APFS Clones
Also known as: apfs clone file, copy on write apple, apfs clones
APFS clones are copy-on-write file copies in Apple File System: cloning a file creates a new directory entry that shares the original's data blocks, so the copy uses almost no extra space until one side is modified.
- Created via clonefile()/FileManager copy on the same APFS volume; the copy shares the original's data blocks.
- Copy-on-write means new blocks are allocated only when one copy is modified.
- Cloned files share storage, so summed file sizes can overstate actual disk usage.
How clones work
APFS is the default file system on iOS, iPadOS, and modern macOS. When you duplicate a file on the same volume -- for example with Finder's Duplicate, or programmatically via FileManager.copyItem which uses the clonefile() syscall -- APFS does not copy the data. It creates a second file record that points at the same underlying data blocks, marking them as shared.
This is copy-on-write: the two files appear independent, but they back onto identical storage. Only when you write to one of them does APFS allocate fresh blocks for the changed portions, leaving the other file untouched. Up to that point the clone is nearly free in terms of disk usage.
Why space numbers look strange
Because cloned files share blocks, the sum of individual file sizes can exceed the actual space consumed on disk. A folder of duplicated videos might report many gigabytes while occupying far less, which is why per-file size and "space you would recover by deleting" are not the same thing.
This matters for cleanup tools. A cleaner like Cleanor focuses on true duplicate files -- byte-identical content stored as genuinely separate copies -- rather than cheap clones, so the space it reports as recoverable reflects real freed blocks, not shared ones.