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How do I properly rotate my tapes for incremental or differential backups?
Incremental Backup Rotation
In this diagram, each tape indicates a single backup process illustrating an "incremental" tape rotation plan. Each backup process could consist of one or more physical tapes. This plan requires a weekly full system backup with intermediate, daily backups that cover data that was changed since the last backup of any type.
This formula, while requiring less tape and time for the daily backups, will require more work on the part of the backup administrator to provide sane recovery after a filesystem crash. In the event of a Thursday crash, the administrator would have to restore the last FULL backup followed by the Monday, Tuesday and Wednesday backups.
Other names for these steps would be:
* Level 0 (Full)
* Level 1 (1st Incremental)
* Level 2 (2nd Incremental)
* Level 3 (3rd Incremental)
Each higher level processes file changes since the immediately lower level.
Differential Backup Rotation
In this diagram, each tape, again, represents a single backup process. This illustrates a "differential" rotation scheme. Also, as before, each backup process could consist of one or more physical tapes. This plan uses the same weekly full system backup with the backup of files that have changed since the last full backup on a daily basis.
While this scheme takes up progressively more tape as each new day of the week passes, a recovery would only involve two tapes - the last FULL backup followed by the previous day's differential backup. Most backup administrators prefer this method as it simplifies recovery and most backups are done overnight.
This process is like performing a Level 0 (Full) followed by only Level 1 backups.
In these examples, the only differences in the two backup scenarios are:
Incremental Backups require fewer tapes while Differential backups require more.
Incremental backups require all of the tapes used for that week in order to do a complete restore, while the Differential backup only requires a maximum of two tapes at any given time.
Neither backup scenario has any effect on the ability to restore single files due to accidental deletion or for file version restoration.
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