How SQL Server operations use Snapshot Client

This topic describes how SQL Server operations use the Snapshot Client.

The selection of a backup methodology, whether standard or Snapshot Client, is dependent on what policy is used. If a policy configured for Snapshot Client is selected, then additional attributes of policy determine the Snapshot Client features. It also determines the specific snapshot methods that are used.

Due to SQL Server limitations certain objects cannot be backed up by snapshots. These are database differentials, filegroup differentials, and transaction logs. If a Snapshot Client policy is selected to back up one of these object types, then NetBackup performs a stream-based backup. NetBackup uses the storage unit that is provided in the policy configuration. If a storage unit is not provided, then NetBackup uses the default storage unit for the server.

The database administrator works exclusively with logical objects, such as databases and filegroups. However, it is useful to understand the differences between file- and stream-based backups in terms of the data content that is archived. For stream-based backups, NetBackup captures the data stream content that is provided by SQL Server. If the user has specified multiple streams, then SQL Server opens multiple streams that NetBackup catalogs as separate images.

For file-based backups, NetBackup creates a file list that consists of all the physical files that constitute the object. This file list is supplied to the Snapshot Client, which is responsible for snapshot creation. If multiple streams are specified, then NetBackup divides the file list into sub-lists. Each sub-list is backed up separately and constitutes a separate image. Users may notice that if multiple streams are specified for a file-based backup and if the number of streams exceeds the number of component files, then the number of file-based streams does not exceed the number of files. With stream-based SQL Server backups, SQL Server always creates exactly the number of streams that the end user specifies.

The file list that is used to back up a SQL Server database consists of the physical files that constitute the primary filegroup. The file list also consists of any secondary filegroups, and the transaction log. Typically, these can be identified respectively by their name extensions, which are .mdf, .ndf, and .ldf. The file list for a filegroup backup consists of the physical files that belong to the filegroup. And, finally, the file list for a file object backup consists of a single physical file. This file is the file that maps to the SQL Server file object.

When a physical file is backed up with the Snapshot Client, the backup consists of the entire extent. This backup contrasts with stream-based SQL Server backups where only the actual data content of the objects are archived. If you intend to use snapshot technology to back up SQL Server, you may want to use the SQL Server dynamic file allocation. This configuration reduces the likelihood that any of the component files contain large areas of empty space.

Also review the other considerations for SQL Server disk initialization.

No special interfacing considerations exist when you perform Snapshot Client backups of SQL Server. A snapshot backup is performed if the backup object is: a database, a filegroup, or a file and a policy is selected and configured for Snapshot Client. If a differential backup or transaction log backup is tried with a Snapshot Client backup, then the operation uses the selected policy. But a standard database backup is performed with the configured storage unit.

Any backup images that were created from snapshots display along with standard backup images. That is, all backup items - without regard to method - display in a time-sequenced ordering that respects the composition of the database hierarchy. In addition, no weighting is given in to determine an optimal recovery that is based on the backup method. To determine what backup method and policy were used when a SQL Server backup was created, right-click the backup image and select Properties.

Figure: Backup method that appears in the backup image properties