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DuoBolt

APFS clones

Duplicate a file in Finder on an APFS volume and macOS does not copy the data. It creates a clone: a second file that points at the same blocks, and starts diverging only when one of them is written to.

Both files are real. Both report their full size. Together they occupy the space of one.

That is a problem for every duplicate finder, because the obvious arithmetic — five copies of 4 GiB, delete four, free 16 GiB — is wrong. Delete four clones and you free nothing at all.

What DuoBolt does about it

Section titled “What DuoBolt does about it”

With Detect APFS clones on — the default on macOS — DuoBolt asks the filesystem for each file’s clone identifier while it walks. Files that share storage share that identifier.

It then counts physical copies rather than files, and computes recoverable space from those:

Removing all but one copy leaves one physical copy behind, so what you get back is (physical copies − 1) × size.

Where you see it

Section titled “Where you see it”

A group made entirely of clones reads 0 B recoverable, even though each file has a real size.

The selection footer shows two numbers whenever they disagree: how much you have selected, and how much is reclaimable. Both are true; the second is what the disk will actually give back.

Selection maths follows the same rule — a clone cluster frees bytes only when every file in it is selected. Leave one behind and the blocks stay allocated.

The limits, stated plainly

Section titled “The limits, stated plainly”
  • macOS and APFS only. HFS+ does not have clones. Neither does NTFS, nor any SMB or NFS share, nor an external drive formatted ExFAT.
  • Where DuoBolt cannot tell, it assumes the worst case for its own numbers. A file whose clone identifier is unavailable is counted as its own physical copy — DuoBolt cannot prove it shares storage with anything, so it does not claim it does. On non-APFS volumes this makes reclaimable space an upper estimate.
  • Symlinks are not resolved for this. A symlink’s clone identifier describes the link, not its target, so DuoBolt does not use it.
  • Turning the option off is available and occasionally useful for comparison, but the reclaimable figure then goes back to being naive arithmetic.

Why it matters more than it sounds

Section titled “Why it matters more than it sounds”

Time Machine local snapshots, cp -c, Finder duplicates, several backup tools and most macOS installers all produce clones. On a working Mac a meaningful share of “duplicates” are clones — which is why a competing tool can promise you 80 GB and deliver eight.