Acronis Backup & Recovery

ToH Example 1

Suppose, 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 cleared
• the Always use a free tape: For each incremental backup option is cleared
• the Always use a free tape: For each differential backup option is cleared.

The figure below shows the tapes’ usage for the ToH scheme combined with the above mentioned tape options. The recurring part of the scheme contains sixteen backup sessions. The figure displays the backup archive state at the moment when the 17th session is finished.

As the Tower of Hanoi backup scheme forces presence of only one backup on each level, all the outdated backups are deleted automatically. In the next figure the deleted backups are drawn as dark-gray rectangles. Actually the deleted backup is still stored on the tapes, but the information about it is deleted from the storage node database.

The figure shows the full backup kept on tape 01 at the moment, which cannot be deleted as it is a base for actual differential (D, C, B) and incremental (A) backups stored on tape 02. The full backup deletion is postponed until all the above mentioned four backups will be deleted.

The next figure demonstrates the tapes’ content at the moment before creation of the new backup on level D:

At the moment the data archive occupies four tapes, and the total size of the backups written up to the current time is maximal for the example. However, if in the future a full backup will be written at the end of a tape, the archive will occupy five tapes.

After the next backup is created on level D, tape 01 is freed and can be reused.

It is noticed that the ToH scheme combined with the specified options has the following properties for the analyzed case:

• the last figure shows that the data recovery requires loading and mounting of up to three tapes (one tape – 16%, two tapes – 72%, three tapes – 12%) as well as rewinding and reading of one (6%), two (50%) or three (44%) backups
• five-level scheme requires up to five tapes for this case.