In-Depth
The Shadow-Copy Knows
Volume Shadow Copy, another new Windows .NET feature, shifts the burden of backups and restores back to users.
- By Bill Boswell
- 11/01/2002
Some of the most significant new features in .NET focus on simplifying
user support. One especially resource-intensive support issue involves
individual file restores. How often have you gotten a phone call that
starts off, “I was making changes to a spreadsheet macro that does a vitally
important, mission-critical calculation and now my spreadsheet doesn’t
work. Can you restore it from last night’s tape backup?” The answer, “I
sure can and you’ll have it in a couple of days” rarely satisfies the
user, so you must take time right then to mount the tape, locate the file
in the catalog (if the user can remember its location and name), restore
the file to a landing pad then copy it to the user’s folder. This imposes
a signficant burden on IT staff and resources.
A new service called the Volume Shadow Copy service shifts the workload
of individual file restores to the user. Volume Shadow Copy creates a
database of changed data files. This database essentially caches historical
revisions and presents them to users so they can be recovered without
any intervention by system administrators.
The Volume Shadow Copy feature is controlled via the Properties window
for a volume. (See Figure 1.) The feature is disabled by default. There
is only one database per volume, so volume shadowing can’t be enabled
or disabled for individual shares. The database is encrypted and can’t
be defragged. You can’t use Volume Shadow Copy on volumes hosted on the
quorum drive of a cluster. The database can be placed on a different driver,
and for best results, you should plan to provide a separate spindle or
RAID array if you want to implement this feature. You can skip the database
when doing tape backups on the theory that you already have historical
copies on previous backup tapes that you can use if the Volume Shadow
Copy database is lost.
Viewing historical file revisions using the Volume Shadow Copy database
requires a special extension to the Explorer interface that only runs
on Windows XP or .NET clients. The feature isn’t supported on any earlier
operating system versions. The client comes with .NET but must be installed
separately on XP desktops via an installer package called Twclient.msi.
The Explorer extension exposes a Previous Version tab on the Properties
page for a file on a Volume Shadow Copy volume that is viewed via a network
share. The second screenshot shows an example. The user can view the contents
of each previous version and select which to restore. A variety of safeguards
prevents accidental file overwrites.
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Figure 1. A user will see this page, showing previous versions
of a document saved using Shadow Copy.
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Figure 2. Using Shadow Copy, the Properties page, shown
here, can cut down on help desk calls.
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The Volume Shadow Copy service also supports backing up locked files.
When a backup utility that uses the .NET Backup API encounters a locked
file, it notifies the Volume Shadow Copy service, which takes a snapshot
of the file that the backup utility can use in place of the original.
This feature works even for volumes that don’t use Volume Shadow Copy
to track historical file changes. If you use a SAN or NAS, the Volume
Shadow Copy feature also supports taking snapshots of dynamic files for
safe transfer to another location.
About the Author
Contributing Editor Bill Boswell, MCSE, is the principal of Bill Boswell Consulting, Inc. He's the author of Inside Windows Server 2003 and Learning Exchange Server 2003 both from Addison Wesley. Bill is also Redmond magazine's "Windows Insider" columnist and a speaker at MCP Magazine's TechMentor Conferences.