HP 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.
HP Volume Shadowing for OpenVMS is available on
HP OpenVMS Integrity servers, OpenVMS Alpha, and on OpenVMS VAX.
|NOTE: This manual covers volume shadowing on OpenVMS
Integrity servers and OpenVMS Alpha. For information on volume shadowing
on OpenVMS VAX, see the OpenVMS Version 7.3-1 HP Volume
Shadowing for OpenVMS manual.|
All volume shadowing features that are available
on OpenVMS Integrity servers are also available on OpenVMS Alpha.
An implementation of RAID 1 (redundant arrays
of independent disks) technology, HP 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.
Any entity that is designated as a disk class device to
OpenVMS is a device that can be used in a shadow set.
You can mount one, two, or three identical-size disk volumes,
including the system disk, to form a shadow set.
Starting with OpenVMS Alpha Version
7.3–2, disk volumes can differ in the number of physical blocks
(see “Supported Devices ”). Starting from OpenVMS
Version 8.4, you can mount a maximum of six disk volumes. 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, as shown in Figure 1-1. 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 prepared for use by placing a new file structure on it.
Figure 1-2 shows how Volume Shadowing for OpenVMS propagates
data through the virtual unit to three individual shadow set members.
Figure 1-2 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
|NOTE: 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.|
Prior to OpenVMS Version 6.2, two forms of volume
shadowing were supported: host-based, also known as phase II shadowing,
and controller-based, also known as phase I shadowing. Starting with
OpenVMS Version 6.2, controller-based shadowing is discontinued ---
Volume Shadowing for OpenVMS is host based only. Consequently, the
term Phase II is no longer used in this manual.
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 non-shadowed I/O operations.
System managers manage and monitor shadow sets using the same commands
and utilities they use for non-shadowed disks. The only difference
is that access is through the virtual unit, not to individual disk.