ConnectX-3 v ConnectX-4 for Dual Port Usage

[nkw]

Since the ConnectX-3 is PCIe Gen3 x 8, my understanding from some back of the envelope figuring is that it is not able to saturate/handle both ports at full rate. (i.e. the bus max is going to be 985MB/s * 8 lanes * 8 bits or approximately 63 mbit/s or so vs 80 mbit/s for both ports). Is this correct? Are the two ports primary intended for failover or less than full simultaneous usage?

I notice some of the ConnectX-4s come in Gen3 x 16 so I assume this is a better choice if you want both ports running at full rate, but the ConnectX-4 advertises QSFP28. Am I correct that QSFP28 and QSFP+ are the same physical form-factor? The ConnectX-4 w/ its QSFP28 port should still be able to successfully connect to a 40G QSFP+ switch via a QSFP+ DAC, right?

 

[i386]

Yes, pcie 3.0 x8 has less than 80 GBit/s (~7.9GByte/s).
Yes, for failover.
The CX-4 with x16 are 100GBit/s cards, the dual port versions are limited by the x16 slot
Yes, the CX-4 should be backward compatible to 40gbe with a qsfp+ dac.

 

[nkw]

Yes, pcie 3.0 x8 has less than 80 GBit/s (~7.9GByte/s).
Yes, for failover.
The CX-4 with x16 are 100GBit/s cards, the dual port versions are limited by the x16 slot
Yes, the CX-4 should be backward compatible to 40gbe with a qsfp+ dac.

Er, yeah. giga instead of mega.

Thanks!