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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:
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.