Shadow Copies; Overview - HP StorageWorks NAS 1500s Administrator's Manual

Shadow Copies

Overview

The Volume Shadow Copy Service provides an infrastructure for creating point-in-time
snapshots (shadow copies) of volumes. Shadow Copy supports 64 shadow copies per volume.
A shadow copy contains previous versions of the files or folders contained on a volume at a
specific point in time. While the Shadow Copy mechanism is managed at the server (see the
"Managing Shadow Copy" section in this chapter), previous versions of files and folders are
only available over the network from clients and are seen on a per folder or file level and not as
an entire volume.
The Shadow Copy feature works at the block level. As changes are made to the file system, the
Shadow Copy Service copies out the original blocks to a special cache file, to maintain a
consistent view of the file at a particular point in time. Since the snapshot only contains a
subset of the original blocks, the cache file is typically smaller than the original volume. In the
snapshot's original form, it takes up no space since blocks are not moved until an update to the
disk occurs.
By using shadow copies, a NAS server can maintain a set of previous versions of all files on
the selected volumes. End users access the file or folder by using a separate client add-on
program, which enables them to view the file in Windows Explorer. Accessing previous
versions of files, or shadow copies, enables users to:
Shadow copies cannot replace the current backup, archive, or business recovery system, but
they can help to simplify restore procedures. Since a snapshot only contains a portion of the
original data blocks, shadow copies can not protect against data loss due to media failures.
However the strength of snapshots is the ability to instantly recover data from shadow copies,
reducing the number of times needed to restore data from tape.
NAS 1500s and 500s Administration Guide
Recover files that were accidentally deleted. Previous versions can be opened and copied
to a safe location.
Recover from accidentally overwriting a file. A previous version of that file can be
accessed.
Compare several versions of a file while working. Use previous versions to compare
changes between two versions of a file.
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