Tower Of Hanoi
Tower of Hanoi is based on a mathematical puzzle. A
series of rings or disks are stacked in size order, the largest on the
bottom, on one of three poles. The object is to move all of the rings to the
third pole. But you can move only one ring at a time, and you can't place a
larger ring on top of a smaller ring. The secret is to shift the first ring
every other move (moves 1, 3, 5, 7, 9, 11...), the second ring at intervals
of four moves (moves 2, 6, 10...), the third ring at intervals of eight
moves (moves 4, 12...), and so on.
The Tower of Hanoi rotation scheme lets you keep
several current copies of data, several week-old copies, and a few month- or
year-old copies.
Each tape set is used a different number of times. When a new tape set
is added, it is slated to be reused every other rotation. Older tape sets
are used every fourth rotation, every eighth rotation, and so on. You can
perform a tape-set rotation daily or weekly. For example, if you have five
weekly tape sets labeled A, B, C, D, and E, your tape rotation would look
like this: A B A C A B A D A B A C A B A E (each letter represents a week of
backups).
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Round Robin
Round Robin uses a single tape set for each day of the workweek. This
ensures that you will never lose more than a day's worth of data, but it
keeps only a week's worth of your information.
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Grandfather, Father, Son (GFS)
GFS is the most common tape rotation method. The
number of tape sets you use is based on the number of workdays that you add
data to your network. It works as follows:
Back up data on a different tape set every working
day. If your backup cycle is based on a five-day workweek, you will need
four daily tape sets (a fifth tape set comes into play later). You can
perform full, incremental, or selective backups during the week.
On the fifth day, you will use a weekly tape set. You
will need three weekly tape sets.
In the fourth week, you will need a monthly tape set.
Since there are 13 four-week cycles in a year, you will need 13 "monthly"
tape sets.
The GFS method is easy to use if you remember to label
your tapes. Also, since the daily tapes are used more frequently than the
weekly and monthly tapes, you will need to replace them more often.
Six-Cartridge Weekly Backup Principle
A simpler and more cost-effective implementation of
Grandfather-Father-Son is called the Six Cartridge Weekly Backup. Perfect
for small businesses, this backup principle requires daily backups and a
single weekly off-site backup copy to provide a data history of up to two
weeks. Friday backups are full backups. Monday through Thursday backups are
incremental. Here are the steps:
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Label each of six cartridges with FRI 1, FRI 2, MON, TUE, WED,
THU.
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Start the cycle on a Friday and backup the entire hard disk onto
cartridge FRI 1.
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On Monday, take the MON cartridge and back up only the files that have
been created or modified since the last backup (FRI 1). This is an
incremental back up and should be stored on-site. (A full backup,
rather than incremental, can be used, if desired).
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Repeat Step 3 on Tuesday, Wednesday, and Thursday, using corresponding
data cartridges.
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On Friday, take data cartridge FRI 2, and perform a
full backup. You have just completed a full rotation of the weekly
principle. Again, be sure to store this data cartridge off-site.
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The weekly process continues by repeating Step 3 and Step 4 using the
same MON, TUE, WED, THU data cartridges. Step 5 is
implemented by alternating cartridges FRI 1 and
FRI 2.
An illustration of the Six-Cartridge Weekly Backup
Principle is located below.
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