This article provides the brief introduction about the important components of Microsoft Windows Volume Shadow Copy Service used by Networker backup.

VSS allows third-party backup applications to perform volume backups of data for applications, like Exchange and SQL Server, file systems or system files, including files currently open and active. By backing up the shadow copy (or snapshot), files that are currently open for writing can be backed up because the backup application is actually backing up a copy of the file. VSS produces clean (uncorrupted) snapshots of a volume by enabling applications to flush partially committed data from memory in a coordinated fashion with the snapshot requestor and the hardware. This ability prevents “torn writes” – the occurrence of the system not being able to complete the write of a block of data to disk.

VSS Components:

There are three components in the VSS framework: the Requestor, the Writer and the Provider:

• Requestor: A requestor is an application that uses the VSS API to request the services of the Volume Shadow Copy Service to create and manage shadow copies of one or more volumes. VSS works with the requestor to gather information to properly save applications, services and other files on the file system. Requestor roles include accepting user requests to make shadow copies, managing the cataloging and archival of shadow copies, and managing the lifetime of shadow copies. A backup application is an example of a requestor.
• Writer: A writer is an application-specific component. A writer provides details of the data to the requestor, such as the location of the data and the method of backup and restore.Writers differentiate VSS from other snapshot solutions in that there is a communications structure between the requestor and writer governing how the snapshot should take place and how the snapshot should be recovered. A writer is available and active only if its application or service is also available and active on the system. If a service or application is present on a system but is not active, information from its writer will not be available. Consequently, writers can appear or disappear from backup to backup.
• Provider: A provider is responsible for the creation and maintenance of the shadow copy. Once the provider has received the XML file describing the application data, it uses an I/O interceptor to intercept write requests and copy blocks of data from the source volume to the virtual volume before allowing the write operation to proceed.

VSS coordinates the actions of the various components required to create a consistent shadow copy of the data that you want to backup. The actions to coordinate are between the backup application, the service or application that contains the data, and the I/O subsystem to get a consistent snapshot of the data. If your operating system, applications, backup software, and SAN manufacturer all support VSS, you can create flexible storage solutions that can easily be protected without the need to stop servicing clients.

How VSS works for Networker backup

The Volume Shadow Copy Service (VSS) manages the creation of a point-in-time shadow copy, or snapshot, of a disk volume or set of data. Let us look at how they work together to create a snapshot.

1. The requestor, such as a backup application, requests VSS to create a snapshot of a particular volume or data set.
2. VSS notifies the application-specific writer to prepare its data for making a shadow copy. The writer creates an XML description of the backup components and defines the restore method. The writer prepares the data by completing all open transactions, rolling transaction logs and flushing caches. VSS then directs the writer to temporarily freeze requestor I/O write requests for the time required to create the shadow copy. VSS flushes the file system buffer and then freezes the file system, which ensures that file system metadata is written and that the data is written in a consistent order.
3. VSS notifies a provider to create and maintain the shadow copy until it is no longer needed. A point-in-time copy of the complete volume mapping is created using XML.
4. Once the snapshot has been successfully created, VSS thaws the file system and instructs the writers to resume normal activities, or thaw. VSS provides the location information for the shadow copy back to the requestor. The requestor uses the snapshot to create the backup. The backup is as of the point in time that the snapshot is taken. The snapshot can also be backed up to secondary storage through a proxy client by either using an existing snapshot or with the use of temporary snapshots.

Author: Fenglin Li

iEMC APJ

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