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Full, Selective and Incremental Backups

There are several different ways that we can select files for backup. The way that you will want to use depends on how you use your system, how often your files change, and your chosen backup method. Many people will in fact end up using a combination of these different techniques:

• Full Backup: Nothing complicated here, a full backup is done by selecting all the files on the hard disk for backup. Only special files that should not be backed up at all are left out. This is the simplest type of backup, and yields the most complete backup image, but it takes the most time and media space to do.
• Selective Backup: In a selective (or partial) backup, you select specific files and directories to back up. This type of backup gives you more control over what is backed up, at the expense of leaving part of the hard disk unprotected (unless of course, you simply do another selective backup later on that covers the area you skipped the first time). Selective backups make sense when some files are changing much more rapidly than others, or when backup space is limited, although in many cases doing an incremental backup is better and easier.
• Incremental Backup: If you perform frequent backups, as you should, you may find yourself backing up the same files over and over, even ones that do not change over time. Instead, you may want to consider a mix of full backups and incremental backups. An incremental backup is one where only the files that have changed since the last backup are selected. It is like a selective backup, but the files are selected based on whether they have changed recently or not, instead of an arbitrary selection based on directory or file names. This gives the time- and space-saving advantages of a selective backup while also ensuring that all changed files are covered.

Incremental backups are supported by most decent backup software. They work using the archive bit that exists for each file and directory. The backup software looks at this bit to determine what files have been changed since the last backup, selects them for backup, and then clears the bit for all the files it backs up. If any files are changed, the software sets the bit again so on the next incremental they are again selected, and so on. You must rely on this bit being managed properly, and I don't always like to do this.

Warning: Programs that change files are not required to set the archive bit. Most well-behaved software will do this, but you cannot absolutely guarantee that all files changed since the last backup will be caught by an incremental selection.

Which type of backup you do depends, again, on what is important to you, in terms of time, media cost, and also ease of restoration. Restoring a system that uses incremental backups can require more steps, as first the full backup has to be restored and then the incrementals, one after the other. You also don't have the redundancy (just in case you need it) that you have when your backup scheme uses only full backups. If you can fit a full backup onto a single media set and it doesn't take too long to do, I still prefer this over incrementals, for this reason. It's just more secure to know that everything is on one tape. The scheduling of backups is discussed here.

Next: Backing Up Programs and Data

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