Windows 7 / Getting Started

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Shadow Copy Feature

One common administrator burden is restoring data from backups. Windows Server 2003 and Windows Server 2008 have a wonderful Volume Shadow Copy Service (VSS). This creates shadow copies, or previous versions. Shadow copies enable users to select the Previous Versions tab of the Properties dialog of a share, see point-in-time copies of the share, and perform self-restoration of data stored on NTFS volumes.

At configurable intervals/times of the day, VSS takes a snapshot of the state of the content of selected volumes (that's right, the entire volume, not just share content). Even at the file level, only the changes are stored. If there is a small change to a 1GB file, only the small change is stored. Up to 64 versions of a share can be made available. This limit is imposed by the previous version client and the amount of disk space available. When the 65th snapshot is taken, the oldest one is deleted. Likewise, if the amount of disk space allocated for shadow copies has run out, the eldest snapshots is deleted to make space for the newest snapshot.

Some clients need a new software component installed that adds a tab to the share's Properties dialog. This tab allows them to select a point-intime view of the share and access its content. Newer operating systems, such as Vista, have VSS built-in. This is great for users and administrators. If a user deletes a file, she doesn't need to trouble an Administrator. The user can view the share as it was at a point in the past and copy over the file. Likewise, if a file has been corrupted, the user can view the share as it was in the past and copy over the uncorrupted version.

NOTE: VSS does not replace performing backups because only the changes are stored. If your file system was lost, the shadow copy information on its own would be of no use. Microsoft has also stated that in times of exceptionally high I/O, shadow copies might be lost.

The amount of disk space used for the shadow copies is not based on the size of the volume, but rather the size and frequency of changes. This is driven by the applications used. For example, if an application only writes changes to the file when data is changed, this uses far less shadow copy space than an application that rewrites the entire file.

When accessing previous states of a share, the ACLs on files and folders still apply. If you did not have access to the file before, you won't have access when viewing it via the Previous Versions client.

The shadow copy file or folder information is stored in the System Volume Information of the volume selected to hold the information and cannot be accessed outside of the shadow copy client.

Finally, although the whole content of the volume is protected by VSS, you can only view previous states via share properties. If you need to recover a file that is not within a current share, create a new share that contains the file and then connect to the share. Notice that if you create a new share, you see a full history of all previous versions equivalent to existing shares. This happens because VSS logs the entire file system, not just existing shares.

You can enable VSS for a drive or volume with the following steps. If you want to create from the command line, use the vssadmin command with the create shadow /for=<volume>: parameters.

1. Right-click the drive and select Configure Shadow Copies from the context menu.
2. Select the volume for which you want to enable shadow copies from the Select a Volume area.
3. Click the Settings button to change the default. The default configuration creates a shadow copy at 7:00 and 12:00 each week day, storing the data on the selected drive.
4. In the dialog, you can choose which drive the shadow copy files are saved to (drive only, you cannot select folders) and the maximum amount of space that can be used (you need at least 100MB). Make your choices and click the Schedule button.
5. The Schedule dialog has a drop-down at the top that shows the current scheduled executions of the shadow copy. You can select one (click the drop-down arrow) and then use the options to change how often it runs. The Weekly option allows a time to be selected and on which days to run. You can also click New to create a new schedule. For example, you might want a schedule that runs just once on Saturday and Sunday at 7:00. Click OK when all options are made.
6. Click OK to the main Settings dialog.
7. The shadow copy is now enabled. (You don't need to click the Enable button.) You can optionally click the Create Now button to create a starting snapshot. The Create Now button can be used at any time to force creation of a shadow copy.

Windows Vista clients (Business and Ultimate editions) have a Restore Previous Versions context menu item for shares. This can be used to view the contents of the shadow copies and restore or copy data from them. Windows XP SP2 also has VSS built-in. For other operating system support, check out www.microsoft.com/technet/downloads/winsrvr/shadowcopyclient.mspx.