I had been nervous that if and when Apple finally got around to making a new file system, they would follow the trend and neither open source it nor document the on-disk format. HFS+ itself has always been open by virtue of being included in the open-sourced core xnu kernel, but Apple's CoreStorage volume manager (the basis for FileVault and Fusion Drive) and DMG disk image format have always lived in closed-source kernel extensions and are not officially documented; Microsoft has acted similarly with its NTFS, ReFS, and exFAT file systems and Storage Spaces volume manager.

The result, of course, is that nothing can access anything else's file system, at least well. Most of the formats I mentioned have been reverse engineered to some extent, but there are limits, especially when you're scared of corrupting data by misinterpreting the format and therefore limit your implementation to read access. If you want read-write access to NTFS from Linux or macOS, a filesystem which is old enough to drink, you have to rely on either a relatively slow FUSE implementation or some proprietary software. If you dual boot Linux on your Mac and want access to the Mac partition, make sure to turn off disk encryption (and hope you don't run into any evil maids) unless you want to use a read-only FUSE tool, and I hope you don't have a Fusion Drive because I don't think anything supports that at all (could be wrong).

To be fair, this situation cannot be blamed entirely on lack of openness. Lack of interest matters too; as I said, HFS+ has an open source implementation, yet Linux's hfsplus driver doesn't support journaling, a feature added in 2002. But if anyone tries to improve the situation, that implementation makes their job a lot easier: for one thing, since filesystem support is in the BSD-based portion of the xnu kernel, it may be possible to port the HFS implementation to other BSDs with relative ease - like the NetBSD rump kernel, which can run in userspace, from which FUSE support wouldn't be that hard... Alternately, if they chose the path of enhancing the native Linux driver, at least they could consult Apple's code to be sure they weren't missing any obscure corners of the format that could cause their driver to corrupt data.

Anyway, what we got with Apple File System is this:

> An open source implementation is not available at this time. Apple plans to document and publish the APFS volume format when Apple File System is released in 2017.

That's pretty good! Open source plus documentation would be ideal, but documentation is much better than nothing[1], and the text of that paragraph doesn't exactly rule out the possibility of the final release being open source too. (I haven't installed the beta yet, but am I right in guessing that APFS is implemented in a kext?)

While there's no guarantee, I hope that that documentation will also come with information about the Core Storage format APFS will usually be wrapped in, so that proper interoperability can be achieved. Rather than my fear coming to pass of one of the last vestiges of openness in proprietary OS storage formats disappearing, the situation may actually be improved compared to today. Maybe in a few years I'll be able to install Ubuntu on my Mac and have full access to the OS X partition out of the box. I can hope...

[1] whether it's better or worse than open source alone is debatable - source can be harder to understand, but it's also never wrong, unlike documentation...