How do StorageCraft® ShadowProtect® and Microsoft Volume Shadow Copy Service (VSS) interact to ensure complete and accurate point-in-time volume backups when VSS-Aware applications reside on these volumes?
What is VSS and what does it do?
VSS technology was first included in Windows Server 2003 to resolve issues associated with backing up open files. Utilizing VSS correctly helps ensure that Administrators do not have to log users off of databases when backups occur. VSS does not backup files but rather the sectors on which the files reside thus giving very accurate and relatively small point-in-time backups.
Which applications are VSS-enabled today?
Type vssadmin list writers in an admin command prompt to list all VSS-enabled applications' respective writers on a system. Examples of Microsoft applications and databases include:
Windows Server 2003 and later
Exchange 2003 and later
SQL 2005 including MSDE and later
Windows Vista and later
How does ShadowProtect use VSS?
StorageCraft ShadowProtect makes a backup call to the VSS framework via its provider. The VSS framework then communicates with the VSS Writers to tell the relevant applications to commit all transactions to disk and then freezes the database so it can gather the list of sectors that needs to be backed up. This freeze process allows the data to be read from the frozen sectors but temporarily blocks writing to these same sectors.
Once the applications communicate to the writers that they are ready the VSS provider creates a snapshot, then informs the framework that the snapshot has been completed. The writers now release (or thaw) the databases and files. The snapshot process typically takes only a few seconds, depending upon the size of the volume and number of changed sectors. Excessively long snapshot times can indicate underlying problems with the application, OS, or hardware of the system. This can also be the byproduct of an extremely busy database-using system such as large Exchange or SQL implementations.
When the entire backup process completes, including writing the backup image to disk, the VSS provider communicates the complete status to the framework. The framework then instructs the writers to perform the defined post-backup operations, such as purging log files.
For more in-depth information on how VSS works, see this article from Microsoft.
The VSS Framework was developed by Microsoft and as such StorageCraft support is limited in troubleshooting these issues. VSS largely relies on the environment that it is being used in rather than the software that is using it. If the VSS Framework needs further diagnosis and troubleshooting after all StorageCraft resources have been exhausted, contact Microsoft or the vendor of the affected application for further assistance. The following StorageCraft Knowledge Base Articles may assist in resolving VSS issues with backups or provide additional information on utilizing VSS in the backup and disaster recovery plan.