On 11/3/18 4:35 am, Gordon Messmer wrote:
On Fri, Mar 9, 2018 at 2:57 PM, Stephen Morris samorris@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.
I can't find the documentation any more, but I have found documentation on how to use copy the create the target as a hard link or as a soft link. It is possible I have incorrectly remembered what I had read, or it is possible over time that the standard copy functionality has changed and now you have to explicitly specify that you want that functionality.
regards,
Steve
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