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I'm having the same problem as the person who asked this question: I cleared a bunch of files out of my Trash and instead of getting free space on the volume, I got 30 gigs of "purgeable space". According to the accepted answer to that question, I can engage in an entirely reprehensible workaround to liberate this space from its ghostly, meaningless existence in purgatory—a strategy to which I may well be forced to resort.

None of the answers to that question, however, deal with the more fundamental issue of why this is happening in the first place. Ideally I'd like to avoid having to do this ever again. Is there some way to tell the filesystem not to mark files as "purgeable" rather than just removing them? I'm running High Sierra, and the system drive (the drive in question) is using APFS.

intuited
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  • Why do you need this behaviour? The linked QA amply demonstrates how this works & how to clear it if space is needed 'urgently'. – Tetsujin May 27 '19 at 18:07
  • @Tetsujin: I guess so I don't have to put whatever I'm in the middle of on hold to dd up a gigantic file and then delete it? I dunno, mostly it just seems super awkward and clunky to have the equivalent of an iPhone in storage space that I can't use doing undefined things that I have no direct control over. – intuited May 27 '19 at 18:13
  • @Tetsujin It's also inconvenient to have to bring up a Finder window or whatever in order to check how much disk space I actually have instead of just being able to trust the results of df -h. – intuited May 27 '19 at 18:20

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Based on this Apple support article, I gathered that the purgeable space might be taken up by local snapshots. I had already backed up since emptying the Trash, but I have more than one backup drive. I removed the backup disks that aren't accessible at the moment and some time later my system drive's purgeable space was shown to have been alchemically transmuted into free space.

Since Apple hasn't bothered to include information on how local snapshotting works when you have multiple backup drives, and since I never managed to discover a way to identify what is currently contained in purgeable space, I can't be entirely certain that this was what fixed the issue. It would be quite helpful if there were to be a post listing the various ways that purgeable space can come about and how to prevent this from happening. I haven't managed to find such a resource; in fact, the support article doesn't even include the word "purgeable"; perhaps comprehending such an arcane detail is considered to be beyond the mental capacity of Apple's target demographic, even for its support pages.

intuited
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    tbh, I sympathise - but this is a side effect of everybody switching from massive HDs to tiny SSDs. You can't really have it both ways. Personally I get round this by having 1TB of SSD & 12 TB of HD in my Mac - an option not really available to laptop users. – Tetsujin May 27 '19 at 18:21
  • Yeah, I have a similar setup in a 2012 MBP; you just replace the optical drive with a hard drive caddy. Max HDD size seems to be stuck at 2TB (7mm width), but it works well. 512GB is big enough of a system volume for me. Personally I don't understand how people can commit to being without a mostly arbitrary set of files if the wifi at the cafe is slow and/or you're somewhere with no mobile service. Though I think I would have been a lot less annoyed if any of this had been explained to me when I upgraded to HS. – intuited May 27 '19 at 19:11
  • Also note that the presumably common use case of only being able to keep so much data on a small storage drive is more obviously an aspect of purgeable data which has not been deleted. Deleted purgeable data, presumably, you'd want to be rid of even faster if you're maxing out at under 1TB. That's the focus of this question — purgeable data that, until purged, counts against your free space rather than purgeable data that could be subtracted from your space used. – intuited May 27 '19 at 23:04