PCI Lanes advice required

cruisinbeatz · 20 Oct 2022 at 20:37

Hey folks. I came in the hope someone can point me in the right direction as I can’t grasp the whole pci lanes things with my pc.

I’m short my setup is:
Asus Hero x570 Wi-Fi
5950X
64GB Ram
2TB Firecuda Pci 4.0 (Top M.2 slot)
2TB Generic Pci 3.0 (Bottom M.2 slot)
3080 in Top x16 slot

From my understanding I have 24 lanes and 4 of which are for the chipset yes? So I should in theory have 20 lanes remaining. If my gpu is taking 16 of those that leaves me 4. But then what does the M.2 drives use? The remaining 4 or the 4 on the chipset that’s shared with the USBs etc or what am I missing here?

Long story short I have an Asus M.2 x16 Card that I was thinking of running with either 2 or more nvme drives in it, in my bottom slot as it’s 4x (I think from the manual) will this be possible without affecting the rest of the system or what’s my best way forward as I’m lost.

Any help will be greatly appreciated as I’m sure this is simple to those who have the slightest idea on this.

MissChief · 20 Oct 2022 at 20:46

I believe both NVME slots and the PCI-E 16x are directly connected to the CPU. The other slots are connected to the chipset so comparatively slower.

Your PCI-E card will work but slightly slower compared to the CPU connected lanes.

cruisinbeatz · 20 Oct 2022 at 21:29

MissChief said:
I believe both NVME slots and the PCI-E 16x are directly connected to the CPU. The other slots are connected to the chipset so comparatively slower.

Your PCI-E card will work but slightly slower compared to the CPU connected lanes.

Ah I see, I don’t mind slower. It’s just when nvme drives are much the same price as sata it seems silly buying sata but perhaps I’m wrong. 4x is still supposed to be able to carry 8gbps though isn’t it? Or there abouts anyhow so the drives while being slower than direct connection should still run faster than sata?

Tetras · 20 Oct 2022 at 22:23

cruisinbeatz said:
Ah I see, I don’t mind slower. It’s just when nvme drives are much the same price as sata it seems silly buying sata but perhaps I’m wrong. 4x is still supposed to be able to carry 8gbps though isn’t it? Or there abouts anyhow so the drives while being slower than direct connection should still run faster than sata?

Looks like in the manual the third slot is PCI-E 4.0 with 4 lanes, but my understanding is that multiple slot M.2 expansion cards usually need the BIOS to support bifurcation if you want to divide the lanes (would be listed under PCI-E config or something like that, where you can select different modes). I suspect your BIOS will not support dividing the lanes in the third slot, so you may find only one M.2 slot on an expansion card will operate when plugged into the 4 lane full-size PCI-E slot. Bifurcation on the second PCI-E slot is more likely, but I think that would still limit you to 2 M.2 slots (8 lanes divided into 2) and you'd lose 8 lanes on the primary PCI-E as a result.

Does the Asus M.2 card have a manual?

cruisinbeatz · 20 Oct 2022 at 23:06

Tetras said:
Looks like in the manual the third slot is PCI-E 4.0 with 4 lanes, but my understanding is that multiple slot M.2 expansion cards usually need the BIOS to support bifurcation if you want to divide the lanes (would be listed under PCI-E config or something like that, where you can select different modes). I suspect your BIOS will not support dividing the lanes in the third slot, so you may find only one M.2 slot on an expansion card will operate when plugged into the 4 lane full-size PCI-E slot. Bifurcation on the second PCI-E slot is more likely, but I think that would still limit you to 2 M.2 slots (8 lanes divided into 2) and you'd lose 8 lanes on the primary PCI-E as a result.

Does the Asus M.2 card have a manual?

Oh that sounds complicated and not worth the hassle.

HYPER M.2 X16 CARD V2 | M.2 Expansion Card | ASUS UK

Power-up your RAID. Hyper M.2 x 16 card V2 is compatible with Intel® Virtual RAID on CPU (VROC) and the AMD Ryzen™ Threadripper™ platform for NVMe RAID support.

www.asus.com

That’s the product page of it. It’s discontinued now however but it did work before a few years back but I can’t test it currently as I’m waiting for my cpu replacement from amd.

Tetras · 21 Oct 2022 at 03:08

cruisinbeatz said:
That’s the product page of it. It’s discontinued now however but it did work before a few years back but I can’t test it currently as I’m waiting for my cpu replacement from amd.

The manual suggests that it is intended for xeon and threadripper boards and it doesn't say much about what happens in consumer boards with limited lanes.

