Microsoft provides an essential service to facilitate backups of many server applications called Volume Shadow Copy Service (VSS). The DSR Backup Agent uses VSS technology to perform all backup operations.

In many application scenarios it is imperative to ensure data integrity. VSS is used to generate a disk freeze and also to ensure the data integrity of transactional applications based on a database layer. This is done using “VSS writers” : this component of VSS is usually delivered by applications like SQL Server, Exchange, Hyper-V.

To ensure the integrity of the backed up data, it is necessary to check the “open files (VSS)” option in the configuration settings of your backup set.

Performing a disk freeze before a backup with the DSR agent on Windows :

• The DSR agent lists the disks that are in the backup perimeter in order to request a freeze from VSS.
• The DSR agent lists the components of the “VSS writers” and determines which ones will be affected by the backup scope.
• The DSR agent asks VSS to prepare for the backup.
• The “VSS writers” perform the necessary operations to make the data they are responsible for consistent.
• The VSS writers and the VSS service notify the DSR agent whether the data is ready to be saved.

Improvements in the latest firmware :

• Regardless of the JDS configuration, VSS is now used. Thus, if the data can be frozen, it will be saved via the VSS freeze.
• If the “open files (VSS)” option is checked, error detection during the VSS freeze interrupts the backup to ensure that only intact data is backed up.
• If the “open files (VSS)” option is not checked, then the degraded backup mode is applied, which consists of backing up everything that can be backed up. Thus two levels of degradation are possible :

Degradation at the level of “VSS writers” :

– During the VSS freeze all “VSS writers” in error will be excluded in order to make the creation of the VSS freeze possible.
– In this case the integrity of the data associated with the excluded “VSS writers” perimeter cannot be guaranteed.

Gel degradation VSS :

– If it is impossible to create a disk freeze, the files are still backed up.
– In the latter case it is impossible to guarantee the integrity of the saved data.