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Flash memory and SSD controllers are reported to spend more time running wear-leveling and/or anti-fragmentation garbage collection algorithms when their storage is near full.

People have reported that clearing some free space on their near full SSD storage, and leaving their iMac or MacBook running overnight (to do something like wear-leveling or storage block garbage collection) increased storage speed/bandwidth performance significantly. Similar things are reported to occur involving UI lag on iOS devices that have almost completely full flash storage.

So, is there a recommended percentage of free space to leave on the SSD on a Mac, or the flash storage on an iOS device, to avoid having these algorithms kick in and possibly affect storage speed or read/write performance.

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I read that things slow down when it is less than 20% free on a SSD or HDD on a Mac. I noticed my read/write speed seemed to improve when I freed up some space to get near the 20%. I don't know about IOS devices but 20% seems reasonable there as well.

  • I've heard this too, but I've also heard that there is no difference. I think the best thing would be (if possible) to try using your devices at 70%, 80%, and 99% and see if you notice a difference. Really what matters in the end is if YOU notice the difference. – NoahL Dec 27 '17 at 21:14
  • absolutely. If you don't notice the difference there really isn't any. – jmh Dec 27 '17 at 22:42
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macOS will tell you how much space it thinks it needs on your startup disk partition. Run

diskutil resizevolume disk0s2 limits |\
perl -0777 -ne '@r=m/\b(\d+\.\d) GB/g;print sprintf("%.1f GB\n",@r[2]-@r[1]);'

where disk0s2 is the identifier of your startup disk. This returns how much space macOS requires on your disk. Testing on a couple of Macs myself, I get 4.3 GB, 5.4 GB and 10.7 GB.

The perl command calculates the difference between the following values output by diskutil limits: ‘Minimum size (constrained by file usage)’ and ‘Recommended minimum size (if used for OS)’.

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