Me and My Volume Shadow Copy
Quick look at this often-ignored but useful Windows 2003 feature, including some gotchas to watch for when deploying it.
- By Bill Boswell
We've just rolled out file servers running Windows
Server 2003 and we want to use Volume Shadow Copy to help us with file
restores. We want to know if there are any gotchas.
Anon: I'm glad to hear that you're going to use one of
the coolest features in Windows Server 2003. I give lots of presentations
on Windows server technologies and it's surprising how few people have
even tried VSS in a lab, much less deployed it in production. I think
it's because nobody quite trusts VSS to be as simple as Microsoft says
it should be.
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If you aren't familiar with Volume Shadow Copy services, here's a quick
rundown. VSS has the capability of "freezing" a volume then
taking a point-in-time copy of the volume's content. Applications such
as backup programs, file copy programs, data integrity verification programs,
and so forth can perform their operations on the point-in-time copy, thus
ensuring data consistency.
To make this magic work, VSS uses three elements. The first element is
the hardware that holds the volume. Many SAN and NAS devices have features
for taking point-in-time copies of a live volume. The device might maintain
a complete mirror image of a volume that can be split away from the source
volume. This is called a "split-mirror" copy. Other storage
devices maintain a block map of a volume and collect original content
in a special repository prior to permitting a block to be modified. This
is called a "copy-on-write" copy. High-end storage devices typically
use a combination of split-mirror and copy-on-write to take point-in-time
copies. A storage device that includes a driver that allows VSS to tell
the device to take a point-in-time copy is called a "VSS Provider."
You don't need a high-end SAN or NAS device to use VSS. Microsoft includes
a software provider in Windows Server 2003 that can do copy-on-write operations
without special storage hardware.
The second VSS element is a group of services and applications that use
the volume to store data. These are called "VSS Writers." Examples
of VSS Writers include SQL Server, Exchange, DHCP, WINS, IIS, the Event
Log service, and others.
The third VSS element is the application that wants to perform a procedure
that requires data consistency. These applications include backup programs,
file copy programs, and so forth.
When a VSS Requestor needs to perform a procedure on a volume, it asks
VSS to freeze the volume. VSS communicates this request to the various
VSS Writers. When all the Writers have stopped their disk I/O, VSS tells
the Provider to make a point-in-time copy. When that operation finishes,
VSS thaws the volume and points the Requestor at the copy.
Windows Server 2003 includes a Requestor called Volume Shadow Copy for
Shared Folders. This requestor allows you to take periodic point-in-time
copies of a volume then present the original content in the form of previous
versions of files and folders. There is a special Explorer add-on called
the Previous Versions client that allows you to view the previous versions
of a file and either make a copy of the previous version or overwrite
the current version.
In other words, Volume Shadow Copy for Shared Folders allows you to do
nearly instant file recoveries. You can even deploy the Previous Versions
client to your users to let them view the file change history, if you
trust them to use this feature wisely. If not, you can restrict client
deployment to administrators.
That's the cool stuff. Here are the gotchas.
First, the VSS software provider needs sufficient storage capacity to
maintain a database of the blocks in use on a target volume and to store
the original content of blocks as they are modified. On a heavily used
volume, VSS might need storage capacity from 10 to 30 percent of the target
volume or more, depending on how long you want to retain previous versions.
If you run out of storage space, the oldest copies are removed first.
In production, you'll want to add more spindles to store the VSS files
or carve out another LUN in your SAN. (The storage must be local to the
server. It cannot be a mapped drive.)
Second, VSS retains a maximum of 64 point-in-time copies of a volume.
This allows you to take two point-in-time copies each working day, which
gives you up to a month-and-a-half of previous versions. This is a hard-coded
limit and cannot be modified.
Third, VSS stores original content in 16K blocks. Unless you specifically
format a new volume to use 16K allocation units, it's possible to overwhelm
VSS with updates during defrag or heavy file loads if the source volume
is formatted in smaller allocation units (typically 4K in size). SP1 has
a fix for this issue. You can also call Product Support Services (PSS)
to get hotfix KB 833167.
Be sure to let me know if you deploy Volume Shadow Copy for Shared Folders
into your production environment. I'll pass on the good and bad to the
Until next week...
More on Access Based Enumeration
If you recall a column my column, "The
Enforcer," I mentioned a new feature in Windows Server 2003 SP1
called Access Based Enumeration. ABE allows you to hide folders and files
from users who don't have access. It's a cool feature but there aren't
any tools in SP1 to take advantage of it. I got a note from the ABE product
manager that there is now a command-line and GUI utility available to
manage ABE and a white paper describing the feature. You can download
the tool by clicking
here and the documentation here.
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.