What operations breaks the hardlinks?

Question

I have a file "a.txt" and its hardlink "ha.txt". If I open either of those files in notepad and save any changes it would appear in both files as they are still linked. But if I overwrite (copy) a.txt with another file with same name the hardlink breaks. What other operations do this?

Also, can I use it to backup a folder? Since if I ever change (overwrite) "a.txt" the hardlink automatically breaks and I have 2 copies. But because it depends on what I use to overwrite it (notepad certainly won't work) it ultimately depends on what operations in windows "breaks" the hardlinks and make copies?

Answer 1

Anything that deletes and recreates the target file will break the link. Some applications will do this, some won't. I don't think there's any way to predict this in advance.

Hard links are not a sensible backup mechanism, since there is only ever one copy of the file. Backups should always be to a different physical disk, anyway, in case the entire disk fails.

Answer 2

Anything that rewrites the filename will break the link. Modifying the file contents will not, nor will changing the filename metadata.