Windows SharePoint Services VSS Writer
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The Volume Shadow Copy Service (VSS), included with Windows Server 2003/2008, is the infrastructure that provides built-in shadow copy capabilities. Shadow copies created by VSS augment the storage administrator’s tape backup archival solutions, providing high fidelity point-in-time copies that can be created and restored easily and effectively, thereby helping to simplify several aspects of storage and data management. Windows SharePoint Services 3.0 uses VSS to simplify backup and restore operations. Following are the Windows SharePoint Services 3.0 VSS solution features and characteristics:
A single VSS reference writer. There has been no easy way for applications to describe their data to backup applications. To successfully back up various Windows platform applications, backup applications have an excessive number of APIs that they need to write specific code for. The Windows SharePoint Services VSS writer (hereafter called "the WSS-VSS Writer") enables backup applications to take advantage of the single writer to backup Windows SharePoint Services.
Full farm backup and restore for catastrophe. The WSS-VSS Writer enables a backup application (requestor) to access the VSS API to request a backup or a restore operation for an entire Windows SharePoint Services farm, including a single box setup or a farm configuration. (The IIS Metabase is not included. It must be backed up and restored separately.)
Database level granularity. The WSS-VSS Writer enables a requestor to select all databases, a segment of the databases (multiple select), or a single database (single select) for both backup and restore operations. All databases, except configuration and the Central Administration content database, are selectable through the writer. The configuration and Central Administration content databases can be backed up and restored only as part of the whole farm. (The IIS Metabase is not included. It must be backed up and restored separately.)
Inventory of databases. Before backup, the WSS-VSS Writer generates a flat list of databases selected for backup within the farm. The list is returned to the requestor so that backup can be run on the location where the database is physically located.
Farm support. The writer understands and provides support to synchronize backup and recovery on a Windows SharePoint Services farm in a limited way. The writer provides the requestor with a list of servers, databases, and files associated with the farm. The requestor is responsible for making a separate connection to each server to call the WSS-VSS Writer on that server to generate the backup or to run the restore operation.
Backup content without interruption. If an application modifies a file while it is being backed up, the file could become corrupt. VSS enables a quick snapshot of the files to the shadow copy, while the application continues to operate at the original location without interruption.
Third-party pluggable database backup and recovery. The WSS-VSS Writer offers pluggable/extensible backup for third-party solutions built on top of Windows SharePoint Services. However, only databases that are registered within the configuration database are included in the writer. Any additional files and unregistered databases are not included.
Search index files backup and recovery. Because search index files are stored in the file system, a separate file writer is needed to them up back up. To resolve this, Windows SharePoint Services includes a separate search writer that handles search index files. To simplify the process for backup application writers, Windows SharePoint Services declares cross-writer dependencies in such a way that search index files are also backed up or restored when backing up registered databases in the farm.
Full rollback. The WSS-VSS Writer handles all components within a Windows SharePoint Services deployment, including the configuration database and the content databases and the Search database and index. (It will also handle any Shared Service Provider [SSP] included with an enhanced service product, such as Microsoft Office SharePoint Server 2007, installed on top of Windows SharePoint Services.) As mentioned previously, the writer also has a dependency on the Search writer, which handles all the Search index files for backup and recovery. At the time of recovery, the writer can roll back the entire Windows SharePoint Services deployment by restoring a previous farm backup. (The IIS Metabase is not included. It must be backed up and restored separately.)
See “Restoring” in VSS Requestors and Windows SharePoint Services for important information about restorations.
Post-restore synchronization of databases. To ensure that all databases are synchronized with the farm after a restore operation is complete, each of the databases are automatically detached and reattached to the farm post recovery. Administrators do not need to run additional procedures to resynchronize the restored databases.
Functions Performed by the WSS-VSS Writer
The WSS-VSS Writer performs the following functions:
Builds Windows SharePoint Services components.
Generates a full list of all components within the Windows SharePoint Services farm.
Is not necessarily tied to backup process or restore process.
Backs up farm or database.
- Requests a Windows SharePoint Services (farm/database) backup via VSS.
Restores a farm or database.
Requests a Windows SharePoint Services (farm/database) recovery via VSS.
Implements postRestore() to synchronize sites table.