UNIX uses the concept of backup levels as a shorthand way of referring to how much data is backed up in relation to a previous backup. It works this way:
A level 0 backup is a full backup. The next backup level would be 1.
Backups at the other numbered levels will back up everything that has changed since the last backup at that level or a numerically higher level (the dump command, for example, offers 10 different backup levels). For example, a level 3 followed by a level 4 will generate an incremental backup from the full backup, whereas a level 4 followed by a level 3 will generate a differential backup between the two.
The following sections examine a few of the many strategies in use today. Many strategies are based on these sample schemes; one of them can serve as a foundation for the strategy you construct for your own system.
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