Microsoft Shadow Copy, also called Volume Snapshot Service (VSS), is a Windows Server data backup feature for creating consistent point-in-time copies of data (called shadow copies).

The type of quiescing used varies depending on the operating system of the backed-up virtual machine, as shown in Driver Type and Quiescing Mechanisms Used According to Guest Operating Systems. ESXi 4.1 added support for Windows 2008 guests using application level quiescing.

File-system consistent quiescing prevents file systems from becoming corrupted, for example, journaled file systems are allowed to write out pending transactions. Crash-consistent quiescing is the ability to restore an application as if it suddenly crashed and lost all stateful information. This involves minimal effort because only data already written to disk is guaranteed safe. Application-consistent quiescing is the ability to restore stateful information as well.

Restore must be done using the backup application’s guest agent. The vSphere APIs for Data Protection provide no host agent support for this. Applications authenticating with SSPI might not work right because HTTP access will demand a user name and password, unless the session was recently authenticated.

When performing VSS quiescing while creating the snapshot of a Windows virtual machine, VMware Tools generate a vss-manifest.zip file containing the backup components document (BCD) and writer manifests. The host agent stores this manifest file in the snapshotDir of the virtual machine. Backup applications should get the vss-manifest.zip file so they can save it to backup media. There are several ways to get this file:

■	Using the datastore access HTTPS protocol, for example by browsing to https://<server-or-host>/folder/ and continuing downward to the snapshot directory until you find the vss-manifest.zip file.
■	By calling the CopyDatastoreFile_Task method in the vSphere API. This method accepts the URL formulated above for HTTPS, or a datastore path. (CopyVirtualDisk_Task is for VMDK files).
■	With the vifs command in the vMA or vCLI.
■	With the Copy-DatastoreItem cmdlet in the PowerCLI (requires PowerShell and VMware snap-in).

Windows 2008 application level quiescing is performed using a hardware snapshot provider. After quiescing the virtual machine, the hardware snapshot provider creates two redo logs per disk: one for the live virtual machine writes and another for the VSS and writers in the guest to modify the disks after the snapshot operation as part of the quiescing operations.

The snapshot configuration information reports this second redo log as part of the snapshot. This redo log represented the quiesced state of all the applications in the guest. This redo log must be opened for backup with VDDK 1.2 or later. The older VDDK 1.1 software cannot open the second redo log for backup.

Application consistent quiescing of Windows 2008 virtual machines is only available when those virtual machines are created in vSphere 4.1 or later. Virtual machines created in vSphere 4.0 can be updated to enable application consistent quiescing by modifying a virtual machine’s enableUUID attribute.

For information about VSS, see the Microsoft TechNet article, How Volume Shadow Copy Service Works. For information about Security Support Provider Interface (SSPI), see the MSDN Web site.