A differential backup contains only the data that is new or has changed since the last full backup. Like incrementals, differential backups, on average, consume less media and place less of a burden on resources than full backups.
For most agents, differential backups are cumulative. This means that each differential backup contains all changes accumulated since the last full backup. Each successive differential backup includes the changes of previous differential backup. The Oracle iDataAgent, however, provides a backup option that allows you to specify whether or not you want the backup to be cumulative. The following illustration demonstrates the nature of differential backups. For simplicity, assume there is a file system that contains six files as represented in the figure.
Note that the File System above could also represent database tables, Active Directory objects and attributes, and so on.
Backup #1 is a full backup and therefore writes all the data to the backup media. Backups #2 through #n-1 are differential backups and only back up those files that changed since the time of the last full backup. For example, files A, B, and E changed after the full backup and were therefore backed up in Backup #2 as well as all subsequent differential backups. File C changed sometime after Backup #2 and was consequently backed up in Backup #3 and all subsequent differential backups. File F did not change; hence, it was not backed up in any of the differential backups, but it was included in both full backups, which, by definition, back up everything.
For the Oracle, DB2, DB2 MultiNode, Informix, SAP and Sybase Agent, the Incremental backup process works as described in this section.
Start a Differential Backup
- From the CommCell Browser, expand Client Computers by double-clicking Client Computers | iDA File System | defaultBackupSet. The default and other subclients (if available) are displayed on the right-hand windowpane.
- To back up the default subclient, right-click the subclient, and click Backup.
- From the Backup Options dialog box, select Run Immediately.
- Select Differential backup.
- Click OK.
You can track the progress of the job in the Job Controller.
Intermediate Differential and Incremental Backups
There is an interaction between differential backups and other incremental or differential backups that have been taken since the last full backup (regular or synthetic). It is helpful to keep this interaction in mind when creating a new differential backup.
When producing a differential backup, the Commvault software compares the current contents of the client with the contents of the last full backup for that client. The resulting backup contains all data that has changed or been added since the last full backup. If other incremental or differential backups have been produced after the last full backup, but before the current differential backup, it is possible to lose some backed-up data (see Data Loss Scenario).
Consider this diagram:
The incremental backup (number 2) backs up a new version of file A because it has changed since the last full backup. The first differential backup (number 3) backs up file D because it has been created since the last full backup was taken. When the second differential backup (backup number 4) is created, it carries forward file A version 2, but does not carry forward file D because it was deleted before backup number 4 was taken. The synthetic full backup (number 5) contains versions 0 and 2 of file A (but not version 1) and does not contain file D even though it existed on the client with backup number 3 was taken. File A version 1 can still be restored from backup 2, and file D can still be restored from backup 3, unless the Do not retain intermediate incremental and differentials before a new differential option is selected.
Important: Although version 1 of file A and file D can be restored from backups number 2 and 3, selecting the Do not retain intermediate incremental and differentials before a new differential option would render them unrestorable. This option saves media by aging intermediate incremental and differential backups once a succeeding differential backup has been created, but without those intermediate backups, version 1 of file A and file D cannot be restored.