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.
No, that’s 630GB of TLC space filled. Writing in SLC mode requires 3x the space. The dynamic SLC caching stops at some point because the controller still needs space left to rewrite 630GB down to 210GB during idle time plus a safety margin so the user won’t run into a situation where the drive has to “freeze” to catch up with the work. There is always some minimal amount of SLC cache available through overprovisioning, but that’s typically only a few GB.