Synthetic Full Backups

Synthetic full backups consolidate the data from the latest full backup or synthetic full backup together with any subsequent incremental backups, instead of reading and backing up data directly from the client computer. Since synthetic full backups do not back up data from the client computer, this operation imposes no load on the client computer.

During a synthetic full backup, a list of objects from previous backups is generated and the latest version of each object is considered to build a new backup image. Furthermore, using extended retention settings for a subclient, synthetic full backups can also retain deleted or older versions of objects. For more information, see Subclient Properties (Retention).

Running a synthetic full with non-deduplicated data requires writing each new object to a new location. For tape media, this means a synthetic full must write the new data to a new tape since you cannot read and write simultaneously to the same tape. As such, a synthetic full using tape media requires at least two tape drives – one for reading and one for writing.

A synthetic full backup done on disk or cloud media will also read and write objects. Unlike tape, disk media can perform concurrent reads and writes to the same media. However, if the source path is full or marked as read-only, objects may be written to other disk media paths available in the storage policy copy.

With deduplicated data, a synthetic full by default utilizes a faster variation of a synthetic full called DASH full where only the deduplication and index records are updated without the need to read each block and generate a signature. Since the associated disk I/O is reduced significantly, a DASH full backup is much faster than a regular synthetic full.

Advantages of Synthetic Full Backups Over Full Backups

Synthetic full backups have the following advantages over full backups:

  • They impose a lighter load on the production environment because they are created on the backup repository.

  • They have the ability to carry forward older or deleted versions of the objects backed up during the previous backup cycles.


Using synthetic full backups can cause unintentional expiration of data since retention periods are defined by the number of full backup cycles. For example, running a synthetic full immediately after a standard full backup does not consolidate any data (as the full already includes all the backup data); storage resources might be unnecessarily consumed.