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five-level scheme requires up to five tapes for this case.
ToH Example 2
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 selected
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 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:
the data recovery requires access to the backups kept on one (25%) or two tapes (75%)
five-level scheme can require up to four tapes.
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.
