For the last 38 days, I have been measuring free and used space in my Mac’s boot container and its backup storage. Here’s what has happened, and how upgrading to macOS 27 changed it.
snapshot
Although they can only grow in size, and mustn’t be left lying around, snapshots can be very useful, provided you manage them carefully, as described here.
You’re running out of free space, so check in Storage settings to see where it has all gone. System Data has reached 50 GB, but what is that, and what should you do to reduce it?
How do you locate part of a document that you removed or changed greatly, and copy that content into the version you’re currently editing? What to do when it’s too much for Undo.
Caching to disk improves performance, but cached data are optional and may be purged when additional free space is needed. This explains how that’s performed by the deleted service.
Further tests on two volumes with over 90 GB purgeable space. One took almost 30 minuted to become available, the other never got there at all. The log reveals who was responsible.
Snapshots made by Time Machine and tmutil should be purgeable, although sometimes they won’t purge, and aren’t shown reliably as purgeable space. An exploration of bugs and missing documentation.
Demonstrated 10 years ago as a major new feature in APFS, and capable of blazingly fast rollback preserving data including document versions, snapshots still aren’t fully accessible or documented.
Can be a Mac-saver if you have problems when your backup storage isn’t available. You don’t need to run Time Machine to make them, and can roll back in Recovery, as shown here.
Should you attempt a repair in Recovery mode? Is it just a warning? How to identify the file or folder with a problem, and what to do to fix or prevent it from recurring.
