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
Guidelines for Using a Shadow Set Member for Backup. 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
Dismounting and Remounting With One Less Member for Backup 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.
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
Using BACKUP/IMAGE on a Shadow Set)
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, consider remounting the shadow set with one less
member (see
Dismounting and Remounting With One Less Member for Backup). Then back up the dismounted member. 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, or minicopy,
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
Using BACKUP/IMAGE on a Shadow Set). 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 removed 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.
Restrictions on BACKUP Procedures 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:
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.
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 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 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.
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 HP OpenVMS System Manager's
Manual 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
Using BACKUP/IMAGE on a Shadow Set.
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:
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.
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 HP OpenVMS System Manager's
Manual and the HP OpenVMS System Management Utilities Reference Manual 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. 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.
$ 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.
Do not attempt to add the restored volume to an existing
shadow set without first dissolving the original shadow set. Mounting
a restored volume into an existing shadow set will result in a copy
operation erasing the restored disk.
$ 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.
If any writes occur between the start of the backup
operation and the dismount of both the volume containing the image
backup copy and the shadow set, the backup image will not contain
all the data on the shadow set. You can prevent any writes from
occurring during this period by mounting the shadow set with the
/NOWRITE qualifier prior to mounting the volume that will serve
as the backup volume.
$ 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.
$ 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.