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Volume Shadowing for OpenVMS

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Chapter 3
Preparing to Use Volume Shadowing

This chapter explains the system management tasks required for using volume shadowing on your system, including licensing, setting system parameters, and booting.

Once you have determined how to configure your shadow set, perform the following steps:

  • If you are running OpenVMS VAX and phase I volume shadowing, migrate to phase II shadowing. Support for phase I shadowing was eliminated as of OpenVMS Version 6.2.
  • Select which of your disk drives you want to shadow. Prepare the selected volumes for mounting by physically placing the volumes in the drives (for removable media disks). Ensure the disks are not write locked.
  • Consider whether or not you want to initialize the volumes you have chosen to shadow. Do not initialize volumes that contain useful data.
    If you are creating a new shadow set, you can initialize one volume at a time, or multiple volumes with one command, which can streamline the creation of a shadow set (see Section 4.3). When you initialize one volume at a time, you can give it a volume label that you want to use for the shadow set. When you later mount additional volumes into the shadow set, each volume will be initialized and will be given the same volume label automatically.
  • Install the Volume Shadowing for OpenVMS licenses. See Section 3.1 for more information.
  • Set the SHADOWING parameter to enable volume shadowing on each node that will use volume shadowing. See Section 3.2 for more information.
    Setting the SHADOWING parameter requires that you reboot the system.
  • Set the ALLOCLASS parameter to 1. This parameter enables the use of allocation classes in device names. You must include a nonzero allocation class in the device name of shadowed disks. For more information, see Section 4.2.
  • Dismount the disk drives you selected for the shadow set and remount them (along with the additional shadow set disk drives) as shadow set members. Note that:
    • You do not need to change the device volume labels and logical names.
    • If you use mount command files, ensure that the commands mount the physical devices using the appropriate naming syntax for virtual units (DSAn:).
  • See Chapter 4 for more information on the MOUNT command.

System disks can be shadowed. All nodes booting from that system disk must have shadowing licensed and enabled.

3.1 Licensing Volume Shadowing for OpenVMS

To use the volume shadowing product, you must purchase the license separately from the OpenVMS operating system even though the volume shadowing software is part of the OpenVMS operating system.

Volume shadowing licenses are available in two options:

  • Per disk license, which enables you to license each disk that you plan to include in the shadow set. One example of how this option might be more cost effective is in a cluster where you intend to shadow only a small number of disks.
  • Capacity license (per CPU), which may be more attractive to those who have larger systems with many more disks to shadow.

Both options work on the same systen or in an OpenVMS Cluster that contains both Alpha and VAX computers.

After licensing the OpenVMS operating system by registering a OpenVMS Product Authorization Key (PAK), you must license Volume Shadowing for OpenVMS with a separate volume shadowing PAK. The PAK provides information that defines the Volume Shadowing for OpenVMS license contract you have with Compaq Computer Corporation. Obtain a PAK from your Compaq sales representative.

When you enter information from the PAK into the online LICENSE database, the OpenVMS License Management Facility (LMF) authorizes the use of volume shadowing.

If you have a per disk license, you must register and activate a license for each shadowed disk. Starting with Volume Shadowing for OpenVMS Version 7.1, a license check for each disk that is shadowed using the per-disk volume shadowing license is included. Per-disk volume shadowing licenses apply to full shadow set members only. When the number of shadow set members exceeds the number of per-disk licenses for five minutes, shadowing issues an OPCOM warning message. You can have this message also sent to an E-mail account by defining the system logical SHADOW_SERVER$MAIL_NOTIFICATION to a standard OpenVMS Mail address or a UNIX (internet) address. An invalid address will not generate a failure message.

Shadowing issues notification again 59 minutes after noncompliant shadow set members are mounted. One minute later, shadow set members are automatically removed from shadow sets until the number of members equals the number of licenses. Members are removed systematically from multiple-member shadow sets; single-member shadow sets will not be affected.

Disks that are the target of a copy operation do not consume a license unit until the copy is complete. Thus it is always possible to obtain a copy of a single-member shadow set.

