Making Sense of Volume Shadow Copy Service

Monitoring VSS Performance
Monitoring the performance of shadow copies using System Monitor on Windows 2003 can help you head off potential problems before they affect users. For example, System Monitor can tell you that the amount of space used for shadow copies is nearing the maximum amount of space allocated. By default, System Monitor doesn't contain objects or counters that monitor shadow copy performance, but you can add some. (See the Microsoft article "Add counters to System Monitor" at http://technet2.microsoft.com/windowsserver/en/library/47a7a162-294d4307-af7e-b679e65858521033.mspx?mfr=true for directions on creating counters.)

Running the Microsoft Windows Server 2003 Resource Kit utility Volperf (with the /install switch) adds the shadow copy objects to System Monitor along with the following counters:

• % Disk Used by Diff Area File: the percentage of disk space used by all difference files on a volume
• Allocated Space (MB): amount of storage allocated for a specific volume
• Maximum Space (MB): maximum amount of space allocated for the shadow copy storage volume
• Nb of Diff Area Files: number of difference files
• Nb of Shadow Copies: number of shadow copies in the shadow copy cache
• Size of Diff Area Files: the total size of difference files for the selected volume
• Used Space (MB): amount of space that's been used in the shadow copy storage volume

Basic-to-Dynamic-Disk Conversion and VSS
There might come a day when you'd like to provide a new level of hardware fault tolerance by creating a mirror set. Mirror sets can be created only on dynamic disks, so a disk that's currently configured as a basic disk must first be converted to a dynamic disk. Most documentation states that converting disks from basic to dynamic doesn't cause any data loss. What the documentation doesn't mention, however, is that improperly converting a basic disk to a dynamic disk might delete existing shadow copies. When the source volume and shadow copy cache are on separate volumes, things can get sticky. (For more information about the differences between basic and dynamic disks, see "Choosing Basic vs. Dynamic Disk Storage for Windows Servers," December 2002, InstantDoc ID 27085.)

The procedure you must follow to convert a VSS-enabled basic disk to a dynamic disk depends on whether the shadow copy cache is stored on the boot volume.

Scenario 1—the shadow copy cache doesn't reside on the boot volume. If the shadow copy cache is not stored on the boot volume, you must first dismount the source volume (the volume you've taken the snapshot of) by using the Mountvol command-line utility with the /P parameter (/P dismounts the volume). Next, convert the volume that contains the shadow copy cache to a dynamic volume. Now, the clock is ticking—you have only 20 minutes to mount the source volume using either the Mountvol utility or the Microsoft Management Console Disk Management snap-in. If 20 minutes pass before you've mounted the source volume, all existing shadow copies are lost. Finally, bring the source volume back online and convert it to a dynamic volume.

Scenario 2—the shadow copy cache resides on the boot volume. If the shadow copy cache is stored on the boot volume, simply convert the volume that contains the shadow copy cache to a dynamic volume—there's no need to dismount the source volume first. Next, reboot the server twice, then convert the source volume to a dynamic volume.

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