Types Of Backups - Red Hat ENTERPRISE LINUX 3 - INTRODUCTION TO SYSTEM ADMINISTRATION Administration Manual

Chapter 8. Planning for Disaster
As you can see, there is no clear-cut method for deciding on a backup system. The only guidance that
can be offered is to ask you to consider these points:
Changing backup software is difficult; once implemented, you will be using the backup software
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for a long time. After all, you will have long-term archive backups that you must be able to read.
Changing backup software means you must either keep the original software around (to access
the archive backups), or you must convert your archive backups to be compatible with the new
software.
Depending on the backup software, the effort involved in converting archive backups may be as
straightforward (though time-consuming) as running the backups through an already-existing con-
version program, or it may require reverse-engineering the backup format and writing custom soft-
ware to perform the task.
The software must be 100% reliable — it must back up what it is supposed to, when it is supposed
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to.
When the time comes to restore any data — whether a single file or an entire file system — the
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backup software must be 100% reliable.

8.2.3. Types of Backups

If you were to ask a person that was not familiar with computer backups, most would think that a
backup was just an identical copy of all the data on the computer. In other words, if a backup was
created Tuesday evening, and nothing changed on the computer all day Wednesday, the backup created
Wednesday evening would be identical to the one created on Tuesday.
While it is possible to configure backups in this way, it is likely that you would not. To understand
more about this, we must first understand the different types of backups that can be created. They are:
Full backups
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Incremental backups
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Differential backups
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8.2.3.1. Full Backups
The type of backup that was discussed at the beginning of this section is known as a full backup. A
full backup is a backup where every single file is written to the backup media. As noted above, if the
data being backed up never changes, every full backup being created will be the same.
That similarity is due to the fact that a full backup does not check to see if a file has changed since the
last backup; it blindly writes everything to the backup media whether it has been modified or not.
This is the reason why full backups are not done all the time — every file is written to the backup
media. This means that a great deal of backup media is used even if nothing has changed. Backing up
100 gigabytes of data each night when maybe 10 megabytes worth of data has changed is not a sound
approach; that is why incremental backups were created.
8.2.3.2. Incremental Backups
Unlike full backups, incremental backups first look to see whether a file's modification time is more
recent than its last backup time. If it is not, the file has not been modified since the last backup and can
be skipped this time. On the other hand, if the modification date is more recent than the last backup
date, the file has been modified and should be backed up.
Incremental backups are used in conjunction with a regularly-occurring full backup (for example, a
weekly full backup, with daily incrementals).
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