Dell NetWorker 19.9 Administration Guide

VSS and the backup process

In VSS terms, NetWorker software is a requestor — an application that needs data from other applications or services. When a requestor needs data from an application or service, this process occurs:

1. The requestor asks for this information from VSS.
2. VSS reviews the request for validity.
3. If the request is valid and the specified application has the requested data, the request goes to the application-specific writer, which prepares the requested data.

Each application and service that supports VSS has its own writer, which understands how the application or service works:

1. After the writer signals that it has prepared the data, VSS directs the writer to freeze I/O to the selected volumes, queuing it for later processing.
2. VSS then calls a provider to capture the requested data.
3. The provider, which is either software-based or associated with particular hardware (for example, a disk array), captures the prepared data, creating a snapshot (or shadow copy) that exists side-by-side with the live volume. Provider support contains more information.

The process of creating a snapshot involves interaction with the operating system. The amount of time it takes to create a snapshot depends on a number of factors, including the writer activity taking place at the time. Once the snapshot is created, the provider signals VSS, which tells the writer to resume activity. I/O is released to the selected volumes and any queued writes that arrived during the provider's work are processed.

The following figure provides a graphical representation of the VSS backup process.

This figure provides a graphical representation of the VSS backup process:

1. NetWorker software (the requestor) asks VSS to enumerate writers and gather their metadata.
2. Writers provide an XML description of backup components and define the recover method.
3. VSS asks which providers can support a snapshot for each of the required volumes.
4. Requestor asks VSS to createsnapshot.
5. VSS tells the writers to freeze activity.
6. VSS tells the providers to create the snapshot of the current state on disk.

VSS tells the writers to resume activity.

NetWorker software backs up data from the point-in-time snapshot that is created during this process. Any subsequent data access is performed on the snapshot, not the live (in-use) file system. The requestor has no direct contact with the provider; the process of taking a snapshot is seamlessly handled by VSS. Once the backup is complete, VSS deletes the snapshot.