Volume Shadow Service

Cobian Backup 9 can now copy files that are in exclusive use by other applications. It does so by using Volume Shadow Services, a technology that is present on Windows XP or a newer MS operative  system.

What is Volume Shadow Copy Service?

VSS is a mechanism for creating consistent point-in-time copies of data known as shadow copies. The Volume Shadow Copy Service can produce consistent shadow copies by coordinating with business applications, file-system services, backup applications, fast recovery solutions, and storage hardware.

The Volume Shadow Copy Service is commonly used in the following scenarios:

Consistent backups of open files and applications: Applications that are running often keep their files open continuously. For backup, this can present a problem because this prevents backup applications from accessing and copying these files to backup media. Even if an application does not have its files open, it is possible—because of the finite time needed to open, back up, and close a file—that files copied to the backup media might not all reflect the same application state at the same point in time.

Additionally, backing up servers that are running critical applications such as databases or messaging services presents a unique challenge. These applications run in a volatile state as a result of extensive optimizations that deal with huge flows of transactions and messages. Because these applications keep their data in a constant flux between memory and disk, it is difficult to pinpoint the data that needs to be archived.

The most straightforward solution is to interrupt the application during backup, which puts the data into a stable state, but might result in unacceptable amounts of downtime, particularly if the applications are large.

For both problems, the Volume Shadow Copy Service provides a solution by enabling a snapshot of the data at a given point in time, while minimizing the interruption to applications. This can involve cooperation from the applications, so that they can notify the operating system that they are briefly quiescing their data for archival. During quiescence, applications make data on the disk consistent. For example, an application might flush its buffers to disk or write out in-memory data to disk.

Considerations

To be able to use VSS you need to install and run the program on Windows XP, 2002, Vista or newer.

Because the program needs to create a snapshot of the volume data before the backup begins, using VSS is slower than a normal backup. You will need also a lot of extra free space for the system to be able to create the snapshot. That's why, you may want to not use VSS if there is no need to. If you know that the files that you are going to backup will not be locked when the backup begins, it's a lot more efficient to copy directly.

You can use VSS for local volumes, so a remote volume will  not be able to use VSS and will always be copied directly.

Be aware that the account that is running the backup may need administrative or backup operator privileges to create a VSS snapshot.

The Shadow Copy will always be created by the original user that is running the engine and not by the user that is impersonated (if impersonation is used)

The Shell method cannot be used with VSS!

Please see the FAQ for other questions related to VSS.

 

Index

©2000-2008 by Luis Cobian
All rights reserved