MacOS Catalina Boot Volume Layout

Howard Oakley, writing at The Eclectic Light Company:

When you upgrade to macOS 10.15 Catalina, your boot volume will effectively be split into two. Assuming it’s the standard internal storage, your existing boot volume will be renamed to Macintosh HD — Data, and a new read-only system volume created and given the name Macintosh HD. However, when your Mac starts up in Catalina, you won’t see the Data volume, as it’s hidden inside the System volume, in what Apple refers to as a 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.

Nice explanation of a complex change in 10.15 Catalina.

For the most part, in the Mac UI (like the Finder), it all just works. You open /Applications and you’ll see all your applications. But when you poke around in Terminal you have to know what’s going on or it won’t make sense. ls in /Applications will show only the contents of the writeable Applications folder; ls in /System/Applications will show you only the system applications on the read-only boot volume.

★ Friday, 22 November 2019