It only states support for PCI-E slots with 8 or 16 lanes available, but there's no mention of how many M.2 slots will function with 8 lanes, perhaps it depends on the bifurcation support of the motherboard.

Seems like it would be trial and error to get it to work, unless you can find some reviews or user feedback somewhere.

EsaT · 22 Oct 2022 at 11:03

MissChief said:
I believe both NVME slots and the PCI-E 16x are directly connected to the CPU.

Wrong, only topmost M.2 slot is connected to CPU in AMD desktop platform boards.
(Threadripper could differ)
And as usual for any desktop platforms, PCIeE x16_1 and x16_2 share 16 lanes for x16 / - and x8 /x8 configs.

MissChief · 22 Oct 2022 at 13:00

EsaT said:
Wrong, only topmost M.2 slot is connected to CPU in AMD desktop platform boards.
(Threadripper could differ)
And as usual for any desktop platforms, PCIeE x16_1 and x16_2 share 16 lanes for x16 / - and x8 /x8 configs.

I thought the benefit of going X570 over B550 was that both NVME slots were CPU lanes? On B550 only the first M.2 one is?

Dg834man · 22 Oct 2022 at 13:53

MissChief said:
I thought the benefit of going X570 over B550 was that both NVME slots were CPU lanes? On B550 only the first M.2 one is?

570 can have multiple nvme gen 4 only the top one via cpu the rest are via chipset, 550 has one nvme gen 4 via cpu and generally one nvme gen 3 via chipset.

MissChief · 22 Oct 2022 at 14:01

Dg834man said:
570 can have multiple nvme gen 4 only the top one via cpu the rest are via chipset, 550 has one nvme gen 4 via cpu and generally one nvme gen 3 via chipset.

So both slots support PCI-E 4.0 but only the first slot is a CPU connected one, the second is connected to the chipset. Ok thanks. I stand corrected!

Jamin280672 · 24 Oct 2022 at 01:30

MissChief said:
So both slots support PCI-E 4.0 but only the first slot is a CPU connected one, the second is connected to the chipset. Ok thanks. I stand corrected!

X570 chipset or south bridge, whatever you want to call it is basically the same IO die that is in your CPU, the 1st and 2nd GPU slot is generally connected to the CPU which takes up 16 lanes (split tto x8x8 if you are using both slots), and the first m2 slot which uses 4 lanes is also connected to the CPU, then you've got your 4 downlink to the chipset, thats all the CPU lanes gone.
The chipset then provides you with......I think another 20 PCI-e 4 lanes (correct me if im wrong) all the other m.2 slots are connected to that, if you have 4 m2 slots in total, 3 of them are connected to the chipset (thats 12 lanes), the bottom one normally shares its lanes with the bottom x16 slot (x4 wired) so its one or the other, 4 are used for uplink back to the CPU, the rest of the lanes are used for things like extra SATA ports as the chipset supports 4 SATA ports anyway and PCI-e x1 slots, so if you've got 2 PCI-e x1 slots then thats only 2 lanes, things like LAN and Audio have there own controllers on the motherboard, USB is integrated into the CPU and chipset.
Different motherboard manufacturers use these lanes differently, so the assignment of lanes could be different between MSI, Gigabyte, ASUS etc.

AMD Ryzen 7 5800X3D review (Page 3)

It's time for another ZEN3 review, this time something extra special for gamers. It's the much-discussed Ryzen 7 5800X3D. The CPU, which is on many people's radar, features eight cores and sixteen ...

www.guru3d.com

And this bring us onto why im now not at all interested in X670E or alike, the chipset is made by ASMedia and not AMD. the chipset only gives out PCI-e 4 (no PCI-e 5), the only thing providing PCI-e 5 lanes is the CPU so thats the 1st and 2nd GPU slot and the first m2 slot, everything else is PCI-e 4, ive already got board wide PCI-e 4 on my X570S, I did see one motherboard manufacturer who was giving you 8x on the GPU slot and then 3 PCI-e 5 m2 slots, but personally when the 5800X3D is keeping up with most of the Ryzen 7000 stuff, I think a lot of the extra performance is probably coming from a slight speed increase and the fact its on DDR5 instead of 4 as AMD chips are very memory dependent.

BongoHunter · 26 Oct 2022 at 15:27

The Hyper M2 cards rely on the motherboard supporting PCIE bifurcation - I have one in my Threadripper system

On the mothboard you have it looks like bifurcation is only supported on the second PCIE16 slot, which only has 8 lanes, so you would only be able to fit 2 drives to that Hyper M2 card. I think it would also be limited to Gen3 speeds.

I think doing this would also reduce the bandwidth to the first PCIE slot on your board to x8 (Gen4).