Protecting your investment: Backing up your data...
Written by Dale Allen, 2005
The Tower of Hanoi
For certain applications, this Tape Rotation method is applicable. The
Tower of Hanoi is named after a game in which you move a number of different
sized rings among three poles. In the game you start out with all the rings on
one pole and must move all the rings to another pole. You can never have a ring
on top of one that is smaller than itself. The idea is that you must move them
in a certain order to accomplish the task. The order of movement of rings is:
A-B-A-C-A-B-A-D-A-B-A-C-A-B-A-E-A-B-A-C-A-B-A-D-A-B-A-C-A-B-A-F
A-B-A-C-A-B-A-D-A-B-A-C-A-B-A-E-A-B-A-C-A-B-A-D-A-B-A-C-A-B-A-G
A-B-A-C-A-B-A-D-A-B-A-C-A-B-A-E-A-B-A-C-A-B-A-D-A-B-A-C-A-B-A-F
A-B-A-C-A-B-A-D-A-B-A-C-A-B-A-E-A-B-A-C-A-B-A-D-A-B-A-C-A-B-A-H
In the case of tape backup, we use the same order to rotate tapes through a
tape drive, each day putting a complete image of the system on the tape.
Although the theories behind why this works as a good rotation method are beyond
our discussion, the benefit of this rotation is that you will always have an
older version of a file on one tape. In the case above, you would have 8 tapes
and considering you used one tape per day you could have a copy from as long as
128 days previous to the backup on tape.
A further example is that since tape "F" will contain a full backup of every
file on the system from every 128 days, if your system was infected with a
virus, you could restore an executable file without a virus as long as you
caught it within 128 days.
There are several variations to this rotation also. If you did backups twice per
day, you would be able to capture work in progress during the day, but you would
also only have versions from as long as 64 days previous in the example above.
It would also be possible to backup to a single tape for a week by doing a full
backup and then only backing up updated files during the week. You could
increase the number of copies this way but may run out of space on the tape. You
could also risk losing up to a weeks worth of data if that tape has a problem or
is damaged.
Related topics on Allen Enterprises Website in Puerto Rico
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