
Tape Rotation Tape backup is wonderful when it comes to cheap storage. Furthermore… Wait! There is nothing else good to say about tape backup. It is fraught with magnetic bits falling off the plastic, stretching, heads going out of alignment, accidental erasure, and the list goes on. The challenge then becomes how to preserve the maximum amount of information for the longest time possible, with the minimum number of tapes. Two popular backup strategies are:
Towers of Hanoi^{421} are based on a puzzle where 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: ABACABADABACABAEABACABADABACABAF ABACABADABACABAEABACABADABACABAG ABACABADABACABAEABACABADABACABAF ABACABADABACABAEABACABADABACABAH In the case of tape backup, label the tapes AH and use the same order to rotate tapes through a tape drive, each day putting a complete backup on the tape. The Towers of Hanoi backup strategy is often used because it creates the longest possible data retention period (the length of time archived data is kept around) with the minimum number of tapes. For example, using the Towers of Hanoi backup strategy with a set of 10 tapes allows for retrieval of data up to 1023 days old. In the GrandfatherFatherSon method, you label tapes by the day of the week with a different tape for each Friday in the month and a different tape for each month of the year. Using a tape for Saturday and Sunday is optional depending on whether you have files updated over the weekend. This would look like the following: MONDAY TUESDAY WEDNESDAY THURSDAY FRIDAY1 MONDAY TUESDAY WEDNESDAY THURSDAY FRIDAY2 MONDAY TUESDAY WEDNESDAY THURSDAY FRIDAY3 MONDAY TUESDAY WEDNESDAY THURSDAY MONTH1 MONDAY TUESDAY WEDNESDAY THURSDAY FRIDAYI MONDAY TUESDAY WEDNESDAY THURSDAY FRIDAY2 MONDAY TUESDAY WEDNESDAY THURSDAY FRIDAY3 MONDAY TUESDAY WEDNESDAY THURSDAY MONTH2 MONDAY TUESDAY WEDNESDAY THURSDAY FRIDAY1 MONDAY TUESDAY WEDNESDAY THURSDAY FRIDAY2 MONDAY TUESDAY WEDNESDAY THURSDAY FRIDAY3 MONDAY TUESDAY WEDNESDAY THURSDAY FRIDAY4 MONDAY TUESDAY WEDNESDAY THURSDAY MONTH1 Because, some months have more than four weeks, it will take over 20 tapes for regular backups. Now that you have a backup, the question becomes where to put it? That is tackled next. __________________ 421. http://www.studyworksonline.com/cda/content/applet/0,,NAV315_SAP48,00.shtml
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