Saturday, August 9, 2025

Does Time Machine on APFS disk use a lot of space for renamed files?

I stumbled upon Does Time Machine Copy Renamed Files?, posted about 10 years ago, and I wondered what the answer would be for the APFS implementation of Time Machine.

The question was: if a file already tracked by Time Machine (HFS+) is renamed, does Time Machine just create a link with the new name to the old file or does it create a new copy, as if the file was completely different? The answer provided there, If I understand it correctly, is that the latter is true and that the same applies to any small change to a file.

My question: Does renaming a (large) file cause a correspondingly large increase in disk usage on the backup disk? Is moving to a different folder any different than renaming in this regard? Also, what about making a small change to a large file? (And why?)

I believe APFS backups work in a completely different way from HFS, with the hard link-based system replaced by something based on APFS snapshots (right?). Also, I think I read somewhere that APFS has the ability to “duplicate” a file without actually creating a full copy, and store only the differences as one of the duplicates is modified. So if I duplicate a 10 GB file and partially modify one of the copies, the actual disk space used would be less than 20 GB (again, is this correct?). So, my guess is that neither renaming/moving nor editing a file causes a fully new copy of that file to be saved on the backup disk, but I’m not sure at all that I understand and even at a basic level how APFS and Time Machine work.


P.S.: I’m asking this purely out of curiosity, I obviously don’t intend to decide whether to rename or edit files based on the disk space taken on my backup drive. Also, I know that Time Machine can delete old backups to free up space, but for this question please assume that the backup disk has plenty of free space left.

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