets from standalone BACKUP is not supported. The command procedures supplied with OpenVMS for building standalone BACKUP kits are designed 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:
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.
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:
The following command adds a second member, $1$DUA11, to the shadow set:
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:
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
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, Volume 1: 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 8.3.4.
The format for a BACKUP command is as follows:
The format is the same as for any BACKUP operation. The following command, for example, designates a virtual unit for the input specifier:
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.
You must take special precautions when you restore a shadow set from a BACKUP/IMAGE save set. (See the OpenVMS System Manager's Manual, Volume 1: 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.
This example shows how to perform a backup on a former shadow set member after you rebuild 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:
This command dismounts 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.
These two commands mount the former shadow set member and a magnetic tape in preparation for a BACKUP command.
This command produces a BACKUP/IMAGE save set from $1$DUA11 while the shadow set is on line with $1$DUA10.
This example shows how to restore a shadow set from an image save set. Restoring an image save set directly to a shadow set is not supported because the BACKUP output medium (the shadow set) must be mounted as a foreign volume.
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.
This command restores $1$DUA10 from the save set.
This command dismounts the restored volume in preparation for mounting into a shadow set.
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.
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:
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.
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.
These commands dismount the target of the image backup and the shadow set, in preparation for re-creating the shadow set.
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
If a multiple-member system disk shadow set is mounted and the system fails, 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).
If the storage controllers attached to your system support minimerge, you can enable a minimerge on a shadowed system disk and write a dump to a nonshadowed, nonsystem disk of your choice by doing the following:
If you accidentally enable a minimerge to a system disk that receives a crash dump and you have not set up DOSD, you may be able to recover if you know which disk contains the valid dump. Remove that member, remount it, and remove the dump from that member.
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 failure.
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, shadowing resolves inconsistent data across the shadow set before the read is returned to SDA.
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.