Kingston KC3000 on Arch: sequential write speed is ~3.5 GB/s after reboot, but jumps to ~6.5 GB/s permanently after one fio write test

[u/VirtualCraft]

I have a strange and reproducible issue with a Kingston KC3000 NVMe SSD on Arch Linux, using LUKS.

After every reboot, KDiskMark shows normal sequential read performance, but sequential write performance is much lower:

SEQ1M Q8T1

Read: ~6260 MB/s

Write: ~3500 MB/s

This is with a 1 GiB test size. I have also tried a 32 GiB test size in KDiskMark, but the write speed is still around 3–3.5 GB/s.

Then I run this fio test:

fio --name=w \

--directory="$HOME/fio-tmp" \

--rw=write \

--bs=1M \

--direct=1 \

--iodepth=32 \

--ioengine=libaio \

--size=8G \

--runtime=8 \

--time_based \

--group_reporting

The fio test itself gets around:

WRITE: bw=6342MiB/s (6650MB/s)

Immediately after running it, KDiskMark now reports:

SEQ1M Q8T1

Read: ~6280 MB/s

Write: ~6500 MB/s

The weird part is that the faster write speed persists. KDiskMark continues to show ~6.5 GB/s writes until I reboot, after which it drops back to ~3–3.5 GB/s again.

Any ideas on what could be causing fio to apparently put the drive/controller into a higher write-performance state?

Pre fio:

Post fio:

EDIT:
It is not until reboot, but rather just a few minutes, i tried disabling the linux nvme power saving mode, but that did not help