How OmniDiskSweeper Reports Free Space
The “purgeable” space is space that the operating system knows how to reclaim when you try to create files that need that space. But it’s not truly cleared up from the disk yet and still shows up in OmniDiskSweeper’s summary list. But even though it shows up in the summary, it won’t show up when you browse the disk looking for files to delete—so OmniDiskSweeper will end up reporting different numbers for space used based on how it scans your disk.
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These snapshots can take nearly zero space at the start (because their contents are exactly the same as the current files on disk), but as files get edited or removed the snapshots start to take up more and more space. In particular, when you delete huge files (because you’re trying to clear up space), they will disappear from your filesystem but will still exist in your snapshots until those are removed. This is where I usually find the bulk of the “purgeable” space reported in Disk Utility.
Also, OmniDiskSweeper doesn’t tell you about APFS cloned files. (I’m not sure how it reasonably could.) So, although it will tell you how much space a given file is using, deleting that file may only increase the free space by a fraction of that amount.
Previously: Dive Into APFS.
Update (2019-04-03): Tyler Loch:
macOS: “Whoa, buddy. You can’t do that. The drive is full!”
Me: “What? Finder says there’s still 50GB available!”
macOS: “Yeah, but that’s actually purgeable data.”
Me: “Ok, so purge it.”
macOS: “No.”
