Synthetic Full Backups
Synthetic full backups consolidate the data from the latest full backup or synthetic full backup together with any subsequent incremental backups into one archive file, 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.
The image at right describes how a synthetic full backup is created.
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 additional spare media available.
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.
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.
Advantages of Synthetic Full Backups
Synthetic full backups benefit in the following ways:
- Consumes fewer resources because it is created from the backup files already on the disk.
- Imposes a lighter load on the production environment because it is created on the backup repository.
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.
To enable DASH full on a storage policy copy, see Modifying a Storage Policy Copy Deduplication Settings.
Optimizing Synthetic Full Backups in Deduplication Appliances
Synthetic full backup operations back up all the data which are recently added or modified and writes a new baseline of items as a new job. This allows prior jobs to be aged, and then the new incremental backups are performed with this baseline as the reference.
Performance of a deduplication appliance such as Data Domain Array is bound by stream or thread performance. The synthetic full operation requires a concurrent read/write operation, which can affect the performance of the device.
It is recommended that you take the following steps to optimize the performance of the device:
- Stagger your backup schedule in such a way that the minimum number of active streams are targeted to the device.
- Alternately, if the synthetic full operation takes too long to complete, you can perform a Full backup followed by daily incremental backup jobs.
- In cases where longer backup cycles are performed in the device, the backup performance may degrade over time. This may be not be due to the concurrent read/write synthetic full jobs, instead it may be due to the large volume of deduplicated data on the deduplication appliance.
Recommendations for Tape Media
When using tape libraries with Synthetic Full backups the following is recommended:
- Make sure to keep at least two media drives available for the same storage policy when the job starts. When tape media is in use synthetic full backups will not work for subclients where the storage policy is associated with a stand-alone drive.
This is because data is read and written simultaneously. Synthetic full backup reads data from one media and simultaneously write (restore) the result to the new active media within the same media group.
- Have only one synthetic full job running to the tape drive for synthetic full backups with multiple streams where the number of streams is more than the number of tape drives.