Storage media has a cost, and the time available to perform a backup is finite. To minimize both factors, administrators have come up with strategies for limiting the scope of backups as much as possible. There are two common strategies: backing up only files that have changed since the last backup, and backing up only dynamic data files.
The first strategy is also called incremental backups. Assume that you have a directory with four files on day one (11/1):
File1 Last modified 10/1 size 1GB File2 Last modified 10/1 size 10KB File3 Last modified 10/4 size 1MB File4 Last modified 9/22 size 2MB
On the first day, you complete a full backup, which backs up all the files on the system. In this example, it backs up all four files. The next day (11/2), you could complete a level one backup, which would back up all the files that had been modified since the last backup at the previous backup level. In this example, that would be the full backup completed on 11/1. A listing of the files at backup time on 11/2 shows:
File1 Last modified 10/1 size 1GB File2 Last modified 11/2 size 10KB File3 Last modified 10/4 size 1MB File4 Last modified 11/2 size 2MB
This means that for the level one backup, only two files (File2 and File4) are backed up. This approach saves the time and space of backing up File1 and File3. It is not necessary because they haven't changed since the previous backup.
The downside to this method is that restoring data involves using as many tapes as you have backup levels. If you had to restore the directory from this example, you would need to recover the last full backup and the last level one backup to return to the most recently backed-up files.
You can also use the modified Towers of Hanoi method from http://en.wikipedia.org/wiki/Towers_of_Hanoi:
One recommended scheme is documented in the dump man page:
After a level 0, dumps of active file systems are taken on a daily basis, using a modified Tower of Hanoi algorithm, with this sequence of dump levels: 3 2 5 4 7 6 9 8 9 9... For the daily dumps, it should be possible to use a fixed number of tapes for each day, used on a weekly basis. Each week, a level 1 dump is taken, and the daily Hanoi sequence repeats beginning with 3. For weekly dumps, another fixed set of tapes per dumped file system is used, also on a cyclical basis.
The second method is to do a full system backup and then to only back up directories that contain constantly changing data (such as application directories). This method also limits the time and size of backups, but it has two problems. The first is that even seemingly static directories have changes from time to time. It would be easy to miss backing up a change to a configuration file if it were not in a common directory. The second issue is that if you don't back up system logs, it would be possible to have them rotate off the system, leaving you without historic data to which to refer.