CPU and PCI-e Lanes, I am a bit confused and need some help.

[enigma27]

Hi All.

Im getting confused when it comes to PCI-e lanes and Ryzen so need a little help.

Basically I am putting a machine together that will run server software (Unraid) and using it to host 2 Windows 10 Virtual machines for the kids to use.

The machine is based on a Ryzen 1700 CPU which I believe has 20 PCI-e lanes in total.

I have been looking at motherboards with both dual PCI 16 slots and 2x Nvme slots

I need to run dual GPU and 2 NVME drives and I understand with most motherboards either x370 or x470 will knock the 16x slots down to 8x which is fine but that only leaves 4 pci-e lanes left for the NVME drives.. will adding two NVME's even work? or like the graphics cards will they be limited to PCI-e 2x?

I could just use a couple of sata drives but already have the NVME's spare.

any help appreciated.

 

[Marak]

You're on the right track chap and X370/470 should do what you need it to.

The Ryzen 7 1700 CPU has 24x PCI-E lanes:

16x for Graphics (which can be split into 8x/8x as you say)
4x for a dedicated M.2 NVME slot
4x to the X370/470 chipset

The chipset then uses those 4x lanes to provide a variety of connections that can be determined by the motherboard maker. SATA ports, USB M.2 etc. This is where you can plug in your 2nd NVME drive.

The top M.2 slot in motherboards is almost always the 4x from the CPU. Any further M.2 slots are normally coming from the chipset, but be aware of a couple of things:

Some motherboards may split off the GPU lanes to provide more M.2 slots, this will mean if you populate the 2nd or 3rd M.2 slots it can disable the 2nd GPU slot.

Some M.2 lanes from the chipset might only be a SATA M.2 slot and not NVME capable. Even if it is NVME it will probably be PCI-E 2 rather than PCI-E 3, and might even just be 2x lanes rather than 4x.

I would have linked a motherboard but OCUK doesn't have a suitable one listed.

 

[Marak]

This motherboard from ASRock will do what you want:

https://www.asrock.com/mb/AMD/X370 Taichi/index.us.asp#Specification

The second M.2 is PCIE gen 2 but that's all you can get with X370/470.

 

[enigma27]

Thank you sir that has been really helpful.

I am guessing though if for instance I wanted to throw into the mix adding a couple of PCI 1x USB 3 cards into other PCI slots as well as both GPU's and both NVME's that would cause issue? I think i can get around this by using the USB headers.

 

[enigma27]

snip

 

[Marak]

Potentially, it depends on how the motherboard manufacturer has laid out the IO devices from the chipset. For example the motherboard I linked his this limitation:

"If PCIE5 slot is occupied, M2_2 will be disabled"

However that relates to just one of the chipset PCIE lanes, which is the bigger x16 slot at the very bottom. The other small 1x slots have no limitation listed.

Generally the PCIE 1x slots are ok in my experience though so you are probably good with them.

 

[enigma27]

perfect thank you.