PCIE backward compatibility/paths
PCIE backward compatibility/paths
Hey everyone, I've been thinking about this for a while. You know how a Gen 4 x8 slot matches the bandwidth of a Gen 3 x16 slot? It's similar when connecting older gear. My X570 board has an x16 port that acts like an x4 slot electrically. If I have a 10Gb NIC that needs a Gen 2 x8 slot, it will only provide 4GB/s. But since the Gen 4 x4 slot offers 8GB/s, would running this device still work? Essentially, are older PCIe devices considered to deliver equivalent bandwidth or lanes on newer boards? Thanks!
The slot with dual lanes assigns the card it connects to the least restrictive PCIe version available between the two. Therefore, inserting a PCIe 2.0 x8 card into a PCIe 4.0 x2 slot will yield the minimum of the two: x2 link and PCIe 2.0 standard. The final choice is always the lower version, considering both electrical requirements and PCIe specifications.
The Intel X520-DA1 NIC supports a PCIE Gen2 x8 connection. I'm wondering if switching it to x4 would have minimal effect on performance, since it currently delivers 2GB/s in both directions. The 2GB/s figure is likely per direction, not total. A 10GB NIC would only achieve about 1.25GB/s each way anyway.
You're correct, 10Gb equals approximately 1.25GB. That means it shouldn't negatively affect performance.
PCIe operates in full duplex, which allows it to transmit information in both directions at the same time without increasing the speed to 4GB/s. Similarly, a full-duplex Ethernet connection doesn't double the bandwidth; it simply eliminates interruptions during data transfer.
Bandwidth involves more than just the connection speed—it also depends on how a device and port are linked electrically. For instance, in Scenario 1, inserting a 1080ti into a PCIe 2.0 x16 slot means it runs at PCIe 2.0 x16, which matches the performance of PCIe 3.0 x8. Benchmarks show no significant slowdown and full utilization of all lanes. In Scenario 2, placing the same card in a PCIe 4.0 x4 slot (like most X570 slots) forces it to operate at PCIe 3.0 x4 speed and bandwidth. Although the slot supports 3.0 x8 theoretically, only four lanes are available, restricting performance to 3.0 and potentially causing issues if the card isn't designed for higher speeds. The key limitation comes from the number of available lanes and the device's maximum PCIe version capability.