NetBackup Snapshot Client for SQL Server overview

Snapshot Client, when used with NetBackup for SQL Server, backs up and restores SQL Server objects with snapshots of the component files. Then it backs up the snapshot version to tape or commits it to another form of persistent storage. The snapshot technology uses SQL Server VDI (virtual device interface) quiescence to affect a momentary freeze on database activity. Then NetBackup takes a snapshot of the identified files.

The term database freeze is interchangeable with database quiescence. It refers to the momentary state in which all activity in the database has been halted while a snapshot (or frozen image) copy is created. The state-change in which the freeze is lifted is referred to as database thaw.

The role of NetBackup for management of the backup images that were taken from snapshots is common for both Snapshot Client and standard database backups. NetBackup provides an identical view for cataloging and browsing images and for accessing them in restore operations. It is transparent whether standard database backups or the Snapshot Client created the images. Operational experience is similar whether the user backs up and restores standard SQL Server database images or any images that were taken from snapshots. However, there are fundamental differences in the technologies.

Snapshot Client backups, in contrast to standard backups, are file-based. NetBackup determines the file list that constitutes the SQL Server object and backs it up asynchronously with respect to SQL Server. On the other hand, standard backups are stream-based, which means that SQL Server provides data to NetBackup buffer-by-buffer that constitutes a backup stream. The key role of SQL Server in file-based backups is to provide the mechanism to freeze database activity. NetBackup can then invoke a so-called snapshot provider that creates volume snapshots of the files.

The snapshot method is the software component or hardware component that creates the snapshot image of the files. The method may be a Symantec product such as Volume Manager or an operating system component such as VSS (Microsoft Virtual Shadow Service). Ia may also be a third-party product. Snapshot methods use a variety of technologies such as copy-on-write, split-mirror, or RAID. They also provide several competing methodologies. These technologies may have different consequences for recovery speed: effect of the backup on the client processor, and the cost of specialized disks or processing units.