If you are using capacity licenses, you must register and activate a license for Volume Shadowing for OpenVMS on each node that mounts a shadow set, including satellites in an OpenVMS Cluster system. If you do not register and activate nodes or disks that will use volume shadowing, subsequent shadow set mount operations will not succeed and will display the error messages like the one in Example 3-1.

Example 3-1 Nodes Not Registered to Use Volume Shadowing

%LICENSE-E-NOAUTH, DEC VOLSHAD use is not authorized on this node
-LICENSE-F-NOLICENSE, no license is active for this software product
-LICENSE-I-SYSMGR, please see your system manager

For more information about the License Management Facility, refer to the OpenVMS Operating System Software Product Description 25.01.xx.

You can also consult the OpenVMS License Management Utility Manual.

After you register the volume shadowing PAK, you must set the shadowing parameters on each node where you want to enable shadowing.

3.2 Volume Shadowing Parameters

Table 3-1 lists the system parameters that you can use to tailor the shadowing software on your system. These parameters were introduced with OpenVMS Version 7.1, except SHADOW_MAX_UNIT, which was introduced in OpenVMS Version 7.3. The term dynamic in Table 3-1 means that the active value can be changed on a running system. See the OpenVMS System Manager's Manual for more information about setting system parameters.

OpenVMS Version 7.3 introduced four write bitmap system parameters (described in Table 3-3) and the volume shadowing system parameter SHADOW_MAX_UNIT. These system parameters support the shadowing minicopy operation, which is described in Chapter 7.

Table 3-1 Volume Shadowing Parameters
Parameter Function Range Default Dynamic
SHADOWING Enables phase II volume shadowing. See Table 3-2 for a description of parameter values. 0, 2 1 0 No
SHADOW_MAX_COPY Limits the number of concurrent merge or copy operations on a given node. 0--200 4 Yes
SHADOW_MBR_TMO Controls the amount of time the system tries to fail over physical members of a shadow set. 1--65,535
120 Yes
SHADOW_MAX_UNIT Specifies the maximum number of shadow sets that can exist on a node. Dismounted shadow sets, unused shadow sets, and shadow sets with no write bitmaps allocated to them are included in this total. 10--10,000 100 on VAX; 500 on Alpha No
SHADOW_SYS_DISK Allows system disk to be a shadow set and, optionally, enables a minimerge to occur. If a minimerge is enabled, the system must also be configured for writing to a nonshadowed, nonsystem disk of your choice. 0, 1, 4097 1 0 Yes
SHADOW_SYS_TMO Controls the amount of time members of a system disk shadow set have to return to the set. 1--65,535
120 Yes
SHADOW_SYS_UNIT Contains the virtual unit number of the system disk. 0--9999 0 No
SHADOW_SYS_WAIT This parameter applies only to shadow sets that are currently mounted in the cluster. Controls the amount of time a booting system will wait for all members of a mounted system disk shadow set to become available. 1--65,535
480 Yes

1All other values are reserved for use by Compaq.

3.2.1 Guidelines for Using Volume Shadowing Parameters

This section provides guidelines for using volume shadowing parameters.


The SHADOWING parameter enables or disables specific phases of volume shadowing on your system.

Table 3-2 describes these settings in detail.

Table 3-2 SHADOWING Parameter Settings
Setting Effect
0 Shadowing is not enabled.

This is the default value.

2 Enables host-based shadowing.

This setting provides shadowing of all disks that are located on a standalone system or on an OpenVMS Cluster system. Set SHADOWING to 2 on every node that will mount a shadow set, including satellite nodes.


The SHADOW_MAX_COPY parameter controls how many parallel copy and merge operations are allowed on a given node. (Copy and merge operations are described in Chapter 6.) This parameter provides a way to limit the number of copy and merge operations in progress at any one time.

The value of SHADOW_MAX_COPY can range from 0 to 200. The default value is specific to the OpenVMS version. You can determine the default value by looking at the parameter setting. When the value of the SHADOW_MAX_COPY parameter is 4, and you mount five multivolume shadow sets that all need a copy operation, only four copy operations can proceed. The fifth copy operation must wait until one of the first four copies completes.

