UBQ000080:  Incremental/Differential Backups and the Archive Bit

UBQ ID Number: UBQ000080
Last Modified: 2010-01-15

Summary:

How to set up incremental and differential backup sets, and how to use the "clear archive bit" function.

Details:

Once the drives from a computer have been loaded into the Backup Set Editor screen, unselect all files by deselecting the blue box at the top of the file tree.

 

Modified files can then be selected by right clicking the desired drive\folder and clicking "Include <drive\folder>..." This will open the "Include/Exclude Files" dialog box (see Fig. 2).

 

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Fig. 1 - Backup Set Editor "Include" option.

 

The "Include/Exclude Files" dialog allows the user to either include or exclude files based on creation/modification date, name, and file extension.

 

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Fig. 2 - "Include/Exclude Files" dialog.

 

The "By Modification Date" option lists:

 

 

The "Since beginning of month" option automatically selects all files modified since the first of the month for backup. This selection is a "pseudo" differential backup based on having performed a full backup at the end of the previous month.

 

The "Since beginning of day" option automatically selects all files modified beginning at "00:01" or "12:01am." This selection is a "pseudo" differential backup based on having performed a full backup at the end of the previous day.

 

The "Since beginning of week" option automatically selects all files modified beginning on Monday. This selection is a "pseudo" differential backup based on having performed a full backup at the end of the previous week.

 

The "Modified files (Archive bit)" option automatically selects any file with its archive bit set.

 

The "In the last <x> days" option automatically selects all files modified beginning "<x>" days ago, the user-specified number determining the selection logic. This selection is a "pseudo" differential backup based on having performed a full backup at the end of the day prior to <x>. For example, a value of 0 means "files modified since today." A value of 10 means "files modified since 10 days ago."

 

The "Modified <x> days ago" option automatically selects all files modified "<x>" days ago. This selection only backs up files modified on the user-specified day.

 

NOTE: The selection logic is relative to when the set starts, not when the backup group is launched. Starting a multiple set backup that spans across midnight will result in all post-midnight sets using the next day as the selection logic base. In other words, when the group/job spans midnight it results in different selection logics being applied to pre-midnight and post-midnight sets.

Clearing the Archive Bit - Scheduled Backups

The option to clear archive bits during scheduled backups can be set in either the Backup Group Options (Fig. 3) to clear the archive bits following all File-by-File backup sets included in the Group, or the Backup Set Options (Fig. 4) to clear the archive bits for the individual set. When the archive bits are cleared during a backup, this sets up the possibility of using the "Modified files (Archive bit)" option to perform an incremental or differential backup.

 

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Fig. 3 - Backup Group Options, "Clear archive bit for all sets."

 

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Fig. 4 - Backup Set Options, "Clear archive bit."

Clearing the Archive Bit - "Run Backup Now" (Ad-hoc) Backups

The setting to clear archive bits during "Run Backup Now" (ad hoc) backups can be set in either the Backup Set Options (Fig. 4), or in the "Backup Options" dialog (Fig. 5) prior to starting the backup. When the archive bits are cleared during a backup, this sets up the possibility of using the "Modified files (Archive bit)" option to perform an incremental or differential backup.

 

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Fig. 5 - "Run Backup Now" Backup Options, "Clear archive bit."

Incremental vs. Differential Backups

The difference between incremental and differential backups is simply when and how often the archive bits are cleared. If you choose to clear the archive bits during the full backups, and then run a series of backups where the "Modified files (Archive bit)" option is selected but the archive bit is not cleared, then this is a "differential" backup. However, if you continue to clear the archive bits during the backups where the "Modified files (Archive bit)" option is selected, this is called an "incremental" backup. There are pros and cons to each type of backup.

 

Incremental backups where the archive bits are cleared during the "modified files" backup means that each backup will be relatively small and take less time. However, it also means that if a restore of the entire disk is needed, the last full backup and then each subsequent incremental backup will need to be restored, in the correct order, to get all files to their most recent backup state.

 

With a differential backup, the archive bits are only cleared during the full backup, but not during each "modified files" backup. This means that each backup will gradually increase in both time and size, but also means that if a restore of the entire disk is needed, only two restores are needed – the full backup, and the last differential backup.

 

Both backup methods reduce backup time and size - an incremental reducing backup time and storage requirements, and a differential reducing restore time but requiring more space. The particular method employed will depend on the client's needs, as both incremental and differential backups have reasons for being used, or not used, as the case might be.

More Information:

See UltraBac User Manual:  Incremental/Differential Backups - Image Agent

See UltraBac User Manual:  Incremental/Differential Backups - File-by-File Agent