So, all the good cards i’ve seen such as Chelsio T6225-CR or the Intel X710 or any other card from the similar lineup/segment, the 10GbE, 25GbE and 40GbE cards are usually X8 lanes. It does not matter whether these cards are Fiber or Base-T (for example X710-T2L). But on other hand newer cards like Aquantia AQC-107 and AQC113 are of X4 lanes. Is this what makes the difference between the performance in these two?
My question is cards like AQC107, AQC113, have one port and its X4 lanes. Is it because of a single port requirement or the chip actually requires X4 lanes? Does this mean the single port is X4 and the dual port is X8 lanes?
Moreover, the onboard Ethernets are lower in performance as compared to the physical NIC?
It is math. A single PCIe lane depends on the version the CPU or chipset can supply. AND what the Network Interface Chip is capable of using.
For example, a single PCIe lane is;
PCIe version 1 = 2.5Gbits/ps, but really only 2Gbits usable
PCIe version 2 = 5Gbits/ps, but really only 4Gbits/ps usable
PCIe version 3 = 8Gbits/ps, and really 8Gbits/ps usable
PCIe version 4 = 16Gbits/ps, and 16Gbits/ps usable
PCIe version 5 = 32Gbits/ps, and 32Gbits/ps usable
Basically, the break between version 2 & 3 speed has to do with the overhead. See the “Line code” column in the link below. In essence, their is 20% loss due to encoding for versions PCIe 1 & 2. While PCIe version 3 uses about 1.5% loss for it’s encoding.
So, check the slot the NIC will plug into, and check the NIC’s capabilities as far as PCIe lane speed. Then compare what Ethernet speed you want. Remember, both PCIe and Ethernet are full duplex, so you don’t have to concern yourself with that aspect.
PCIe Gen 2 has got nominal 5GB/s per lane. Real world data transfer is less, say 4GBit/s. This is enough for a single 2.5GB/s port. For 5 GBit/s you would need 2 lanes for full speed. A single 10G port needs at least 3 lanes, but since there are no 3 lane cards you need 4 lanes. Two 10G ports need 6 lanes, so you choose 8 lanes for your card.
PCie Gen 3 hat 8GBit/s nominal and 6-7GBit/s real world. Same calculations as before.
These are only rough estimates, and many parameters like the gaps between PCIe packes etc are left out. Reality is much more complex. This is just to give you a rough idea.
Performance is also affected by offload capability etc.
