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.
Tom’s Hardware usually does longer write tests with iometer to analyze cache size and performance however I think it may only be a 15 minute run. Here’s their review of the 2tb samsung 990
However it’s becoming less of an issue with modern ssd’s that incorporate hybrid caches and faster NAND. As long as the drive has a large enough static cache to absorb most of what you throw at it or fast enough NAND to at least be bearably fast it’s not that big of a deal. You just have to know the NAND’s limitations. Even drives like the crucial p3 are fine as long as you understand eventually writes will slow down to 80MB/s or so.
For secondary storage it’s less important but I would still try to leave some free space on OS drive for fast swap and temp file performance