## Using the Tower of Hanoi tape rotation schemeThe 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 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. 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. 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. 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. |