Why mdadm RAID does **NOT** boost reading performance?

I add 2 NVMe SSDs(Samsung 980 Pro) into my PC as tablespace of postgresql, while the OS boots from another SATA SSD(Samsung 860 EVO) as before.

These 2 nvme m.2 SSDs have a basic performace about reading 6GB/s and writing 600MB/s, that I got in Gnome-Disks.

I made a soft RAID from these two nvme by means of mdadm, and then tested again.

The writing rate reaches 1.3GB/s as expected, but the reading rate still is 6GB/s.

I'm wondering why the reading performance does NOT change?

Thanks!

CPU:AMD-5900X

Motherboard: Asus Pro WS X570-ACE

Chipset: AMD-X570

OS:Kubuntu-20.04.2

6GB/s is a common max rate for disk controllers.

Very likely, the speed bottleneck is not the disks but the disk controller, so if both disks are on the same controller, you won't get faster than 6GB/s.

Looking at your motherboard specs, it has 4x SATA 6Gb/s ports, 2x 8GB/s M.2 slot. The specs on the nvme disk are max 7GB/s according to Samsung's web page.

It appears that the first M.2 slot should get 4 lanes, which would support ~8GB/s. The second M.2 slot only gets 2 lanes, and one of those is shared with a PCI slot, so it only gets both lanes if the PCI slot is empty.