I've been experimenting with different combinations of NICs between my PC and NAS to get the fastest possible connection. Because they're both running PCIe 3.0 x4, I know I can't get 50 or even 40Gbps, but I'd like to get as close as possible. I recently tried these Mellanox ConnectX-4 NICs, which use QSFP28, with a QSFP28 DAC. They established nominal 40GbE and 50GbE links, but both ran at only 25Gbps. Likewise, these have 2x25GbE ports, but they were limited to a combined throughput of 25Gbps. This therefore appears to be a chipset limitation: when running these x8 cards in an x4 slot, the maximum throughput is 25Gbps.
Next I'm going to try some older 40/56GbE ConnectX-3 NICs, which connect using QSFP+. Do I need a new DAC, or can I use the QSFP28 one? I know that a QSFP28 cable won't work for 100GbE on QSFP hardware, but will it work for 40GbE?
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