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backup deletion frees a tape so it can be reused.
The main drawback is the large number of required tapes that is used 5-10%.
If we have to keep a daily backup for a week (4 backups) and a weekly backup for a month (4
backups), the total number of required tapes will be equal to 4+4+13+1 = 22.
Using the Tower of Hanoi tape rotation scheme
The ToH scheme requires fewer tapes for rotation as compared with the GFS scheme. So the ToH
scheme is the best choice for small tape libraries, especially for autoloaders.
Once the ToH backup scheme is selected, it is possible to specify the scheme schedule and the
number of levels.
Best practice suggests that five levels should be used if you are applying the Tower of Hanoi to
weekly backups and eight levels if you are applying it to daily backups. In the first case the rotation
includes 16 weekly sessions so it ensures that the roll-back period (the minimum number of days that
you can go back in the archive) is 8 weeks. Tape rotation for the second case includes 128 daily
sessions, i.e. it allows the roll-back period equal 64 days. The roll-back period is always half the
number of sessions.
Each additional level doubles not only the number of sessions but also the oldest backup age.
Let’s return to the analyzed case described in the Case to analyze (p. 148) section, and suppose the
ToH settings are the following:
Schedule:
Start the task every 1 day at 11:00 PM. Repeat once.
Number of levels: 5
The Tower of Hanoi scheme with five levels ensures that the roll-back period is 8 days. Let’s
designate the backups of the levels with numbers from 1 to 5 by letters A, B, C, D, and E respectively.
Then the rotation template for the backup sequence in the archive is the following: E-A-B-A-C-A-B-A-
D-A-B-A-C-A-B-A. In the five-level ToH scheme all the backups on the 1st level (A) are incremental, on
the 5th level (E) – full, and other backups on levels 2, 3, and 4 (B, C, and D) are differential.
Tape rotation for the ToH scheme substantially depends on the tape options, whose default settings
do not always provide optimal usage of tapes and the whole tape library.
The goal is to choose the tape options requiring the minimal number of tapes in the rotation.
There are analyzed examples showing how the ToH scheme can be combined with different tape
options in the following sections:
ToH Example 1 (p. 156). The
Use a separate tape set
option is selected. All the
Always use a free
tape
options are cleared. It requires 5 tapes in rotation.
ToH Example 2 (p. 157). The
Use a separate tape set
option is selected. The
Always use a free
tape: For each full backup
option is selected. The other
Always use a free tape
options are
cleared. It requires 4 tapes in rotation.
ToH Example 3 (p. 158). The
Use a separate tape set
option is selected. All the
Always use a free
tape
options are selected. It requires 7 tapes in rotation.
ToH Example 2
requires 4 tapes, which is the minimum for the case. So its tape options settings are
the best in comparison with options for other examples.
