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Unix Backup and Recovery by W. Curtis Preston

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Deciding When to Back Up

This might appear to be the most straightforward topic. Everybody backs up their system every night, right? What’s the big deal? Actually, this could more aptly be titled “What levels do I run when?” It’s always a big question. How often do you run a full backup? How often do you run incremental backups? Do you run various levels of incrementals that back up just today’s changes or continuous incremental backups that back up everything since the last full backup ? Everyone has his own answers to these questions. The only thing that is a definite is that there should be at least some level of incremental backup every night. Before any further discussion on the topic, let’s define some terms.

Backup Levels

The following are various backup levels:

Level 0

A full backup.

Level 1

An incremental backup that backs up everything that has changed since the last level backup.

Levels 2-9

Each level backs up whatever has changed since the last backup of the next lowest level, e.g., a level 2 backs up everything that changed since a level 1, or since a level 0, if there is no level 1.

Incremental

Usually, a backup that behaves like levels 1-9. Also used by some products to mean the same as a level 1, backing up all changes since a level 0.

Differential

A type of “incremental” backup, in the generic sense, which backs up only what has changed since the last differential. This term is usually found in software that does not use the numbered level concept. Such software would ...

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