MCX312A-XCBT or X520-DA2 in PCIe v2 x8 slot?

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ChasW

New Member
Nov 3, 2019
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I think I already know, but I could use clarification regarding bandwidth requirements for these cards.

The main board slot is physically PCIe 2.0 x16.
The max link speed for this slot is x8.

The two cards being considered for this slot:

1) Mellanox MCX312A-XCBT Specifications
PCI Express Gen3: SERDES @ 8.0GT/s, 4 lanes (2.0 and 1.1 compatible)
PCI Express Gen2: SERDES @ 5.0GT/s, 8 lanes (1.1 compatible)

2) HPE NC560SFP X520-DA2 665249-B21 Specifications
PCIe v2.0 (5.0 GT/s), Speed & Slot Width 5.0 GT/s, x8 Lane

Given that an x8 link in a v2 slot is 40Gbit, either card should work in a PCIe 2.0 x8 slot at full speed with both ports active simultaneously (20Gbit/s), right?

Aside from the Mellanox using less power and being PCIe v3, should either be expected to perform comparably in a v2 x8 slot?

I'd be interesting in hearing about any experiences with either card anybody has had particularly regarding bandwidth, CPU utilization, and any other cost related factors like cabling or transceiver requirements.

Regards.
 

Kal G

Active Member
Oct 29, 2014
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Yes, either card should work fine.

5 GT/s translates to about 4 Gbit/s for each lane. So the theoretical maximum is 32 Gbit/s for 8 lanes with the 8b/10b encoding scheme used by PCIe v2.