When backing up open files, VSS relies on VSS-aware applications and their Writers to prepare the associated files for backup (for example, the Microsoft Exchange Writer is responsible for preparing Microsoft Exchange files and the MSDE Writer is responsible for preparing Microsoft SQL files). If no Writer is available for a certain application, any open files of that type cannot be reliably backed up.
Use the Client for VSS Software Snap-Shot or the Enterprise Option for VSS Hardware Snap-Shot when the files you are backing up are associated with a Writer. Because of the way in which Writers communicate with their applications and the application’s files, VSS can obtain an in-depth knowledge of the transactional behavior of a Writer's files. When file activity is very high, the Agent for Open Files may take much longer than VSS to find a transactionally safe period in which to back up the open files.
Use the Agent for Open Files to guarantee that all open files not associated with a Writer are reliably backed up. If open files not associated with a Writer are backed up using VSS, there is no guarantee of transactional integrity and you run the risk of invalidating the entire backup.
The Agent for Open Files operates independently of other applications. An application does not need to know about the agent to back up open files, and an application can write to its files during a backup job without having to communicate with the agent. Everything is handled by the agent.
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