How Synthetic Full Backups Work
You can start a synthetic full backup at the subclient, backup set or agent level. If you start the synthetic full backup at the backup set or agent level, then an individual job starts on each subclient. In other words, if you start a synthetic full backup on a backup set that contains two subclients, then two synthetic full backup jobs run, one for each subclient. If you choose to use a new media for synthetic full backups, then you might need to have several spare media available.
A synthetic full backup extracts the index data of each participating subclient. Using this index data and the previously backed up user data images, it builds new full backup images, one for each subclient. The new backup images consolidate the index and user data stored in the related incremental, differential, and latest full backups. As the synthetic full backup for each subclient proceeds, the system writes an archive file to the storage policy for each subclient from which the backup data originated. Since each archive file represents one synthetic full backup for each subclient, a synthetic full backup of a backup set containing three subclients would initiate three operations, each resulting in an archive file.
The following illustration demonstrates the operation of a synthetic full backup that was initiated at the backup set level. For simplicity, assume:
- The backup set contains two subclients; default and subclient1
- Each subclient has the same backup schedule
- Index and user data of the backups are directed to the same storage policy. (This is the default behavior of standard backups.)
- Backups for each subclient are directed to separate and dedicated storage policies (i.e., groups that service no other subclients.)
From this figure, we can see that from a single request on 4/7, two synthetic full backups were created: one for each subclient. When we requested a synthetic full backup for the backup set, the system identified all the backups that were created for each subclient back to the most recent full (F), which in this case occurred on 4/1. Using the index data from these backups, the system created new consolidated index and user data images and wrote them to new archive files (SF) on the same storage policies.
In this example, we assumed that the standard backup was written to a dedicated storage policy and that the user data and corresponding index were both backed up to the same storage policy. Note that these conditions do not affect the overall operation of a synthetic full backup. In either case, (i.e., non-dedicated storage policies or divided index/user data) the system locates the applicable index and user data and creates a new archive file.
An important point to remember is that a synthetic full backup consolidates data; it does not back up data from the client computer. Consequently, the data in a synthetic full backup always reflects the data of the previous backup, regardless of when the synthetic full backup was requested. In this example, the synthetic full backup was started on 4/7; however, the data it contains is from the 4/1 through 4/6 backups. To secure data for 4/7, we would need to run a conventional (i.e., full, incremental, or differential) backup on 4/7.
Also, keep in mind that synthetic full reads the data from the copy whose copy precedence is set to 1. And like a real backup, synthetic full is always created in the primary copy. i.e., its destination is always a primary copy.
The following figures show two examples of using synthetic full backups in standard backup schedules. The figures show a series of full backup cycles where each cycle is a week in length. The longer lines indicate full backups while the shorter lines indicate incrementals. Note that both sets of backups begin with the required standard full backup.
In the first figure, both synthetic and standard full backups are used to define the full backup cycles. Synthetic full backups are used more often because they do not affect the operation of the client computer. In the second figure, synthetic full backups are used exclusively. To ensure that client data is properly secured, each synthetic full backup is followed by an incremental backup on the same day. If you decide to incorporate synthetic full backups into your standard backup schedules, the approach you take is entirely dependent upon your needs.
If a synthetic full backup fails to complete, we suggest that you run a standard full backup in its place.
If a tape library is being used for synthetic full backups with multiple streams where the number of streams is more than the number of drives on the tape, then its recommended to have only one synthetic full job running to the tape drive.