VSS: Description, Compatibilities, and Troubleshooting Resources

Topic

This article explains the Volume Shadow Copy Service, and how it interacts with the Datto backup solution. Use this article as an aid when troubleshooting backup issues.

Environment

  • Datto ALTO
  • Datto SIRIS

Description

Microsoft's Shadow Copy Provider's Volume Shadow Copy Service, also commonly known as VSS, is used to take image-based backups. VSS is composed of the following:

  • VSS Writer: This writer tells the backup tool how to back up the application and its data. A VSS writer must be present for the Volume Shadow Copy Service to quiesce (freeze) the program to take an application-aware backup.
  • VSS Requestor: This is the writer that initiates the backup process.
  • VSS Provider: This writer allows the VSS backup process to work with the system's hardware and operating system. The VSS provider is responsible for taking a snapshot. The VSS provider and its associated driver are required for this snapshot to take place.

For VSS backup solutions, only one process on the production machine can use the VSS writers at a time. If more than one of these processes are running in the same environment, a conflict between the solutions will happen, resulting in one or more VSS Writers failing.

The difference between VSS backups and DBD backups

On Windows systems, Datto's agent-based backup software uses the Microsoft Shadow Copy Provider to perform the initial step of the backup process. If the Microsoft Shadow Copy Provider's VSS writers encounter an error, the Datto Windows Agent will fail over to the Datto Snapshot Driver, referred to as the Datto Backup Driver, bypassing the VSS backup process. The Datto Backup Driver generates backups in a crash-consistent state. Data in processing at the time of the backup may not save correctly.

  • Crash-consistent backups are a concern for systems running any services that include a VSS writer for backups (such as database services). For instance, if the VSS writer for a database fails, recent transactions for that database may not back up.

Application-Aware VSS refers to the Microsoft Shadow Copy Provider. This VSS provider interacts with supported applications to provide backups as the applications are running.

Additional Resources

The following external knowledge articles provide resources for troubleshooting and optimizing VSS writers in the Windows environment.