Recommendations for synthetic backups and restores
The synthetic full backup is a scalable solution for backing up remote offices with manageable data volumes and low levels of daily change.
If the clients experience a high rate of change daily, the incremental backups are too large. In this case, a synthetic backup is no more helpful than a traditional full backup.
Synthetic backups are supported on all media server platforms and tier one master server platforms.
The following items describe recommendations to use synthesized backups to full advantage, and situations under which synthesized backups are not supported:
Recommendations concerning backups:
Do not multiplex any backups that are to be synthesized because it is inefficient. To synthesize multiplexed client images requires multiple passes over the source media - one per client.
Performance issues can also occur if multiple streams are selected for synthesized backups. The issues are similar to those encountered while multiplexing synthesized backups. Back up to disk whenever possible to improve multiple stream performance issues.
Veritas recommends that you not enable Expire after copy retention for any storage units that are to be used with SLPs with either of the following: Accelerator or synthetic backups. The Expire after copy retention can cause images to expire while the backup runs. To synthesize a new full backup, the SLP backup needs the previous backup image. If the previous image expires during the backup, the backup fails.
Reduce 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 there is a concern to reduce a potential gap in backup coverage, run an incremental backup before the synthetic backup.
You can create multiple copies with synthetic backups by using the multiple copies synthetic backup method.
Although synthetic backups do support the use of storage lifecycle policies, SLPs cannot be used for the multiple copy synthetic backups method.
Synthetic backups are not supported if any of the component images are encrypted.
A user-generated backup cannot be used to generate a synthetic image. A backup that is generated from a User Backup schedule or a User Archive schedule cannot be used as one of the components of a synthetic backup.
Synthetic backups and optimized synthetic backups do not support Auto Image Replication.
Recommendations concerning restores:
The time that is required to perform a restore from a synthetic backup does not increase significantly over time.
The restore times for both a complete synthetic backup and for a single file is the same. It is the same whether the restore is from a traditional backup or from a synthetic backup.
The restore time of a single directory may increase over time when sourced from synthetic backups. The restore time depends on the pattern of file changes within the directory.
Contrast a traditional full backup, which stores the files in file system order with a synthetic full backup, which stores the files in last-file-accessed order. The synthetic full contains the newest files at the front of the media and the unchanged files at the end. Over time, the processing order introduces the potential for fragmentation of a single directory across the synthetic full image.
Note that the scenario is limited to single directory restores. Single file restores and full image restores from synthetic fulls are equal or better than from traditional full backups, as noted in previous bullets.
If checkpoint restart is indicated for the policy, the backups that are produced with the synthetic backup schedule are not checkpointed. The option is enabled if Take checkpoints on the policy Attributes tab is enabled. If the option is enabled for a synthetic backup, the property has no effect.
Table: Recommendations when using disk storage or tape storage for synthetic backups
Storage unit type | Recommendations |
|---|---|
Disk storage units | Disk-based images are more efficient for synthesizing. NetBackup processes the newest component images first in a synthesized backup, followed by sequentially older images. When two or more component images are written to the same tape, the tape movement can be inefficient compared to disk-based images. Synthetic full backups are generated more quickly when built from disk-based incremental backups. If the synthetic full backup is also generated on disk, the run time is even faster. The disk copy then can be duplicated to tape. |
Tape storage units | If tape is used instead of disk, the tape for the synthetic image must be different from the tape where the component images reside. The maximum drive usage applies only to the drive that is needed for writing the synthetic backup. If any of the component images reside on tape, an additional drive is needed for reading. If a single tape drive device is used to generate synthetic images, place component images in a hard drive location first. In that way, a synthetic image can be generated with the single tape drive device. |