Recommendations for Synthetic Backups

Scenario in which synthesized backups would be most beneficial:

The set of NetBackup clients to be backed up experience a moderate rate of change in their file systems every day. (Approximately 5% of the files change every day, for example.)

If the clients experience a high rate of change daily, the incrementals will be too large and a synthetic backup would not be any more helpful than creating a traditional full backup.

Refrain from multiplexing backups that will be synthesized:

While synthesizing multiplexed backups is possible, it is not recommended because of its inefficiency. Synthesis of multiplexed client images requires multiple passes over the source media---one per client.

Synthesized backups and multistreaming:

Performance issues similar to those encountered while multiplexing synthesized backups problems will occur if multiple streams are selected for synthesized backups. Multiple stream performance issues can be improved by backing up to disk whenever possible.

Reducing the gap between the last incremental backup and the synthesized backup:

Since a synthetic backup does not involve direct contact with the client, a synthetic backup is only as current as the last incremental backup. If this is a concern in a NetBackup environment, to reduce a potential gap in backup coverage, consider running an incremental backup just prior to the synthetic backup.

Disk-based images are more efficient for synthesizing:

While synthesizing a backup, NetBackup processes the newest source images first, followed by sequentially older images. When two or more component images have been written to the same tape, the tape movement may be somewhat inefficient compared to disk-based images.

Storage unit considerations:

• When performing tape backups, a tape outside of the tapes where full and differential images reside, is required for the formation of a synthetic image.
• If the maximum drive usage is set to utilize only a single drive, the generation of a synthetic image results in the maximum drive usage being ignored and multiple drives being utilized in the creation of synthetic images.
• If a single tape drive device is used to generate synthetic images, component images (full, differential, or cumulative images) should be placed in a hard drive location first. In that way, a synthetic image can be generated with the single tape drive device.