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Differential

Another approach to data backup is the differential backup method. While the incremental backup only archived those files that had changed since any backup, the differential method will back up any files that have been changed since the last full backup. The following is an example of a backup schedule using tapes, with weekly full backups on Sunday night and daily differential backups.

Each Sunday, a full backup is performed. For Monday’s differential backup, only those files that have been changed since Sunday’s backup will be archived. On Tuesday, again those files that have been changed since Sunday’s full backup, including those backed up with Monday’s differential, will be archived. Wednesday, Thursday, Friday, and Saturday would all simply archive all files that had changed since the previous full backup.

Given this schedule, if a data or disk failure occurs and there is a need for recovery, then only the most recent full backup and most recent differential backup are required to initiate a full recovery. Though the time to perform each differential backup is shorter than a full backup, as more time passes since the last full backup the length of time to perform a differential backup will also increase. If much of the data being backed up regularly changes or the time between full backups is long, then the length of time for a backup might approach that of the full backup.

Archive Bits

Some file systems, such as Microsoft’s NTFS, support the archive bit. This bit is a file attribute used to determine whether a file has been archived since last modification. A full backup will archive all files (regardless of each individual file’s archive bit setting), and then reset all archive bits to 0 (indicating each file has been archived).

As files are modified, the associated archive bits are set to 1 (indicating the file has changed, and needs to be archived). An incremental backup will archive each modified file and reset the archive bit to 0. A differential backup will archive each modified file and leave the archive bit set to 1.