ToH Example 3

Imagine the backup plan has the following tape options:

• the Use a separate tape set option is selected
• the Always use a free tape: For each full backup option is selected
• the Always use a free tape: For each incremental backup option is selected
• the Always use a free tape: For each differential backup option is selected.

The figure shows tape rotation for the ToH scheme with these options.

Maximal number of tapes used in the rotation is seven that is more than in classical five-level ToH scheme.

Two additional tapes used for:

1. keeping an old full backup (postponed deletion) as it is a base for other level backups
2. keeping an old backup on a level until a new backup has been successfully created on the level.

The example demonstrates that the tapes’ usage efficiency is reduced. Moreover, the data recovery requires access to the backups kept on one (full backups, 6%), two (differential backups, 44%) or three (incremental backups, 50%) tapes. So on average the operation takes more time than in the previous examples.