I've seen recommendations for the minimum amount of free space one should have on an SSD that range from 20%—33%. For instance, this answer (Can MacBook Pro get slower after SSD is almost full?) cites a 2012 Anandtech study (https://www.anandtech.com/show/6489/playing-with-op) that concluded:
"Whatever drive you end up buying, plan on using only about 75% of its capacity if you want a good balance between performance consistency and capacity."
The Anantech study was, however, pre-APFS. I understand that APFS is optimized to work with SSDs, so my question is whether that optimization changes (reduces) the minimum free space needed to obtain, using Anandtech's words, "a good balance between performance consistency and capacity."
Further, it's also possible that APFS's different design might mean that the minimum free space needed to obtain a "good balance" should be expressed in absolute, rather than relative (percentage) terms.
I currently have a late-2014 MBP with a 1 TB 4-lane PCIe SSD, with 200 GB free out of 940 GB available on my main partition (I have a 2nd partition with 60 GB available).
I understand APFS can reduce the total storage space needed to store a given set of files (see https://appleinsider.com/articles/17/06/08/inside-macos-1013-high-sierra-apfs-benefits-end-users-with-space-speed), but that's a different attribute from the one I'm asking about. Please also note that I know I can move little-used files to other disks (obviously), but that's not my question. I'm purely interested in learning if APFS affects the minimum free space needed for "a good balance between performance consistency and capacity" and, if so, why it does this.
Best Answer
If we assume that the Anandtech study's conclusions are correct, then no, there's no change in the needed free space when using APFS instead of HFS+. You would need no more and no less free space.
It is important to note that the "free space" that Anandtech talks about is not the free space inside the file file system (i.e. part of the filesystem structure that is not currently used for storing user data), but rather free space outside the file system (i.e. part of the disk that has never been written to).
As both HFS+ and APFS support the TRIM command, the essential here happens outside the file system, and thus the change from HFS+ to APFS does not affect the result.
However, you might still observe a performance difference when using APFS compared to HFS+. This is not caused by the free space issue itself, but rather that APFS and HFS+ simply have different performance characterics. Some things APFS does faster than HFS+, and vice versa.