Yesterday I updated from Big Sur beta 1 to beta 2, which went smoothly except for the fact that the update doubled the size of my read-only system volume to over 27 GB, which didn’t leave me enough free space on the partitioned external disk to install (the enormous) Xcode. Without Xcode, it’s impossible to adequately beta test Big Sur as a developer. It turns out that Big Sur always boots from a snapshot (an APFS snapshot that you can see with
diskutil apfs listSnapshots
, not a Time Machine snapshot that you’d see withtmutil listlocalsnapshots
), and updating to beta 2 apparently added a new snapshot without deleting the old snapshot. I tried to delete the old snapshot, which was listed as “Purgeable” bydiskutil
, but Big Sur claimed I lacked permission, even with SIP disabled.
Not only does Big Sur use a snapshot of the current system for its Signed System Volume, but it makes and keeps one of your previous installation too. When your startup disk has half a terabyte free, that may escape notice, but for us lesser mortals, and anyone trying to manage Big Sur in less than 100 GB, it really gets in your way. I’m sure that those figures will improve by the time that Big Sur is ready for release, but that isn’t the point: once the system has made that snapshot of the previous installation, there seems no easy way to discover how much disk space the snapshot is occupying, and the only way to free up that space seems to be performing a clean install of macOS.
[…]
If you really want to know what’s going on with your snapshots and why free storage space is disappearing, the best tool at present is Carbon Copy Cloner.
[…]
We need Disk Utility to be able to list all snapshots stored on a volume, and their current effective size, together with a command to delete those which are purgeable. Is this really too much to hope for three years after Apple introduced APFS and snapshots?
Previously:
- Installing the macOS 11.0 Beta
- APFS and Time Machine in Big Sur
- APFS Snapshots and Large Files
- What You See in the Finder Should Always Be Correct
- Quantum Computing and APFS: Free and Used Space
Update (2020-07-30): Erik Gomez:
Worse is you actually can’t use time machine in recovery OS to revert to any of these snapshots. Every time I try, it fails.