[_rend]

> Which means if you actually edited those files, you might fill up your HD much more quickly than you expected.

I'm not sure if this is what you intended, but just to be sure: writing changes to a cloned file doesn't immediately duplicate the entire file again in order to write those changes — they're actually written out-of-line, and the identical blocks are only stored once. From [the docs](^1) posted in a sibling comment:

> Modifications to the data are written elsewhere, and both files continue to share the unmodified blocks. You can use this behavior, for example, to reduce storage space required for document revisions and copies. The figure below shows a file named “My file” and its copy “My file copy” that have two blocks in common and one block that varies between them. On file systems like HFS Plus, they’d each need three on-disk blocks, but on an Apple File System volume, the two common blocks are shared.

[^1]: https://developer.apple.com/documentation/foundation/file_sy...

[jonhohle]

The key is “unmodified” and how APFS knows or doesn’t know whether they are modified. How many apps write on block boundaries or even mutate just in disk data that has changed vs overwriting or replacing atomically? For most applications there is no benefit and a significant risk of corruption.

So APFS supports it, but there is no way to control what an app is going to do, and after it’s done it, no way to know what APFS has done.

[mort96]

For apps which write a new file and replace atomically, the CoW mechanism doesn't come into play at all. The new file is a new file.

I don't understand what makes you think there's a significant risk of corruption. Are you talking about the risk of something modifying a file while the dedupe is happening? Or do you think there's risk associated with just having deduplicated files on disk?

[jonhohle]

The vast majority of apps using structured data and not block oriented data formats. A major exception is databases, but common file formats that most people work with - images, text, etc. often aren't best mutated directly on disk, but rewritten either to the same file or a new file. Without some transactional capability, mutating a file directly on disk can corrupt a file if the writer fails in the middle of the write. More than a few text editors use this as their method of saving to ensure that there is never an inconsistent state of that file on disk.

[bsimpson]

Thanks for the clarification!