Originally Posted by zylicyde
I've looked through the BIOS four times now at every single option, and there are only two related to M.2, and all of the PCIe stuff is set to gen 3. *shrugs*
The devil is always in the details with this stuff. On page 1-7 of the MSI X99S SLI PLUS manual
there is a diagram that shows how the PCIe lanes are divided up from the CPU. However in the 'fine print' you can see the diagram applies to a 40-lane CPU only.
The 5820K is a 28-lane CPU, so I'm thinking that the board is only feeding 2 PCIe lanes to the M.2 socket, even though you've only got 1 GPU installed. Perhaps it's not even feeding any PCIe lanes from the CPU in your configuration, just 2 lanes from the PCH switch.You would need a 40-lane CPU (5930K or 5960X) to get the results you are looking for on that MSI board.
Edit: I just received an update on this issue from Luke, one of our Amazon.com customers who was noticing low performance numbers on his SM951. Now I'm 100% certain that this board WILL support M.2 x4 lanes when using a 28-lane 5820K cpu.
Here's what he said:
The name of the option is "M.2 Source Link", and the two values are "Auto" and "PCH", and you can see the option description in the picture attached.
I now recall that I might have changed the value of this when I was diagnosing a seagate HDD some days ago, and forgot to change it back to the proper value, causing the lane downgrade issue.
To my surprise, the proper value for SM951 to work at maximum speed is "Auto" instead of "PCH". I thought that PCH should represent something related to PCI-E and chose that, but "Auto" is the value that gives 4x lane to SM951.
Thus, X99s SLI Plus with a 28-lane 5820K does work well with SM951.
My desktop's configuration is:
Core i7 5820K, 4x4 RAM, MSI X99S SLI PLUS, MSI GTX 970 4G GAMINGx1, SM951 256G on M.2 slot, 850 EVO 500G on SATA. An Intel 7260ac WiFi card is plugged to a PCI-E slot.