pci-e PCI Express Lane Allocation Issues

[Gordon_4]

I've recently been researching adding another SSD to my system and I think I'm about to run head long into an issue. To whit, the idea is to use a PCIe expansion card to add a WD_Black SN770 as my gaming drive, or a WD_Black AN1500 addin card for the same.  Both will be the 2TB capacity.

 

However, I'm currently wondering if my current setup has not already bifurcated my PCI Express lanes to the point that adding either of those to replace the SATA SSD that I currently use won't work correctly,

 

Some data:

 

CPU: AMD Ryzen 9 3900XT

Mobo: ASUS ROG STRIX B550-E Gaming

GPU: ASUS ROG STRIX GTX1080

M.2_1: WD_Black SN750 - 500GB

M.2_2: WD_Black SN750 - 500GB

SATA SSD: Samsung 860 PRO

 

The manual for the motherboard indicates - although not clearly enough I'd swear to it in a court of law - that the top most slot, PCIe 4.0 x16_1, is meant to be PCI-E with full Generation 4, 16 lane support.  Likewise M.2_1 is meant to be Gen4 x4 for the drive.  Currently my research says that I should be able to get the full x16 for the GPU and (in my case) up to x4 speed for the NVMe drive at PCIe Gen4.  Now according to WE Dashboard, both NVMe drives are Gen3x4 and they are currently running at that speed so no problems there.  However the first issue is that according to GPUz my GTX1080 is only running at PCIe Gen3 x8 speed.  Now I accept with my luck of late that its probably been that way for ages and I'm hardly at the cutting edge of gaming so I can live with it.

 

However, my question is that if I install either the expansion card (acknowledging that I won't get Gen4 performance out of the SN770 but the difference between NVME and SATA is near unnoticeable) at Gen3 x4 OR the AN1500 which is Gen3 x8, am I going to cut my GPU's lanes in half, again?  Or am I better off just leaving the configuration as is?

 

Also if someone could advise me what pulled the GPU down to Gen3 x8 instead of x16, I'd likewise be most grateful.

 

[Needfuldoer]

If all you need is bulk storage, and you haven't filled all your SATA ports yet, an additional SATA SSD will be fine.

 

Here's what your motherboard's manual has to say about slots sharing bandwidth:

 

 

(Page 6 of the PDF)

 

(Page 10 of the PDF)

 

So if I'm reading this right:

 

M.2_1 gets 4 dedicated lanes no matter what.

 

If you populate PCIEX16_1 and PCIEX16_2, they'll each get 8 lanes. PCIEX16_1 gets all 16 lanes if PCIEX16_2 is empty.

 

If you put anything into either PCIEX1_1 or PCIEX1_2, PCIEX16_3 will only run at 2 lanes instead of 4 (which is electrically a 4x slot at most). These all hang off the chipset's dedicated 4 lanes.

 

If you populate M.2_2, you lose SATA ports 5 and 6.

 

 

Do you have anything in PCIEX16_2?

 

I think if you install a riser into PCIEX16_2, you could add two M.2 SSDs that would each get their own 4 lanes, but don't quote me on that.

[Gordon_4]

7 hours ago, Needfuldoer said:

If all you need is bulk storage, and you haven't filled all your SATA ports yet, an additional SATA SSD will be fine.

 

Here's what your motherboard's manual has to say about slots sharing bandwidth:

 

 

(Page 6 of the PDF)

 

(Page 10 of the PDF)

 

So if I'm reading this right:

 

M.2_1 gets 4 dedicated lanes no matter what.

 

If you populate PCIEX16_1 and PCIEX16_2, they'll each get 8 lanes. PCIEX16_1 gets all 16 lanes if PCIEX16_2 is empty.

 

If you put anything into either PCIEX1_1 or PCIEX1_2, PCIEX16_3 will only run at 2 lanes instead of 4 (which is electrically a 4x slot at most). These all hang off the chipset's dedicated 4 lanes.

 

If you populate M.2_2, you lose SATA ports 5 and 6.

 

 

Do you have anything in PCIEX16_2?

 

I think if you install a riser into PCIEX16_2, you could add two M.2 SSDs that would each get their own 4 lanes, but don't quote me on that.

First of all thank you for taking the time and interest to assist me with this conundrum.

 

The reason for the addition is not bulk storage, but to move the old Samsung drive into my wife's laptop because it has NO bulk storage at all.  Semi related note, I've just re-seated my GPU's power cables and now GPU-Z is reporting its running at 16 speed so yay there.

 

As for the question RE: additional PCIe devices, there are none but since I've solved the primary bifurcation issue by just re-seating the power, if I stick the SN770 on the PCIe card into slot PCIEX 6_3, I should at least get that to run at full PCIe Gen3 x4 which is fast enough for my minuscule needs without impacting the GPU.