Question on X570 PCIe x16 slot bandwidth

[Zinn]

I'm looking at a few X570 boards, for example this one: https://www.newegg.com/msi-mpg-x570-gaming-pro-carbon-wifi/p/N82E16813144260

I'm wondering if I can use the second PCIe x16 slot to run a PCIe 3.0 x4 fiber networking interface card without limiting the PCIe 4.0 bandwidth of the top slot, in other words my setup might look like:

• Top PCIe x16 slot: AMD 6900XT
• Bottom PCIe x16 slot: x4 fiber NIC card

Ideally this would run at 16x / 4x, but will the X570 chipset limit this to 8x / 4x? Or does this depend on the mobo?

 

[faugusztin]

First rule i would recommend is to never take shop listings as face value, and take time to look at product page and/or manual. In your example, you would go to https://www.msi.com/Motherboard/MPG-X570-GAMING-PRO-CARBON-WIFI, go to Specification and then Detail.
And then you would see this in Expansion Slots section:

• 1x PCIe 4.0/3.0 x16 slot (PCI_E1)
  • 3rd Gen AMD Ryzen™ support PCIe 4.0 x16 mode
  • 2nd Gen AMD Ryzen™ support PCIe 3.0x16 mode
  • Ryzen™ 4000 G-Series support PCIe 3.0x16/x0/x0, x8/x0/x8, x8/x4/x4 mode
  • Ryzen™ with Radeon™ Vega Graphics and 2nd Gen AMD Ryzen™ with Radeon™ Graphics support PCIe 3.0 x8 mode
• 1x PCIe 4.0/3.0 x16 slot (PCI_E3), supports x4 mode
• 2x PCIe 3.0 x1 slots

That means this board doesn't split the CPU lanes to x8/x8 mode, has PCI-E 4.0 x16 main slot electrically and the second slot is PCI-E 4.0 x4 electrically, x16 physically. So yes, in your use case it would work just fine.

Any board with x16/x0 & x8/x8 split usually also has a third x16@x4 slot.

 

[lopoetve]

I would avoid that board; tends to have bad VRMs - jump up to the Unify / Ace / Tomahawk on the MSI side.

That being said - yes, what you want to do (third slot) should be fine. That third slot tends to take it's x4 link from the chipset instead of the CPU, but you won't really notice for storage traffic (I do that all the time on various systems). For that matter, benchmarks show that x8 is fine for video cards to (1-2 fps), so doing the second slot (which tends to set the first to x8) won't matter either.