Overview of Windows SharePoint Services and the Volume Shadow Copy Service

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For backup vendors, the Volume Shadow Copy Service (VSS) simplifies backing up Microsoft server solutions by using a centralized API. Windows SharePoint Services 3.0 includes a referential VSS writer (hereafter, called "the WSS-VSS Writer") that integrates with the Windows VSS backup framework, enabling backup applications to back up and restore Windows SharePoint Services data. It supports a catastrophic overwrite scenario for the entire farm (search index included). On recovery, it hooks up databases and synchronizes site mappings.

The following figure shows the main components in the system: Microsoft Windows Server 2003/2008 (and the Volume Shadow Copy Service), Windows SharePoint Services 3.0 (and the WSS-VSS Writer for the Windows Server 2003/2008 Volume Shadow Copy Service), and the third-party (or custom) backup/restore application (including the requestor and the provider).

The VSS communicates with the Windows Server 2003/2008 file system and with the mass storage device driver through a third-party (or custom) provider. The hardware provider must determine where the shadow copy is created. The VSS abstracts the hardware-specific shadow copy so the backup/restore application can access the shadow copy in a uniform manner without knowing the hardware implementation details.

The Windows SharePoint Services store is a component of Windows SharePoint Services and accesses Windows SharePoint Services storage groups through the Windows Server 2003/2008 file system. Within the file system, each Windows SharePoint Services storage group includes configuration, content, Search databases, and any third-party databases registered in the configuration database and search index files. Also included is any Shared Service Provider (SSP) that comes with an enhanced service product installed on top of Windows SharePoint Services, such as Microsoft Office SharePoint Server 2007.

To support the VSS, Windows SharePoint Services includes the WSS-VSS Writer that is built into the Windows SharePoint Services store. The WSS-VSS Writer coordinates with the Windows SharePoint Services store (operating on behalf of the requestor) to freeze and dismount the storage group before backing it up, and then to unfreeze and mount the storage group after the backup is complete.

During a recovery, the backup/restore application instructs the WSS-VSS Writer to coordinate with the Windows SharePoint Services store (operating on behalf of the requestor) to dismount the storage group, replace the database files, and mount the storage group.

A requestor is a third-party (or custom) application designed to use the VSS to properly back up and restore Windows SharePoint Services data. The requestor communicates with the VSS to obtain information about Windows SharePoint Services, command the creation of shadow copies, and gain access to the data for backup.

When restoring, the requestor also communicates with the VSS to prepare the system for the restore operation, and then to put the data back onto the mass storage device. The backup/restore application is also responsible for working with Windows Server 2003/2008 to read data from and write data to the backup storage media, whether a tape archive, a storage area network, or other backup medium.

Information needed to successfully complete backup and restore operations among Windows SharePoint Services, the VSS, and the backup/restore application is transferred as part of the WSS-VSS Writer metadata.

The following is the high-level sequence of events during backup or restore operations:

1. The backup program (or agent) runs a scheduled job.

2. The VSS requestor in the backup/restore application sends a command to the VSS to take a shadow copy of the selected Windows SharePoint Services storage groups.

3. The VSS communicates with the WSS-VSS Writer to prepare for a snapshot backup. Windows SharePoint Services prohibits administrative actions against the storage group, checks volume dependencies, and suspends all write operations to database and transaction log files while allowing read-only access.

4. The VSS communicates with the appropriate storage provider to create a shadow copy of the storage volume that contains the Windows SharePoint Services storage group.

5. The VSS releases Windows SharePoint Services to resume ordinary operations.

6. The VSS requestor verifies the integrity of the backup set prior to signaling that the backup succeeded. Windows SharePoint Services records the time of the last backup for the database.