Volume Shadowing for OpenVMS

This manual, "Volume Shadowing for OpenVMS" (April 2001), is designed for system managers and users of OpenVMS Alpha and VAX systems (specifically versions 7.3 and later) to understand, configure, and manage data replication for high availability.

The primary purpose of Volume Shadowing is to transparently replicate data on multiple disks to prevent interruptions from disk device failures, such as media deterioration, communication path issues, or controller/device failures. It functions as a RAID 1 implementation, creating "shadow sets" (groups of one, two, or three compatible disk volumes) that are logically presented as a single "virtual unit."

Key areas covered include:

• Core Functionality: Explains how shadow sets duplicate data, allowing I/O requests to continue even if a disk fails, and how data integrity is maintained through copy and merge operations.
• Minicopy (Alpha-specific): Details a streamlined copy operation, introduced in OpenVMS 7.3, which uses "write bitmaps" to copy only changed blocks. This significantly reduces the time required to return a removed disk to a shadow set, especially for backup purposes.
• System Configuration: Illustrates various shadow set configurations, from single-node systems to highly available OpenVMS Cluster systems, including the use of shadowed system disks and support for different disk and controller types (DSA, SCSI, Fibre Channel).
• Management: Provides instructions for creating, mounting, dismounting, and dissolving shadow sets using DCL commands (MOUNT, DISMOUNT, SHOW DEVICE) and OpenVMS system services ($MOUNT, $DISMOU, $GETDVI).
• System Parameters: Guides users on setting crucial system parameters (e.g., SHADOWING, SHADOWMAXCOPY, SHADOWSYSDISK) to enable and optimize volume shadowing.
• Data Consistency & Recovery: Describes how copy and merge operations ensure data consistency and availability during membership changes or system failures, and outlines repair and recovery mechanisms for various error types.
• System Management Tasks: Includes procedures for upgrading OpenVMS on shadowed system disks, performing backup operations (including considerations for data consistency), and managing crash dumps.
• Performance: Discusses factors influencing shadow set performance during steady-state, copy, and merge operations, offering guidelines for resource management.
• Licensing: Covers the separate licensing requirements for Volume Shadowing (per disk or capacity).

The manual is intended for those familiar with the OpenVMS operating system, its Mount utility, system services, and parameter settings.