macOS Catalina is the first version of macOS which boots not from a single volume, but from two: a read-only System volume named by default Macintosh HD, and a writeable Data volume with the default name of Macintosh HD - Data. If you upgrade to macOS 10.15 Catalina, your boot volume will effectively be split into those two. When your Mac then starts up in Catalina, you won’t see the Data volume, as it’s hidden inside the System volume, in what is an APFS Volume Group.
Although new to macOS, this scheme is already in use in iOS, and specifies the read-only system volume as having the role APFS_VOL_ROLE_SYSTEM, and the writeable user volume has the role APFS_VOL_ROLE_DATA. In that, the volume with the System role is normally mounted at the root /, and that containing both user and mutable system data is then mounted in /System/Volumes and accessed from there using several firmlinks.
Volume layout
A bootable Catalina APFS container normally contains at least five APFS volumes:
- the read-only system volume, by default on the internal disk named Macintosh HD, which forms the root of the boot file system,
- the writable data volume, by default on the internal disk named Macintosh HD – Data, which is normally hidden from view at /System/Volumes and accessed via firmlinks,
- Preboot, a small volume of around 40-50 MB,
- Recovery, the Recovery Volume, of around 500 MB,
- VM, containing virtual memory caches, which is upwards of 20 KB depending on use.
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