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New mac user here (Linux background). I'm running macOS Catalina (Beta5). I've noticed there are 5 volumes in my main disk container. One is mounted at /, that I understand, it's the system, another is /System/Volumes/Data, which I tough it would be the Linux /home, but it's not, so what it it?

Also what are the other two unmounted volumes? One might be the recovery one, but the other? Thanks!

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Catalina introduces a new file system layout. Where Mojave and earlier had one filesystem that combines the system and user data, Catalina has a read-only system volume and a read-write user volume interleaved on a folder by folder basis using firmlinks.

The easy way to move forward is just save your additional files to /usr/local and other traditional places where Apple expects user modifications to their default system to be saved.

Some of the implementation is quite normal for Unix/Linux like sparse files not being allocated and copy on write and cloning of an entire file system / snapshots. Other items like Firmlinks that act as “wormholes” between two containers / filesystems to present an unified file tree, System Integrity Protection and APFS specific features are quite new still to everyone.

You can see this better with df or diskutil apfs list command line tools than the Disk Utility view.

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    Why are there copies of my applications in there? They don't seem to be simlinked, as best as I can tell. If I was to delete one, should it be the ones in /Applications?
    – jktravis
    Oct 11, 2019 at 22:36
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    @jktravis Applications is even weirder than any other folder. Most redirection use firmlinks (entire folders crossing the volume border), but Finder mixes them within one directory - a merged “virtual folder” of apps from two locations. This “virtual folder” doesn’t express as an actual folder to command-line tools but two distinct folders See the paragraph on Applications here eclecticlight.co/2019/10/11/… this will confuse all sorts of tools and scripts like find and du for quite some time I expect.
    – bmike ♦
    Oct 12, 2019 at 3:26
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    I wouldn't say that Firmlinks is a new and unknown concept for Linux users actually.. it's used very much the same way as you use overlayfs (or other similar features such as unionfs and aufs) on Linux. For example you can do the "blending" that you see in the Applications folder, where some files are from the read-only volume, and others are from the read-write volume.
    – jksoegaard
    Oct 13, 2019 at 17:08
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    It seems to me du cannot handle firmlinks correctly. When showing a grand total sudo du -d 1 -m -c / it seem du is counting storage in /Applications and /System/Volumes/Data/Applications twice.
    – asmaier
    Jan 28, 2021 at 10:31
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    @asmaier I always add the -x option so du doesn't cross mount boundaries.
    – pedz
    Jan 7, 2022 at 0:20

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