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Synthetic vs differential backup

Created: 17 May 2012 • Updated: 07 Aug 2014 | 7 comments
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can explain what is the differance between Synthetic and differential backup?

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revarooo's picture
A differential backup only backs up the files that have changed since the last backup.
Synthetic backups are created from a full backup and from all subsequent incrementals.
When you do a synthetic full, you roll up the incrementals back to the last full and create in esscence an aritificial new Full backup without actually having to do any backup on the client.
This is useful if the full backups take a long long time to run. You only need to have the initial hit of the backup taking a long time on the first full backup.
Editted: for correction
Andy Welburn's picture

but found that I just couldn't get the wording right!

Really, have a look in the Admin Guides - they have good explanations of each, but to summarise from same:

Cumulative incremental backup
A cumulative incremental backup backs up the files that are specified in the backup selections list that have changed since the last full backup.

Differential incremental backup
A differential incremental backup backs up the files that have changed since the last successful incremental (differential or cumulative) or full backup.

Synthetic backup
A synthetic full or synthetic cumulative incremental backup is a backup assembled from previous backups.

Omar Villa's picture

If you are under NBU 7.5, try this:


Omar Villa

Netbackup Expert

Twiter: @omarvillaNBU

revarooo's picture

Accelerator is a neat little edition, but it is restricted to specific storage units at the moment.

muhanad.daher's picture

Revaroo :"Accelerator is a neat little edition, but it is restricted to specific storage units at the moment"

if i have understood you this mean the syn backup add the last syn-inc backup to full backup.

but if i have a full backup before 2 week as full-syn, and daily as inc-syn and i want to restore the files that lost today, what i must to do?

restore the full and the last inc-syn...


Muhanad Daher

Senior Data Center Engineer

revarooo's picture


No Synthetic rolls up the last full backup with all of the incrementals, to create a brand new Synthetic Full backup. That Synthetic backup will take the 'time' from the last incremental backup.

So if you did a Full normal backup on Sunday, followed by incrementals Mon-Fri, then a synthetic full on the Sunday, all of the backups from Sun-Fri are used. Saturdays Synthetic Full backup takes the backup time from the last incremental on the Friday. Hope this makes sense.

From the Admin Guide:

Synthetic backup (schedule attribute)
Asynthetic full or synthetic cumulative incremental backup is a backup assembled
from previous backups. The backups include one previous, traditional full backup,
and subsequent differential backups, and a cumulative incremental backup. (A
traditional full backup means a non-synthesized, full backup.) A client can then
use the synthesized backup to restore files and directories in the same way that
a client restores from a traditional backup.
Synthetic backups can be written to tape, to disk storage units, or to a combination
of both.

Edit: Just found this article, take a read

imashu's picture

Synthetic backup:-

It is a backup assembled from previous backups,the backup includes one previous  traditional full backup & subsequent differential backup and/or cumulative incremental backup. The client need not be running to form the new full synthetic backup. The new full synthetic is an accurate representation of the client’s file system at the time of the most recent full backup.

The policy type must be either Standard or MS-Windows.

When a synthetic backup is scheduled , Netbackup starts the bysynth  program to manage the synthetic backup process bpdm to cause tape  & disk images to be read or written.

The Synthetic full backup contains the newest file at the front of the media and the unchanged files at the end (last file accessed order).

Synthetic backups can be written to tape storage units or disk storage units or a combination of  both.

A synthetic backup must be created in a policy with the True Image Restore with Move Detection option selected.

A synthetic job is distinguished from a traditional full backup by the notation that is indicated in the Data Movement field of the Activity Monitor. Synthetic jobs display Synthetic as the Data Movement type while traditional backups display Standard.

Like a traditional backup, a synthetic backup is typically initiated by nbpem. Nbpem submits to nbjm a request to start the synthetic backup job. nbjm starts bpsynth. bpsynth controls the creation of the synthetic backup image and controls the reading of the files needed from the component images. bpsynth executes on the master server. If a directory named bpsynth exists in the debug log directory, additional debug log messages are written to a log file in that directory.

bpsynth makes a synthetic image in several phases:

  1. Prepare catalog information and extents

  2. Obtain resources

  3. Copy data

  4. Validate the image

How to create Synthetic backup policy:

Use the NetBackup interface to create a backup policy that includes full, incremental, and synthetic full backup schedules.

 When you configure the backups, make sure to configure them in the following order:

 On the Change Policy Wizard dialog box's Attributes tab, check Collect true image restore information and check with move detection.

On the Add Schedule dialog box's Attributes tab, check Synthetic backup.

On the Scheduling tab, make sure to configure the full backup and the incremental backup to run before the synthetic full backup schedule runs.

Benefits of Synthetic backup:

  1. Reduces the network traffic – Network traffic is reduced because the files are transferred over the network only once.

  2. It can reduce the number of tapes or disk space in use.

  3. Synthetic backups can be created in the environments that are comprised exclusively of disk storage units.

  4. Uses drives more effectively-The backups can be synthesized when drives are not generally in use. For Example; if the backup occurs primarily at night then drives can synthesize full backups during the day.

    Differential incremental backup : A differential backup back up data that has changed since the last full or incremental backup.

    Cumulative Incremental backup : A cumulative incremental backup backs up data that has changed since the last full backup.

    I hope this clears all your doubts..