Document revision date: 30 March 2001
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Volume Shadowing for OpenVMS

Volume Shadowing for OpenVMS

Order Number: AA--PVXMG--TE

April 2001

This manual explains how to use Volume Shadowing for OpenVMS to replicate data transparently on multiple disks and provide high data availability.

Revision/Update Information: This manual supersedes Volume Shadowing for OpenVMS, OpenVMS Alpha Version 7.2-1 and OpenVMS VAX Version 7.2.

Software Version: OpenVMS Alpha Version 7.3 OpenVMS VAX Version 7.3

Compaq Computer Corporation
Houston, Texas

© 2001 Compaq Computer Corporation

COMPAQ, VAX, VMS, and the Compaq logo Registered in U.S. Patent and Trademark Office.

OpenVMS and DECnet are trademarks of Compaq Information Technologies Group, L.P. in the United States and other countries.

All other product names mentioned herein may be trademarks of their respective companies.

Confidential computer software. Valid license from Compaq required for possession, use, or copying. Consistent with FAR 12.211 and 12.212, Commercial Computer Software, Computer Software Documentation, and Technical Data for Commercial Items are licensed to the U.S. Government under vendor's standard commercial license.

Compaq shall not be liable for technical or editorial errors or omissions contained herein. The information in this document is provided "as is" without warranty of any kind and is subject to change without notice. The warranties for Compaq products are set forth in the express limited warranty statements accompanying such products. Nothing herein should be construed as constituting an additional warranty.


The Compaq OpenVMS documentation set is available on CD-ROM.

Contents Index


Intended Audience

This book is intended for system managers and system users who want to:

Although you do not need any previous volume shadowing experience to use the volume shadowing software or this documentation, you do need a familiarity with the OpenVMS operating system, the OpenVMS Mount utility or OpenVMS system services, and setting system parameters.

Document Structure

The Volume Shadowing for OpenVMS manual consists of the following chapters and appendix:
Chapter Contents
Chapter 1 Introduces Volume Shadowing for OpenVMS and describes how it provides high data availability.
Chapter 2 Illustrates various shadow set configurations.
Chapter 3 Describes how to set up a volume shadowing environment, including information about setting shadowing system parameters, booting a system that uses a system disk in a shadow set, and booting satellite nodes from a shadowed system disk.
Chapter 4 Describes how to use DCL commands to create, mount, dismount, and dissolve shadow sets. The chapter also describes how to use the SHOW DEVICES command, the System Dump Analyzer, and the F$GETDVI lexical function to obtain information about shadow sets on a running system.
Chapter 5 Describes how to use the OpenVMS system services in a user-written program to create and manage shadow sets. The chapter also describes how to use the $GETDVI system service to obtain information about shadow sets.
Chapter 6 Describes how the copy and merge operations maintain data consistency and availability during changes in shadow set membership.
Chapter 7 Describes how the minicopy operation can be used, in a carefully controlled environment, to shorten the time required for a member to be returned to a shadow set. Typically, the member is removed for backing up data.
Chapter 8 Describes how to perform system management tasks on shadow sets, including performing backup and upgrade operations, performing shadowing operations in OpenVMS Cluster systems, and handling crash dumps on the shadow set.
Chapter 9 Includes helpful information and guidelines for achieving better performance from shadow sets.
Appendix A Lists messages related to volume shadowing that are returned by the Mount utility and the VOLPROC, shadow server, and OPCOM facilities.

Related Documents

The following documents contain information related to this manual:

For additional information about OpenVMS products and services, access the Compaq website at the following location: 

Reader's Comments

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The following conventions are used in this manual:
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Chapter 1
Introduction to Volume Shadowing for OpenVMS

This chapter introduces Volume Shadowing for OpenVMS and describes how volume shadowing, sometimes referred to as disk mirroring, achieves high data availability.

1.1 Overview

Volume Shadowing for OpenVMS ensures that data is available to your applications and end users by duplicating data on multiple disks. Because the same data is recorded on multiple disk volumes, if one disk fails, the remaining disk or disks can continue to service I/O requests.

An implementation of RAID 1 (redundant arrays of independent disks) technology, Volume Shadowing for OpenVMS prevents a disk device failure from interrupting system and application operations. By duplicating data on multiple disks, volume shadowing transparently prevents your storage subsystems from becoming a single point of failure because of media deterioration or communication path failure, or through controller or device failure.

You can mount one, two, or three compatible disk volumes, including the system disk, to form a shadow set. Compatible disk volumes are those with the same number of physical blocks (see Section 1.3.2). Each disk in the shadow set is a shadow set member. Volume Shadowing for OpenVMS logically binds the shadow set disks together and represents them as a single virtual device called a virtual unit. This means that the multiple members of the shadow set, represented by the virtual unit, appear to applications and users as a single, highly available disk.

Note that the term disk and device are used interchangeably throughout this manual to refer to a disk volume. A disk volume is a disk that was prepared for use by placing a new file structure on it.

