HP OpenVMS Systems Documentation

Content starts here 7.1 Introduction
HP OpenVMS I/O User’s Reference Manual: OpenVMS Version 8.4 > Chapter 7 Shadow-Set Virtual Unit Driver

7.1 Introduction

HP Volume Shadowing for OpenVMS ensures that data is available for 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. This ability to shadow disk volumes is sometimes referred to as disk mirroring.

Volume shadowing supports the clusterwide shadowing of a variety of storage systems. Volume shadowing also supports shadowing of all mass storage control protocol (MSCP) served disks. For more information about Volume Shadowing supported devices, see the Volume Shadowing for OpenVMS Software Product Description.

You can mount multiple compatible disk volumes, including the system disk, to form a shadow set. Each disk in the shadow set is known as a shadow set member. Volume Shadowing for OpenVMS logically binds the shadow set devices together and represents them as a single virtual device called a virtual unit. This means that multiple members of the shadow set, represented by the virtual unit, appear to applications and users as a single, highly available disk.

Volume Shadowing features include:

  • Controller independence. Shadow set members can be located on any node in an OpenVMS Cluster that has Volume Shadowing enabled.

  • Clusterwide, homogeneous shadow-set maintenance functions.

  • Ability to survive controller, disk, and media failures transparently.

  • Shadowing functions that do not affect application I/O.

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. Volume shadowed sets are managed and monitored using the same commands and utilities that are used for nonshadowed disks. The only difference is that access is through the virtual unit, not to individual devices.

SHDRIVER, the driver that controls the virtual unit functions, is described in “Driver Functions”.

For more detailed information on HP Volume Shadowing for OpenVMS, see the Volume Shadowing for OpenVMS manual.