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PCIe bifurcation (lemy.lol)

submitted 2 months ago* (last edited 2 months ago) by [email protected] to c/[email protected]

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Hey y'all, I bought a x4x4x4x4 4 slots M.2 PCIe card foolishly thinking that if it fits in the slot, it would surely work. In the end, I got 2 of the 4 SSDs working on my old AM4 X470 chipset. I'm coming to you for advice what the cheapest way to get to use this would be. I've noticed that a lot of CPUs have a PCIe lane limitation of 28, just short of what I need (I'd like to run the SSDs but also a x16 GPU). I'm not too keen to buy a threadripper setup for this occasion...

Cheers!

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[–] [email protected] 8 points 2 months ago (2 children)

It’s all down to your motherboard. But the tldr is no, at least not without compromises.

The top x16 slot is almost always connected directly to the CPU. The others are limited to X8 or X4.

Your CPU has 20 lanes. 16 for the GPU, and 4 for your “main” NVME SSD. Your chipset typically has like 4 or 8 more (the very bottom slot). And you have to figure out some sort of combo where that works, and they’re not combineable. What’s worse is a lot of motherboards hard wire those extra x4 pcie lanes to the M.2 SSD slots so you get no choice.

[–] [email protected] 2 points 2 months ago (1 children)

ur “main” NVME SSD. Your chipset typically has like 4 or 8 more (the very bottom slot). And you have to figure out some sort of combo where that works, and they’re not combineable. What’s worse is a lot of motherboards hard wire those extra x4 pcie lanes to the M.2 SSD slots so you get no choice.

I've put the bifurcation card next to the CPU now and it only offers two x4 lanes. I thought zen2 should be able to handle bifurcation, so if it was directly hooked up with the CPU, I should get all four. Seems chipsets still have some influence over the first PCIe slot.

[–] [email protected] 2 points 2 months ago

What mobo do you have? That's the most important thing here. Your mobo will almost always be the limiting factor in bifrucation.

When I was looking around before I saw that Asus supports 4 way bifrucation. I got bored so I asked chatGPT and it thinks only two x470 mobos support 4 way bifrucation. ASUS ROG Crosshair VII Hero (Wi-Fi) and MSI X470 Gaming M7 AC. It also seems to think x570 has better support, but I think that's more just that they support two way vs no bifrucation at all.

4 way bifrucation isn't a very common thing for a regular user to do which is probably why so few boards support it. 4 way plus a full fat GPU is even less common. To be honest I'm kinda surprised any 400 series boards support bifrucation at all.