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.
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). 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
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.
Figure 1 Virtual Unit
Elements of a Shadow Set shows
how Volume Shadowing for OpenVMS propagates data through the virtual
unit to three individual shadow set members.
Figure 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 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.
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 was 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 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 disk.