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..