Consider the following when choosing a value for the SHADOW_MAX_COPY parameter:

  • CPU power
  • Disk controller bandwidth
  • Interconnect controller bandwidth
  • Other work loads on the system

For example, the default value of 4 may be too high for a small node. (In particular, satellite nodes should have SHADOW_MAX_COPY set to a value of 0.) Too low a value for SHADOW_MAX_COPY unnecessarily restricts the number of operations your system can effectively handle and extends the amount of time it takes to merge all of the shadow sets.

SHADOW_MAX_COPY is a dynamic parameter. Changes to the parameter affect only future copy and merge operations; current operations (pending or already in progress) are not affected.


The SHADOW_MAX_UNIT specifies the maximum number of shadow sets that can exist on a node. The important thing to note about this value is that any shadow set that has been created, regardless of whether it is in use, is included in this total. Because this is not a dynamic system parameter, you should be very careful when determining the value to use. If you need to change this parameter, you must reboot the system. Furthermore, any attempt to mount more shadow sets than the value specified will fail.

Note that this parameter does not affect the naming of shadow sets. For example, with the default value of 100, a device name such as DSA999 is still valid.


The SHADOW_MBR_TMO parameter controls the amount of time the system tries to fail over physical members of a shadow set before removing them from the set. SHADOW_MBR_TMO is a dynamic parameter that you can change on a running system.

With the SHADOW_MBR_TMO parameter, you specify the number of seconds, from 1 to 65,535, during which recovery of a shadow set member is attempted.


The value of SHADOW_MBR_TMO should not exceed the value of the parameter MVTIMEOUT.

If you specify zero, a default delay is used. The default delay is specific to the version of OpenVMS running on your system. For shadow sets in an OpenVMS Cluster configuration, the value of SHADOW_MBR_TMO should be set to the same value on each node.

Determining the correct value for SHADOW_MBR_TMO is a trade-off between rapid recovery and high availability. If rapid recovery is required, set SHADOW_MBR_TMO to a low value. This ensures that failing shadow set members are removed from the shadow set quickly and that user access to the shadow set continues. However, removal of shadow set members reduces data availability and, after the failed member is repaired, a full copy operation is required when it is mounted back into the shadow set.

If high availability is paramount, set SHADOW_MBR_TMO to a high value. This allows the shadowing software additional time to regain access to failed members. However, user access to the shadow set is stalled during the recovery process. If recovery is successful, access to the shadow set continues without the need for a full copy operation, and data availability is not degraded. Setting SHADOW_MBR_TMO to a high value may be appropriate when shadow set members are configured across LANs that require lengthy bridge recovery time.

Shadowing uses a timer to adhere to the number of seconds specified by the SHADOW_MBR_TMO parameter. For directly connected SCSI devices that have been powered down or do not answer to polling, the elapsed time before a device is removed from a shadow set can take several minutes.


A SHADOW_SYS_DISK parameter value of 1 enables shadowing of the system disk. A value of 0 disables shadowing of the system disk. A value of 4097 enables a minimerge. The default value is 0.

If you enable a minimerge of the system disk, you must also configure your system to write a dump to a nonshadowed, nonsystem disk of your choice. This is known as dump off system disk (DOSD). For more information on DOSD, see the OpenVMS System Manager's Manual: Tuning, Monitoring, and Complex Systems manual.

In addition, you should specify a system-disk, shadow-set virtual unit number with the SHADOW_SYS_UNIT system parameter, unless the desired system disk virtual unit number is DSA0.


You can use the SHADOW_SYS_TMO parameter in two ways: during the booting process and during normal operations. SHADOW_SYS_TMO is a dynamic parameter that you can change on a running system.

During the booting process, you can use this parameter on the first node in the cluster to boot and to create a specific shadow set. If the proposed shadow set is not currently mounted in the cluster, use this parameter to extend the time a booting system will wait for all former members of the system disk shadow set to become available.

The second use of this parameter comes into effect once the system successfully mounts the shadow set and begins normal operations. Just as the SHADOW_MBR_TMO parameter controls the time the operating system waits for failing members of an application disk shadow set to rejoin the shadow set, the SHADOW_SYS_TMO parameter controls how long the operating system will wait for failing members of a system disk shadow set. All nodes using a particular system disk shadow set should have their SHADOW_SYS_TMO parameter equal to the same value, after normal operations begin. Therefore, after booting, this parameter applies only to members of the system disk shadow set.

