I always hear people saying you need to leave ~20% of the space on your SSD free otherwise you'll suffer major slowdowns. No way I'm buying a 4TB drive and then leaving 800GB free on it, that is ridiculous.
Now obviously I know it's true. I have a Samsung 850 Evo right now that's 87% full, and with a quick CrystalDiskMark test I can see some of the write speeds dropped to about a third of what they are in reviews.
I'm sure that the amount of performance loss varies between drives, which to me would be a big part in deciding what I'd rather buy. AnandTech used to test empty and full drives as part of their testing suite (here, for example), but they don't have any reviews for the more interesting drives that came out in the last couple of years, like 990 Pro, SN850X, or KC3000.
Is there anyone else doing these kinds of benchmarks, for an empty and filled drive? It would be a lot better knowing just how bad filling a drive is instead of throwing 20% of it away (some suggest to keep 50% full at most) as some kind of rule of thumb.
The slowdown is write speed, if you read from the drive mostly, don't worry about it much.
I feel this gets stated but I do wonder how true it is as opposed to people basically just assuming due to lack of testing.
Testing reads in this manner and especially accounting for SSD background activity is much more involving and time consuming than the typical write to full tests which are done. So are just assuming writes are affected just because the latter is the common test?
See counter data on reads being affected -
https://www.anandtech.com/show/13512/the-crucial-p1-1tb-ssd-review/6
https://www.anandtech.com/show/13078/the-intel-ssd-660p-ssd-review-qlc-nand-arrives/5
https://www.anandtech.com/show/2738/14