Tower
of Hanoi is a complex tape backup strategy that's useful for archiving
data for an extended period of time in an economical manner. The strategy,
which is based on a mathematical puzzle invented by the French mathematician
Edouard Lucas, uses a cycle of exponential retention periods instead of a large
number of tapes.
Lucas,
who is well-known for his study of the Fibonacci sequence and his work
with prime numbers, loved recreational mathematics. His Tower of Hanoi
puzzle, which is still marketed as a toy for children, has a platform with
three poles. There is a stack of disks or rings on the first pole. The stack
looks like a pyramid; each disk going down the pole is a little larger than the
one above it. To solve Lucas' puzzle, the player must move all the discs from
the first pole to the third pole in the fewest possible moves. There are two
rules: only one disk can be moved at a time and a larger disc cannot be placed
on top of a smaller one.
There
are several ways to solve the puzzle, but one of the easiest ways is to start
with only one ring -- and then figure out how to solve it with two rings -- and
then figure out how to solve it with three rings. Once you have solved the
puzzle using small numbers, patterns will begin to emerge. This is known as a
recursive solution. (See recursion.) A recursive pattern uses the
information gained from one step to figure out the next step.

Number
of disks
|
Fewest
number of moves
|
1
|
1
|
2
|
3
|
3
|
7
|
4
|
15
|
5
|
31
|
Like
the puzzle, Towers of Hanoi backup uses a recursive pattern for scheduling
tapes. It enables an administrator to restore data from backups that are one,
two, four, eight and sixteen days old, yet only use five tapes. This strategy,
which requires backup software to support the complex rotation schedule, is
good for small businesses that need to be able to do full restores.
Comments
Post a Comment