HP OpenVMS Systems Documentation

Content starts here Creating a Shadow Set
HP Volume Shadowing for OpenVMS: OpenVMS Version 8.4 > Chapter 4 Creating and Managing Shadow Sets Using DCL Commands

Creating a Shadow Set

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 Example 4-1.

Example 4-1 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 name 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 must 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. Note that you cannot use more than 3 characters for the ddc portion of the name. Failure to observe this requirement results in failure to mount the shadow set.

    • u is the unit number of the device.

    Note that you cannot use more than 3 letters for the ddc portion of the name.See HP 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 ”.)

You can also streamline the process of creating a shadow set by initializing multiple devices in one command, using INITIALIZE/SHADOW/ERASE, as described in “Using INITIALIZE/SHADOW/ERASE to Form a Shadow Set (Integrity servers and Alpha)”.

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”.