Base of a Differential Backup
This topic is relevant for all types of databases.
A conventional database, partial, or file backup upon which a differential backup is based is known as the base of the differential, or the differential base. A copy-only backup cannot serve as a differential base. The base backup for a file differential backup can be contained within a full backup, a file backup, or a partial backup. For more information, see Backup Under the Simple Recovery Model or Backup Under the Full Recovery Model.
A differential backup records only the data that has changed since the last differential base. Differential backups are smaller and faster than differential bases, facilitating frequent backups, which decrease the risk of data loss.
Information about the differential base of each file in a database is maintained in a catalog in the primary filegroup, except in a read-only database. For every database, the differential base information is also stored in the master database. The information about the current differential base is stored in the following columns.
Column name |
Description |
|---|---|
differential_base_lsn |
Base for differential backups. Data extents changed after this LSN will be included in a differential backup. |
differential_base_guid |
Unique identifier of the base backup on which a differential backup will be based. |
differential_base_time |
Time that corresponds to differential_base_lsn. |
If a database is read/write and is online, you can view these columns by querying the sys.database_files catalog view. If the database is read-only or offline, query the sys.master_files catalog view, instead.
Multibase Differentials
Under the simple recovery model, a differential backup must have one base. Multibase differential backups are prohibited, and trying to use a multibase differential backup displays an error and fails.
However, under the full recovery model, a differential backup can include files that have different differential bases. Such a backup is known as a multibase differential backup. Multibase differential backups can be difficult to administer and maintain. For that reason, we recommend that you use single-base differentials whenever possible. For more information, see Working with Multibase Differential Backups.