Copy-on-write on APFS

APFS (like many modern file systems but unlike its predecessor HFS+) supports copy-on-write. This means you can logically copy a file - it looks and behaves like a distinct file - but it doesn't immediately copy the file's contents on disk - it merely shares them with the original. Only if and as you modify either version do they start to diverge on disk, with APFS dynamically allocating new storage for the modified parts1.

This is kind of a sister function to hard links, which similarly [...]

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