The default value is OpenVMS version specific. You can set a range of up to 65,535 seconds if you want the system to wait longer than the default for all members to join the shadow set.


The SHADOW_SYS_UNIT parameter, which must be used when the SHADOW_SYS_DISK parameter is set to 1, contains the virtual unit number of the system disk.

The SHADOW_SYS_UNIT parameter is an integer value that contains the virtual unit number of the system disk. The default value is 0. The maximum value allowed is 9999. This parameter is effective only when the SHADOW_SYS_DISK parameter has a value of 1. This parameter must be set to the same value on all nodes that boot off a particular system disk shadow set. SHADOW_SYS_UNIT is not a dynamic parameter.


Use the SHADOW_SYS_WAIT parameter to extend the time a booting system will wait for all current members of a mounted system disk shadow set to become available to this node. SHADOW_SYS_WAIT is a dynamic parameter that you can change on a running system (for debugging purposes only). The shadow set must already be mounted by at least one other cluster node for this parameter to take effect. The default value is 256 seconds. Change this parameter to a higher value if you want the system to wait more than the 256-second default for all members to join the shadow set. This parameter has a range of 1 through 65,535 seconds.

3.3 Write Bitmap System Parameters

Starting with OpenVMS Version 7.3, system parameters are available for managing update traffic between a master write bitmap and its corresponding local write bitmaps in an OpenVMS Cluster system. Another system parameter controls whether write bitmap system messages are sent to the operator console and, if they are to be sent, the volume of messages. These system parameters are dynamic; that is, they can be changed on a running system. They are shown in Table 3-3 and described in detail in Section 7.9. These system parameters support the minicopy operation (see Chapter 7).

Table 3-3 Write Bitmap System Parameters
Parameter Meaning Unit Min Max1 Default
WBM_MSG_INT In single-message mode, the time interval between assessment of the most suitable write bitmap message mode. In buffered-message mode, the maximum time a message waits before it is sent. msec 10 -1 10
WBM_MSG_UPPER The upper threshold for the number of messages sent during the test interval that will initiate buffered-message mode. msgs/interval 0 -1 100
WBM_MSG_LOWER The lower threshold for the number of messages sent during the test interval that will initiate single-message mode. msgs/interval 0 -1 10
WBM_OPCOM_LVL Controls whether write bitmap messages are provided to the operator console: 0 means messages are turned off; 1 means messages are provided when write bitmaps are started, deleted, and renamed, and when the SCS message mode (buffered or single) changes; 2 means that all messages for a setting of 1 are provided along with detailed messages for debugging purposes. n/a 0 2 1

1 The maximum value of -1 corresponds to the maximum positive value that can be represented by a longword.

3.3.1 Setting System Parameters

To set or modify volume shadowing parameters, edit the [SYSn.SYSEXE]MODPARAMS.DAT file or the appropriate AUTOGEN include file. After editing the file, execute SYS$UPDATE:AUTOGEN as described in the OpenVMS System Manager's Manual, Volume 2: Tuning, Monitoring, and Complex Systems. If you have an OpenVMS Cluster system, ensure that the system parameters are updated on each node. Example 3-2 illustrates a MODPARAMS.DAT file that includes assignment statements to set shadowing parameters.

Example 3-2 MODPARAMS.DAT File

! Volume Shadowing Parameters:
SHADOWING=2             ! Enables phase II shadowing

SHADOW_SYS_DISK=1       ! Enables system disk shadowing

SHADOW_SYS_UNIT=7       ! Specifies 7 as the virtual unit number
                          of the system disk

SHADOW_MAX_COPY=4       ! Specifies that 4 parallel copies can occur at one time

SHADOW_MBR_TMO=120      ! Allows 120 seconds for physical members to fail over
                        ! before removal from the shadow set


See the OpenVMS System Manager's Manual, Volume 2: Tuning, Monitoring, and Complex Systems for complete information about invoking AUTOGEN and specifying the appropriate command qualifiers to perform the desired AUTOGEN operations.

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