A synthetic full backup is a synthesized backup, created from the most recent full backup and subsequent incremental or differential backups. The resulting synthetic full backup is identical to a full backup for the subclient.
Unlike full, incremental, and differential backups, a synthetic full backup does not actually transfer data from a client computer to the backup media and does not use any resources on the client computer.
Synthetic full backups are media-based; they read backup data from one media and simultaneously write (restore) the result to the new active media within the same media group. As a result, synthetic full backups require that at least two media drives for the same storage policy be available at the time the job is started. Synthetic full backups cannot be performed on subclients where the storage policy is associated with a stand-alone drive.
If any IntelliSnap backups have not yet been backup copied, a synthetic full backup job fails. Synthetic full backups cannot run on snap copies.
Note
For Forever Incremental backups with the Virtual Server Agent, create a separate Snapshot copy on the storage policy and specify a data aging rule that includes the Retain Snaps by Number of Jobs option with a number of jobs that matches your backup cycle. This ensures that the previous backup jobs are pruned on a regular basis. For more information, see Data Aging for IntelliSnap Snapshots.
Accelerated Synthetic Full Backups (DASH Full)
When a storage policy copy is deduplicated, synthetic full backups can be created in an accelerated mode to significantly reduce the copy duration. This is done by identifying and transferring the data signatures (instead of the data itself) to the target wherever possible.