ToH Example 2
Suppose, the backup plan has the following tape options:
The only difference between ToH Example 2 and ToH Example 1 is that the Always use a free tape: For each full backup option is selected.
The first figure 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.
In the figure below the backups deleted at the moment are drawn as dark-gray rectangles.
The figure indicates that there are two full backups on level E because at the moment the first full backup is a base for differential backups D, C and B are a base for incremental backup A. So the full backup deletion is postponed until all the D, C, B and A backups will be deleted.
The next figure shows the tape usage at the moment before creating a new backup on level D:
At the moment the backup archive occupies four tapes. It is the maximal number of tapes required in the example.
After the next backup on level D is created, both tapes 01 and 02 are 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:
So in this specific case the selection of the Always use a free tape: For each full backup option considerably increases the usage efficiency of the tapes in the library.