Applications and users read and write data to and from a shadow set using the same commands and program language syntax and semantics that are used for nonshadowed I/O operations. System managers manage and monitor shadow sets using the same commands and utilities they use for nonshadowed disks. The only difference is that access is through the virtual unit, not to individual disks.

Figure 1-1 shows how Volume Shadowing for OpenVMS propagates data through the virtual unit to three individual shadow set members.

Figure 1-1 Elements of a Shadow Set

An additional benefit of volume shadowing is its potential role in repairing data. For example, if data on a shadow set member becomes unreadable, the shadowing software can read the data from another member. Before the good data is returned to the process, it is written to the member that could not originally read it.


Remember that volume shadowing protects against hardware problems that cause a disk volume to be a single point of failure for both applications and systems that use the disk. Volume shadowing does not provide for recovery from software-related incidents, such as the accidental deletion of files or errant software corrupting the contents of a disk file. Do not use volume shadowing as a substitute for regular backup or journaling.

Volume Shadowing for OpenVMS is sometimes referred to as phase II shadowing or host-based shadowing. Support for phase I shadowing (controller-based shadowing) was discontinued as of OpenVMS Version 6.2.

1.2 Volume Shadowing Tasks and Operations

The primary volume shadowing operations used to create shadow sets and to maintain consistent data on each member are mount, copy, assisted copy, minicopy (introduced with OpenVMS Version 7.3), merge, and minimerge. When these operations are in progress, the system continues to process read and write requests, thus providing continuous availability.

All volume shadowing operations, except for merges and minimerges, are under the control of the system manager. Merges and minimerges are started automatically by the volume shadowing software if a hardware or software failure occurs that could affect the consistency of the data on the shadow set members.

System managers turn on volume shadowing with the SHADOWING system parameter. They can control the number of concurrent merge or copy operations on a given node by the SHADOW_MAX_COPY system parameter. These volume shadowing system parameters, and all other system parameters used with volume shadowing, are described in Section 3.2 and in Section 3.3.

Volume Shadowing for OpenVMS is never invoked directly. Instead, you invoke the DCL commands MOUNT and DISMOUNT. The MOUNT command works with the volume shadowing software to create shadow sets. The DISMOUNT command works with the volume shadowing software to remove shadow set members and to dissolve entire shadow sets.

HSJ and HSC controllers, when present in a configuration, provide software assists for the minimerge and assisted copy operations.

OpenVMS also provides a programming interface for creating and managing shadow sets with the $MOUNT, $DISMOU, and $GETDVI system services. This programming interface is described in Chapter 5.

Table 1-1 shows the main volume shadowing tasks, the operations associated with them, and the software used to perform the operation. These operations are described in more detail in Chapter 4, Chapter 6, and Chapter 7.

Table 1-1 Main Volume Shadowing Tasks, Operation Name, and Related Software
Task Operation Software Used
Create a shadow set. Copy MOUNT/SHADOW command with the SHADOWING system parameter set, which starts the copy automatically. When a second or third member is added, the shadowing software starts a copy operation.
Remove a member from a shadow set. Dismount a disk DISMOUNT command.
Dissolve a shadow set. Dismount the shadow set (specify its virtual unit name) DISMOUNT command.
Ensure that the data is identical on all shadow set members in the event of a hardware failure. Merge or minimerge Shadowing software does this automatically when it detects a hardware or software failure. If an HSJ or HSC controller is present in the configuration, a minimerge might be done.
Return a dismounted shadow set member to the shadow set. Copy, assisted copy, or minicopy MOUNT command, with shadowing software that initiates either a copy or, if properly configured, a minicopy.

1.3 Hardware Environment

Volume Shadowing for OpenVMS does not depend on specific hardware in order to operate. All shadowing functions, with the exception of minicopy, can be performed on Alpha and VAX computers using the OpenVMS operating system. The minicopy operation can be performed only on an OpenVMS Alpha system. However, an OpenVMS VAX system can be a member of an OpenVMS Cluster system that uses this feature.

Volume shadowing requires a minimum of:

The following sections generically describe hardware support. See the most recent Volume Shadowing for OpenVMS Software Product Description 27.29.xx for more information.

1.3.1 Memory Requirements

Starting with OpenVMS Version 7.3, the following additional memory is required to run Volume Shadowing for OpenVMS:

These memory requirements are cumulative. For example, a system with 10 shadow sets mounted, with each shadow set consisting of 50-GB member disks, would require an additional 1,119 KB of memory. The calculation follows:

24 KB per node (regardless of whether you use volume shadowing)
45 KB (10 shadow sets x 4.5 KB per unit mounted on the system)
1050 KB (50 x 2.1 KB (per GB of disk size) x 10 shadow sets
1119 KB total memory required

1.3.2 Supported Devices

The compatibility requirements for the physical disks that form a shadow set follow:

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