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I have started an incremental back up, on an external hard drive, of my Fedora 20 data etc using the scheme from here, which produces a full back up on day 1 followed by incremental backups using hard links on following days, so that only changes are recorded. I intend to keep 28 days backups, by deleting each as it becomes 28 days old.

What happens when the original full backup up becomes the next to delete? It seems to to be possible that then there is nothing left to link back to. Surely I have not appreciated fully what happens, but this is not explained in the excellent site quoted, and my understanding of it is not sufficient for me to predict what will happen.

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The backup system you link to uses hard links. Hard links are special. Files are really just pointers to inodes. With hard links it is possible to have multiple identical "files" all pointing to the same inode. This means that all of your backups are essentially full backups, but because of the hard linking, the additional space taken up by each new backup is small. You can delete any backup you want without worrying about losing anything other than what is in that backup. This is very different from "delta" based backups where the incremental backup only contains the information needed to change to last full backup, after all the previous delta have been applied, into the current state. If you lose any piece of information along the way, your backup is essentially worthless.

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