There are basically three backup modes for use with rotation schemes, either custom or GFS:
The first weekly backup is a full backup. Subsequent backups (until the next full backup) can either be incremental or differential (but all one mode).
In incremental backup mode, on the first day after the last full backup, only those files that have changed since the last full backup are backed up. With subsequent incremental backups, only those files that have changed since the previous incremental backup are backed up.
In differential backup mode, every file that has changed since the last full backup is backed up during every differential backup.
Thus incremental backups are, on average, much faster than differential backups because fewer files need to be backed up. However, restoring data backed up with a rotation scheme using incremental backups is much slower, since data from the last full backup plus data from all the incremental backups must be restored. Restoring data backed up in differential mode only requires the last full backup plus the last differential backup.
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