Can't get to correct sustained writes in benchmarks

Preamble: This was all tested on 3 different motherboards with 3 different CPUs. It’s not a fluke on my hardware setup other than the drive model or if it is, i’m very, very unlucky. Ryzen 5950x/5900x/5800x with 64GB of ram on different x570 motherboards and adequate cooling. I understand sustained writes can vary depending on block size and that sort of things but this is not what i’m seeing here.

So i have 4 2TB SN750. These drives have a 1400MB/s sustained write speed after the cache is all used up. That’s what the reviews say, and I can measure that in windows. In Ubuntu 20, if I benchmark with the gnome disk utility, it caps to around half that after the cache is used (before the cache is used, it works as expected, no cap, so not a bus bandwidth issue). So i investigated and I found a bunch of people that had the same issue on the web. One of them said he could get the right speed with Linux Mint 18 (pretty old). I first tried a live usb build of recent mint, and no dice. Same result. I went for his version and… Bingo, same results as Windows, straight line at 1400MB/s after the cache expired. This build is so old it comes with an old kernel and with yahoo as home page and might I’m pretty sure it won’t make the best out of my modern CPU.

Guys, have you actually benchmarked your NVME drives with the gnome disk tool past the cache to see if you get the sustained writes they should have? I’m getting 2 980 PROs on the mail tomorrow and i’m eager to see if they’ll behave the same.

Whether that’s makes an impact on plotting I’m not sure, but i would like to be able to explain it. I went ahead and tried like 5 modern distros and they all got up to 700MB/s like ubuntu. I tinkered with the mount settings, scheduler, etc. I couldn’t get modern distros to measure the same values as old ones or windows or the reviews. I’m now running 3 of these drives in the same computer and it weirds me out that the write speed seems to be capped at the same total, but the disks themselves wiggle a lot… Like you can obviously tell the 3 drives are never writing at 100% of their capacity at the same time. Can this be explained?

If you have nvme drives that you can benchmark and compare with the theoretical sustained writes from reviews it’d be interesting.

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I also have the same issue using gnome disk tool: Benchmarking indicates very low write. I also get very different results for two identical Samsung 980 Pros, one on a PCI-e x4 card, the other on a PCI-e x16. The results should not be different, both slots having much higher bandwidth than the drives require

Don’t use the built-in gnome disk benchmark… use FIO and run a sustained write test, you’ll see the correct speeds.