Microsoft SQL Server Virtual Backup Device Specification
VDC_PrepareToFreeze
When BACKUP WITH SNAPSHOT is used, and the VDF_SnapshotPrepare configuration bit is set, the VDC_PrepareToFreeze command is sent prior to the freeze of the database files. Refer to SNAPSHOT Support for more information on the use of this command.
This command was added in Service Pack 2 for Microsoft® SQL Server™ 2000. In earlier versions the VDF_SnapshotPrepare configuration bit is ignored and the VDC_PrepareToFreeze command is never issued.