Volume Shadow Copy Service (VSS) for virtual machines

This option controls the creation of quiesced snapshots for protected virtual machines.

The preset is: Enabled.

When this option is enabled, the transactions of all VSS-aware applications running in the virtual machine are completed before taking a quiesced snapshot.

If a quiesced snapshot cannot be taken for some reason, the outcome of the backup operation depends on the application backup settings as follows.

• If application backup is set to Enabled and a quiesced snapshot cannot be taken after the number of re-attempts specified in the "Error handling" option, the backup fails.
• If application backup is set to Disabled and a quiesced snapshot cannot be taken after the number of re-attempts specified in the "Error handling" option, a crash-consistent backup is created.

  To make the backup fail instead of creating a crash-consistent backup, select the Fail backup if taking a quiesced snapshot is not possible check box.

When this option is disabled, a non-quiesced snapshot is taken. The virtual machine will be backed up in a crash-consistent state.

The following table summarizes the available settings and their results.

Settings	Quiesced snapshot was taken successfully		Quiesced snapshot was not taken
	Application backup enabled	Application backup disabled	Application backup enabled	Application backup disabled
Volume Shadow Copy Service (VSS) for virtual machines enabled Fail backup if taking a quiesced snapshot is not possible not selected	Quiesced snapshot is taken. Application-consistent backup is created.	Quiesced snapshot is taken. Application-consistent backup is created.	Backup fails.	Non-quiesced snapshot is taken. Crash-consistent backup is created.
Volume Shadow Copy Service (VSS) for virtual machines enabled Fail backup if taking a quiesced snapshot is not possible selected	Quiesced snapshot is taken. Application-consistent backup is created.	Quiesced snapshot is taken. Application-consistent backup is created.	Backup fails.	Backup fails.
Volume Shadow Copy Service (VSS) for virtual machines disabled	Non-quiesced snapshot is taken. Crash-consistent backup is created.	Non-quiesced snapshot is taken. Crash-consistent backup is created.	Non-quiesced snapshot is taken. Crash-consistent backup is created.	Non-quiesced snapshot is taken. Crash-consistent backup is created.

Enabling Volume Shadow Copy Service (VSS) for virtual machines also triggers the pre‐freeze and post‐thaw scripts that you might have on the backed-up virtual machine. For more information on these scripts, refer to Running pre‐freeze and post‐thaw scripts automatically.

To take a quiesced snapshot, the backup software applies VSS inside a virtual machine by using VMware Tools, Hyper-V Integration Services, Virtuozzo Guest Tools, Red Hat Virtualization Guest Tools, or QEMU Guest Tools, respectively.