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[Solved] Purgeable Space?  

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Following on from my earlier question (regarding the time to move my photo files to a flash drive), I'm now satisfied that they're fully there, and being picked up by CCC in my backups.  So, I decided I could delete them from my SSD.

In the process, I also pointed Google Photos to my new flash drive, to make sure it was also picking the files up from it.

Prior to deleting the files from Trash, Finder said I had 99 GB free; now it says I have 51. ?? System Information tells me my "System" files are 296.41 GB, but Disk Inventory X tells me my System files are 10.6 GB (see screen shot)

Screen Shot 2018 12 06 at 1.36.19 PM

Daisy Disk says System files are 8.1 GB, but it also reveals purgeable space of 246.4 GB.  Is this the source of the discrepancy in reporting the size of my System files, and is it likely that this is part of the reason my overall free space declined rather than increasing when I deleted the photos from my SSD and emptied the Trash?

I remember a discussion some months ago on MGG regarding purgeable files, and whether or not they could be deleted safely, but have yet to dig that conversation out.  What is the consensus?  It sure would be good to free up almost half the capacity of my drive.

I also get confused about the way different products report space usage - recently the new category of "purgeable" and previously iPhoto & Photos libraries that use hard links to share files.

I don't have a full answer, but certainly for the screenshot you've included the designation of "System" files is different. What you have highlighted in Disk Inventory is simply the System folder - whereas the System Information app is using "System" as a category of files which may be spread around.

My first guess, and it's only a wild guess, is that much of that purgeable space is being used by APFS snapshots - where if you wanted to revert to s snapshot prior to your "deletion event" APFS effectively has to retain all those files (up until it needs the space for something else, at which time the snapshot and associated files are removed).

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Thanks for the thoughts, Graham.  Given that this huge block of data (which is growing, BTW) may be APFS snapshots, it raises the question: will these snapshots simply grow in number/size until one’s storage is exhausted, kind of like Time Machine backups?  Kind of puts my OCD in overdrive...

 
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I'm pretty certain that anything tagged as "purgeable" is automatically managed by macOS (and/or APFS) such that if space is needed for real files the purgeable stuff will be removed automatically as required.

I'm running some scans on my own boot drive at the moment - and one thing I can see is that the System Information coloured bar changes as background processing works through the categorisation of files - so I think that the "System" category is effectively the space that hasn't been, or isn't able to be, otherwise categorised.

 

As I say, I also remain confused by different numbers being reported by different tools!!

 
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There may be other tools (or Terminal commands) for this, but I know that Carbon Copy Cloner will show you a listing of APFS snapshots and their sizes. eg:

CCC snapshots

 
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Does any of your space get recovered if you restart your Mac? What about if you run a Time Machine backup?

 
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Ben - restarting, either in safe mode or routinely, didn't make any difference.  I don't use Time Machine, as I'm on a laptop and in the past, TM backups to network-attached drives gave me problems.

I ran Onyx's maintenance scripts, clearing caches, rebuilding the Spotlight index, etc.  No change.

Riffing on Graham's comments above, I looked in Carbon Copy Cloner.  I had four Snapshots; the three older ones showed a total of ~100 GB.  After sitting on this a couple days, I crossed my fingers and deleted those.  My Finder-reported free space immediately went from ~50 GB to ~295 GB; System Information showed my "System" files falling from >300 GB to 59 GB.

It's puzzling that deleting Snapshots reporting roughly 100 GB frees up over twice that on my drive, but I'll be watching these numbers going forward to see what happens when more Snapshots are taken.

All in all, an interesting exercise.

This post was modified 22 hours ago by bkmiller
 
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Got it. I think that if you were using TM, the snapshots would have cleared after running a backup. In your initial post, you showed that you had 50GB free, which probably isn't too little for the computer to run with ease.

A notable experiment might be to see if snapshots get deleted and/or created subsequently and how low your free space might go. Not that you actually need to figure this out.

 
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No, I agree that 50 GB is OK in reality, though I’m not sure about the old rule of thumb that said one should keep at least 10% of your HDD’s capacity free for overhead and such. I don’t know how that relates in the SSD world.  I’ve long had roughly 100 GB or so free, and kind of freaked out when I moved 178 GB off and ended up with only 50 GB left!

I’ll be watching my Snapshots and free space over the next few weeks to see what happens when I’m not moving large amounts of data around.

 
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I don't think SSDs are quite as sensitive but recommend keeping at least 5–8% free.

Indeed I think one of the reasons for leaving a percentage of free space on rotational drives was to allow breathing room for background "defrag" operations to happen. This isn't necessary on SSDs. And with the big rotational drives now I don't believe using percentage is appropriate - eg. leaving 150GB free on a 3TB drive? 

So my general rule, for machines with an SSD, is to keep 5-20GB free to allow for swap file and sleepimage file creation - dependent on how much memory you have installed. The more memory you have the larger a sleepimage file may be. On my own MBP with 8GB of memory I have set my threshold warning at 12GB. And I frequently have it operating with less than 20GB spare.

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