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Tape Rotation



Tape Rotation

The Backup Patterns chapter gives an overview of different ways to store data on tapes. In case of the full/differential and full/incremental patterns, it is easy to see that you will run into problems with versions of the backup data.

Let's take a full/incremental pattern example. If in week 2, you will need to restore a file that was backed up in week 1, you will need to have these tapes still available. This means that the number of tapes needed increases significantly. That is why rotation schedules  are a very important part of tape management. Tape rotation schedules will provide you with different versions of files, without having a large number of tapes.

A commonly used tape rotation schedule is the 'grandfather-father-son' schedule. How does it work?

This schedule will make use of three sorts of tapes:


Let's start our backups:
  1.  On Sunday, a full backup is taken to a tape labeled 'Week_1'.
  2.  From Monday to Saturday, backups are taken to tapes labeled 'Monday',  'Tuesday', etc.
  3.   The next Sunday, a full backup is taken to a tape 'Week_2'.
  4.   On Monday, we reuse the tapes labeled with the names of the week  (the same tapes as used in week 1). These tapes are called the son tapes.
  5.   For the next two weeks, we take weekly full backups to separate  tapes, and store daily backups on the son tapes.


At the end of the month, this leaves us with four father tapes, labels 'Week_1', 'Week_2', 'Week_3', 'Week_4'.

This gives us the possibility to restore a version of a file that is one month in age. The last day of the month, a backup is taken to a grandfather tape, labeled 'Month_1'. After this, the 'Week_1' through 'Week_4' tapes can be reused to do the weekly full backup.

So, you will have a set of 5 son tapes reused weekly, a set of 4 father tapes reused monthly, and a set of 4 or 12 grandfather tapes (depending on the amount of time you want to cover).
 
Figure 3. Grandfather-Father-Son Media Rotation Schedule

Another possibility, which typically works with applications using the incremental pattern (see Incremental Pattern), is one in which tape rotation is not necessary.

In the policy of the backup environment, you will define how many or how long (depending on the application) you want to keep the versions of files. The tape management subsystem of the application will then only reuse tapes when they no longer contain needed data.

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