The backup database, or index, keeps track of which files were backed up to which volume. Since the backup system can’t restore anything without this index, it becomes the single most important database in your environment. It is also the single point of failure in any backup system. As mentioned earlier, even if a volume is made with a format that is readable by a native utility, you still need the index to know what’s on it. The backup index is the greatest invention since someone created volume labels, but if it goes bad, you are out of luck.
Backup indexes usually are located on the central backup server, but they can be spread out to what is sometimes called "slave servers.” (A slave server would be one that is allowed to have backup devices.) One of the first questions you might ask is, “How big will this thing get?” The typical answer is .5 percent to 1 percent of the amount of data that is being backed up. That answer is very misleading, completely wrong, and totally irrelevant. The total size of the data that is being backed up has absolutely nothing to do with the database size. Let me state that again.
The total size of the data that is being backed up has absolutely nothing to do with the size of the backup database. It is the number of files being backed up, not their size, that determines the size of the database.
Each file that is backed up becomes a record in the index. That record will be the same size, regardless of how big the ...