To create a shadow set,
you must use the MOUNT command with the /SHADOW qualifier to mount
at least one physical disk into a shadow set and assign a virtual
unit name to the set, as shown in
Creating a Shadow Set.
Example 1 Creating a Shadow Set
$ MOUNT DSA23: /SHADOW=$4$DUA9:volume-label logical-name
This example forms
a shadow set represented by the virtual unit DSA23, and includes
one shadow set member, $4$DUA9. To create a shadow set, you must
observe the following rules:
Use the DSAn: format to specify the
shadow set virtual unit, where n represents a unique number from
0 through 9999. If you do not include a number after the DSA prefix,
MOUNT automatically assigns the highest unit number available. Numbering
starts at 9999 and decrements to 0; the first virtual unit mounted
is numbered 9999, the second 9998, and so on.
Each virtual unit number must be unique across the
system, regardless of whether or not the unit is mounted for public
(mounted with the /SYSTEM qualifier) or private access. Virtual
units are named independently of the controllers involved.
The /SHADOW qualifier is
required when specifying a physical device. You must name at least
one physical device as a parameter to the /SHADOW qualifier. Although
one-member shadow sets are valid, you should mount one or two additional
disks in order for the shadowing software to maintain duplicate data.
Adding disks to an existing shadow set is discussed in
Adding Shadow Set Members.
Use a nonzero allocation class for each physical
device in the shadow set. Use the allocation class naming format
$allocation-class$ddcu, where:
allocation-class is a numeric value from 1 to 255.
dd describes the device type of the physical device
(for example, DU, DK, or DG).
c is a letter from A to Z that represents the controller
designation.
u is the unit number of the device.
See OpenVMS Cluster Systems for more information about allocation classes.
Specify a 1- to 12-character
volume label for the virtual unit.
Optionally, specify a 1-
to 255-alphanumeric-character logical name string for the shadow
set.
In addition, you can specify /SYSTEM, /GROUP, or /CLUSTER
to make the shadow set available to all users of a system, all members
of a group, or all nodes in a cluster on which shadowing is enabled.
To create a three-member shadow set, you can add two members
in a single MOUNT command to an existing one-member shadow set.
This method optimizes the I/O operation because both members are
copied at the same time. (See the example in
Creating a Shadow Set With /SYSTEM and With /CLUSTER.)
Upon receiving a command to create a shadow set, the volume
shadowing software may perform a copy or a merge operation to reconcile
data differences. If you are not sure which disks might be targets
of copy operations, you can specify the /CONFIRM or /NOCOPY qualifiers
as a precaution against overwriting important data when you mount
a disk. These and other MOUNT command qualifiers are discussed in
MOUNT Command Qualifiers for Shadowing.