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The determination of a complete data plus operating system backup is somewhat dependent on the environment.
Complete operating system failures typically donít happen as often as data failures. Operating systems also donít typically change as often as the data.
Catastrophic operating system failures are often caused by hardware errors, and if specific hardware needs to be replaced, it is often the case more than not, that a complete re-installation of the operating system may need to be done anyway.
As part of a backup strategy, it may only be worthwhile to backup the operating system periodically such as on a weekly basis.
Complete System Backup vs. Partial
When practical, the most prudent backup strategy is to perform a complete system backup every working day.
If there are 22 working days in the month, 22 tapes should be used for the daily backups whenever possible.
This insures the ability to recovery any file for at least one month.
Furthermore, if weekly and monthly processing is performed, a partial backup should be performed of the transaction files before processing and then a full backup after the period has been processed.
Keep 5 weekly tapes and 13 monthly tapes.
When disaster strikes and a critical data volume is lost, being able to restore the entire volume from last night's backup reduces the liability of attempting to restore the volume from multiple incremental and base line backups.
If you are performing full backups every night and last night's backup failed, and a critical data volume is lost before the next backup, then the volume can be restored from the backup created the night before (and so on).
As a standard rule, do not append to tapes without just cause. Prudent disaster recovery planning conflicts with the practice of leaving a tape in the drive and using it all week. The last five backups are all resident on just one extremely thin strand of tape.