Often only some files are changed between backups; other files in the data directory are not updated. Nevertheless backups contain all files. Such backups are called normal.
To decrease backup size it is possible to include only updated files in new backups. Differential backup contains only updated files in comparison with the latest normal backup. You can make differential backups from time to time creating normal backups (otherwise changes would accumulate and differential backups will tend to have the size of normal ones).
To restore a differential backup you need a normal backup which served as a basis for the differential one. This normal backup is called base for the differential backup.
A succession of backups including normal and differential ones (made on the basis of the normal one) is called a series. Number of backups in the series including normal backup is called series length.
To create differential backups during section backup you should select "Differential" mode in the section options dialog on the "Backups" tab. You should specify maximum series length and maximum number of series stored at a time.
Note. Changing mode ("Only normal backups" or "Differential"), series length and number of series is unavailable if there is at least one backup in the section. This helps to avoid problems with changing backup mode for old backups.
Backup type — normal or differential — is determined before start according to the following rules:
If backup collection is full (number of series is maximum) a new backup will overwrite the oldest one considering that:
the oldest differential backup in the oldest series is deleted, if the oldest series contained one backup, this series is deleted (with normal backup as well). Maximum number of backups for a section is a maximum series length multiplied by maximum number of series.
Filename template for differential backups:
NNN — series point; DD — point of the differential backup in the series.
Base normal backup has usual filenames:
When a differential backup is created $FILECNG.INF file is created in the data directory. It contains file properties and status: unchanged, updated, new, deleted. This file is automatically included in the backup. When viewing backup properties this information is displayed at the side of the files in the backup.
When restoring a differential backup first the base normal backup is restored; then the differential backup is restored; then updates are calculated and superfluos files are deleted. Thus the result data directory is exact like it was during creation of the differential backup.
Note. It is not recommended to restore differential backups manually by external programs like PkZip or WinZip. You will have to manually reproduce the above-described procedure.
When exporting a differential backup or making it stationary the differential backup will be automatically transformed into a normal backup.