SharePoint and Windows Volume Shadow Copy Services: Overview

For backup vendors, the Windows Server 2003 Volume Shadow Copy Service (VSS) simplifies backing up Microsoft server solutions by using a centralized API. Microsoft Office SharePoint Server 2007 includes a referential VSS writer that integrates with the Windows VSS backup framework, allowing backup applications to back up and restore SharePoint 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 (and the Volume Shadow Copy Service), Microsoft Office SharePoint Server 2007 (and the Microsoft Office SharePoint Server 2007 writer for the Windows Server 2003 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 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 SharePoint store is a component of Office SharePoint Server 2007 and accesses Office SharePoint Server 2007 storage groups through the Windows Server 2003 file system. Within the file system, each Office SharePoint Server 2007 storage group includes configuration, content, Shared Services Provider (SSP) and Search databases, and any third-party databases registered in the configuration database and search index files.

To support the VSS, Office SharePoint Server 2007 includes a SharePoint writer that is built into the SharePoint store. The SharePoint writer coordinates with the SharePoint 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 SharePoint writer to coordinate with the SharePoint 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 Office SharePoint Server 2007 data. The requestor communicates with the VSS to obtain information about Office SharePoint Server 2007, 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 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 Office SharePoint Server 2007, the VSS, and the backup/restore application is transferred as part of the SharePoint writer metadata.

Following is the high-level interaction among the VSS, the SharePoint writer, and Office SharePoint Server 2007 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 Office SharePoint Server 2007 storage groups.

3. The VSS communicates with the SharePoint writer to prepare for a snapshot backup. Office SharePoint Server 2007 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 Office SharePoint Server 2007 storage group.

5. The VSS releases SharePoint Server 2007 to resume ordinary operations.

6. The VSS requestor verifies the integrity of the backup set prior to signaling that the backup succeeded. Office SharePoint Server 2007 records the time of the last backup for the database.