Old Dual 10Gbe NIC in new PCIe 4.0 Slot

[mcginnie]

I have a X520-T2 dual 10Gbe NIC (uses 8xPCIe 2.0 lanes) which has a x8 footprint.
Will this work properly in a PCIe 4.0 x16 slot which states it "supports x4 mode"?
I think this means that the big slot (x16) has 4 PCIe4.0 lanes associated (x4) so that is equivalent to 16 x PCIe 2.0 lanes - more than enought, but will the mismatch in pins make a difference?

Many thanks for guidance!

Regards,
Paul

 

[Orcvader]

PCIe is backwards compatible. "Supports x4 mode" usually means while it can physically fit x16 cards, it's only been electrically wired for x4, regardless what speed it's running at. That means plugging in your x8 2.0 NIC into it will mean it'll end up running in x4 PCIe 2.0.

What motherboard is it? I'm guessing the main PCIe slot is being used by something else? Some motherboard's second PCIe x16 slot will share lanes with the first one, meaning it will drop to x8 when something is plugged into the second slot. This is still enough for GPUs and you'll only lose 1-3% odd of performance.

 

[hernaldo]

I have a X520-T2 dual 10Gbe NIC (uses 8xPCIe 2.0 lanes) which has a x8 footprint.
Will this work properly in a PCIe 4.0 x16 slot which states it "supports x4 mode"?
I think this means that the big slot (x16) has 4 PCIe4.0 lanes associated (x4) so that is equivalent to 16 x PCIe 2.0 lanes - more than enought, but will the mismatch in pins make a difference?

Many thanks for guidance!

Regards,
Paul

No.

The card will run at the lower of the multiplier the slot or card supports at the gen of the card.

So pcie 2.0 4x in your case.

This is enough for a single 10gbit port but not two port cards.

 

[Tetras]

but will the mismatch in pins make a difference?

Yeah, unfortunately it doesn't matter if you 'technically' have more bandwidth available, it is limited by lanes and the PCI-E standard.

What is the motherboard?

According to the Intel datasheet for the controller, it supports 1 - 8 lanes, so at least should it work if the motherboard accepts PCI-E 2.0 operation in the slot.

 

[Quartz]

I think this means that the big slot (x16) has 4 PCIe4.0 lanes associated (x4) so that is equivalent to 16 x PCIe 2.0 lanes

That's correct, but the card will only use those 4 lanes at v2 speeds. PCIe v2 bandwidth is 500 MB / 4 Gb per lane so 4 lanes will give you 2 GB or 16 Gb. However, that's before any overheads. You may well be okay for one channel. You have nothing to lose by trying it.