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I've seen some suggestions about not allowing free disk space to go critically low or

... to have no less than about 15% of your drive's capacity free ...

Currently, I've been silencing the "disk full" notification for a day now, but haven't observed any performance degradation.

"The risks involved with low disk space" is the theme of the question. More specific questions are:

  • What processes will see a problem in low storage space (except spotlight, assuming I don't use it.)?

  • Is there an official backing for the claim?

MacBook Air 2017, Mojave.

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    MBP won't startup, disk is full Source of the quote. – anki Oct 2 '19 at 15:12
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    "The problem is that the system goes into a fail-safe mode when the directory structure cannot be presumed to be safe for a new write." Is in the most helpful answer . So that I wouldn't have to write another question about why does this happen, I'd love if this thread contains its explanation too. – anki Oct 4 '19 at 11:46
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There are a number of issues you could have with low disk space, but I wouldn't consider them high-risk. On a SuperUser post, the accepted answer cites a lot of information that is very old. The 10%-15% free space adage may even be out of date with APFS and modern SSDs. There is an older post on AskDifferent regarding the 10% free space limit with answer mentioning certain system process impacted by low disk space.

Typically, these are the main factors I've heard mentioned concerning having not enough free space:

  1. macOS uses virtual memory. When more space is needed in RAM, macOS will swap some data from RAM to the hard disk. This is explained in an older Apple developer guide. If RAM is full and the disk is full, applications can crash.
  2. Data that normally stays in purgeable storage, like caches and local Time Machine backups, will be unavailable for use. If you use the Store in iCloud feature, you could see many files unavailable locally that you would have to download again.

Anecdotally, I've run into these issues:

  1. Large projects in Photoshop or Premiere couldn't be saved because there's not enough free space. This is risky since a crash could lead to losing data, even if I'm diligent at saving.
  2. System updates unable to be installed due to lack of space.
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    Regarding virtual memory: Actually applications won't run slower in this case - they'll simply crash. – jksoegaard Oct 3 '19 at 23:44
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    @jksoegaard I think they'd crash not only because virtual is full(not available), but rather that RAM is also full. Full HD and empty RAM won't have applications crashing. As for drivec, lovely dog! And: I won't eagerly consider 4th and Half of 3rd in performance degradation, but I framed my bullet question in a way that it would attract those observations. I have gone through both of them actually, but reserved file structure related degradations for the question. – anki Oct 4 '19 at 4:51
  • @ankii Your sentence is hard to understand. What I wrote is that if RAM is full and disk is full, then applications start crashing because of out of memory errors. They won’t slow down as written in the answer here. Obviously if RAM is not full, then nothing bad happens. – jksoegaard Oct 4 '19 at 7:54
  • @jksoegaard despite the difficulty, you got it right! Clarified. – anki Oct 4 '19 at 8:09
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    @ankii I was right from the start, yes. The answer here is still wrong though - but I guess I’ll just edit it myself then. Was just giving drivec a chance to correct it. – jksoegaard Oct 4 '19 at 8:23
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I think the main risk is that you inadvertently completely fill your disk drive. There have been several questions on this site from people who filled their disk drives and then ran into problems trying to delete files. You can search on Ask Different for something like "disk drive full" and you'll see several questions about problems encountered with a full drive. Two previous questions are here and here.

I'd be very aware of free disk space and not fill your drive. It may cause problems.

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    "The problem is that the system goes into a fail-safe mode when the directory structure cannot be presumed to be safe for a new write." Is in the most helpful answer and I hope is the summary in almost all of them. So that I wouldn't have to write another question about why does this happen, I'd love if this thread contains its explanation too. – anki Oct 4 '19 at 4:49
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    Thanks for your input. I've seen too many questions about trouble from a full disk but I didn't know the details. – Natsfan Oct 4 '19 at 4:54

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