On Fri, Mar 9, 2018 at 2:57 PM, Stephen Morris <samor...@netspace.net.au> wrote:
> No, what I was mentioning here is what I have read as standard linux
> functionality with copying, when a file is copied, and it doesn't matter
> where to, rather than create a 2nd copy of the file, the "copy" is created
> as a hard link to the original file, for storage efficiency, and then when
> one of the files is updated the hardlink is broken and both files become
> physical.

The only process I can think of that works even remotely like that is
rsnapshot (and similar backup systems).  In those setups, rsync will
create new files for the first backup.  On the second backup, it
creates hard links to all of the files in the set, first, and then
runs rsync to refresh the directory holding the newest backup.  When
rsync runs, it does not modify files in place, it creates a new file
and merges data from the local file and data from the remote file in
order to save bandwidth, according to its specialized algorithm.  When
it's done, it moves the new file into place, atomically updating the
path in the backup.

As you describe, rsnsapshot uses hard links to conserve disk space and
breaks links when it updates files.  My guess is that at some point in
the past, you read about rsnapshot or a similar backup system, and
later remembered their mode of operation as standard functionality.
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