Question Motherboard's 2nd M.2 slot has much slower 4k random write performance

[Dave3000]

CrystalDiskMark Random 4K Q1T1 write I get:

2TB Samsung 980 Pro in 1st M.2 slot: 251 MB/s
2TB Samsung 980 Pro in 2nd M.2 slot: 150 MB/s

My motherboard is an MSI Tomahawk X670E with two 2TB Samsung 980 Pros installed. Now the question I have is, why is the 2nd M.2 slot in my motherboard much slower at the 4K Q1T1 performance in CrystalDiskMark even though they both are operating at PCI-E 4.0x4 mode? I can understand if it would be about 5-10% slower for that category because the chipset lanes have higher latency than the CPU lanes but 40% slower, I don't get it. All other categories in CrystalDiskMark are within 5% difference in performance between both these M.2 slots. I have installed the latest chipset drivers downloaded from MSI for my specific motherboard which are newer than the ones directly from AMD.

 

[Dave3000]

AMD just released an updated chipset driver today for motherboard and a few minutes ago I installed those ones. I'm still getting the 150 MB/s in the 4K random Q1T1 write in the 980 Pro that's installed in the 2nd M.2 slot. So it's not a driver issue or at least trying a newer driver didn't speed up that category for the 2nd M.2 slot.

 

[Hail The Brain Slug]

It's probably just chipset vs cpu lanes causing it. There's some latency going across the chipset, and it's not uncommon for the m.2 to be off the second chipset, so it has to make 2 hops each way to the CPU. It shouldn't affect sequentials much, but you might see it limited to ~6000MB/s as well.

Real world performance shouldn't be super badly affected by this.

 

[Dave3000]

So is it better for me to have my games installed on the 980 Pro in the 1st M.2 slot (PCI-E 4.0x4 CPU lanes) in a dedicated games partition than having them installed in the 980 Pro in the 2nd M.2 slot (PCI-E 4.0x4 Chipset lanes) and use the 980 Pro in the 2nd M.2 slot for data storage? So would this be optimal in my case:

1st 2TB Samsung 980 Pro in 1st M.2 slot: C partition for Windows and D partition for Games
2nd 2TB Samsung 980 Pro in 2nd M.2 slot: E partition for User lIbraries such as Downloads, Documents, Pictures, and Music

or would this be better:

1st 2TB Samsung 980 Pro in 1st M.2 slot: Only Windows and basic apps installed here on C partition
2nd 2TB Samsung 980 Pro in 2nd M.2 slot: Games, Downloads, Documents, Pictures, and Music on D partition

 

[Hail The Brain Slug]

So is it better for me to have my games installed on the 980 Pro in the 1st M.2 slot (PCI-E 4.0x4 CPU lanes) in a dedicated games partition than having them installed in the 980 Pro in the 2nd M.2 slot (PCI-E 4.0x4 Chipset lanes) and use the 980 Pro in the 2nd M.2 slot for data storage? So would this be optimal in my case:

1st 2TB Samsung 980 Pro in 1st M.2 slot: C partition for Windows and D partition for Games
2nd 2TB Samsung 980 Pro in 2nd M.2 slot: E partition for User lIbraries such as Downloads, Documents, Pictures, and Music

or would this be better:

1st 2TB Samsung 980 Pro in 1st M.2 slot: Only Windows and basic apps installed here on C partition
2nd 2TB Samsung 980 Pro in 2nd M.2 slot: Games, Downloads, Documents, Pictures, and Music on D partition

Windows and any applications you use regularly should be on the CPU drive. Games are absolutely fine to be on the chipset drive - it won't meaningfully affect load times because games tend to be sequential reads where the bottleneck tends to be decompression.

 

[SteinFG]

Try other slots, there are 4 on the motherboard, all support at min Gen4 x4

 

[CP5670]

I have this board too and the last M.2 slot drops to half speed if the last PCIe slot has anything in it, but otherwise they are supposed to work at full speed. I keep the OS drive (990 pro) in the top slot, but doubt you would notice any difference in games.

 

[Dave3000]

Try other slots, there are 4 on the motherboard, all support at min Gen4 x4

The problem with that is that I would have to remove my graphics card which is huge and heavy as that drive is installed on the 2nd M.2 slot which is located under the graphics card's heatsink. Also I had a hard time installing the M.2 heat shield onto my 2nd 980 Pro SSD that is installed in the 2nd M.2 slot because the heat shield for that M.2 slot shares the same screw hole as a 2280 sized NVMe drive that screws into the M.2 standoff and it's the same way with the 3rd M.2 slot and I don't want to go through that again if I can avoid it.

 

[BoomerD]

First slot is gen 5.0 from the CPU.

Remaining m.2 slots are gen 4.0 from the chipset. Those will always be slower than slot 1.