Microsoft’s Volume Shadow Copy Service (VSS) was created to provide a backup and recovery infrastructure for the Windows Server 2003 operating system. It includes a mechanism for creating point-in-time copies of data to preserve relational integrity. While the Service can provide coverage for some missed files during backups, there are limitations because many business applications are either non-VSS aware or IT administrators are using VSS to backup file-only servers. With the integration of Open File Manager the problem of files being skipped or inaccurately captured is solved. Open File Manager can find a point-in-time where all data is in a consistent state even if the data from the application is not VSS aware. This eliminates "crash consistency" when interacting with legacy and non-VSS compliant applications.

Open File Manager Solves the Problem

Open File Manager contains features that enhance VSS by providing the capability to support legacy writer applications within the VSS framework. The components of this feature integrate with VSS and use a time-based paradigm to insure that no legacy writer transaction is in progress when a VSS snapshot is created. When components are used with VSS, the data that is backed up with VSS will have transactional integrity and be self-consistent regardless of whether or not the applications running on the system have been integrated with VSS. This functionality is critical if you are using VSS and your business applications (writers) are not “VSS compliant.”

How VSS Works
The VSS process is as follows:

• The backup application (requestor) is run to make a backup.
• The requestor sends a command to VSS to take a snapshot.
• VSS communicates with the business applications (writers) to finish existing transactions and pause new transactions.
• VSS communicates with the storage snapshot technology (provider) to create a snapshot.
• The provider creates a snapshot of the volume set.
• VSS communicates with the writers to resume normal operations.
• The requestor copies the snapshot creating a backup of the volume set. Since the snapshot is not being accessed by any other application, all files on the volume will appear as closed and available to the requestor even though application programs are accessing the files on the original volume. This is an apparent, but flawed, solution to the open file backup problem because this method can compromise relational data integrity.
• Upon completion of the backup, the requestor communicates with VSS to delete the snapshot.

The VSS Dilemma

The VSS framework coordinates VSS compliant backup applications (requestors), business applications (writers), and storage snapshot technology (providers) to enable reliable storage backup and recovery. Backup applications and business applications must be VSS compliant in order to benefit from the VSS framework. However, many applications are not VSS compliant and may never be updated to be compliant because of development and maintenance costs. Also, VSS cannot predict or control the I/O flow of such legacy applications. The VSS solution to legacy writer support is to provide "crash consistency". This means that VSS only guarantees file system integrity and does not guarantee legacy writer transaction integrity. If the VSS snapshot is created while a legacy writer transaction is in progress, the snapshot will contain a partial transaction and will be corrupt.

Don't Take Chances, Use Open File Manager

If you are unsure about any applications' VSS-compliancy status, don't take chances; use Open File Manager to enhance VSS and insure data integrity during backups. If other servers in your network are managed by anything other than Microsoft's Windows Server 2003, VSS will not be supported without the functionality of Open File Manager. If you have file-only servers or applications and related files on separate servers and you want to guarantee the integrity of your backups, use Open File Manager in conjunction with VSS.