Hi jlzhou - please see this thread for more information:

https://www.dell.com/community/XPS-Desktops/XPS-8940-and-Samsung-EVO-970-Plus-Slow-Performance/td-p/7859990

I installed a second SSD in the x4 PCIe slot and it was recognised by Windows - but there were other performance issues I was trying to diagnose.

The 16 + 4 PCIe lanes you are referring to are for direct attach GPU and a M.2 SSD.  However, it appears that Dell have not implemented the +4 lanes, at least not on my motherboard.  They didn't even exist on Gen 10.

The M.2 slot would appear to be attached via the PCH chip.  The limitation is how many PCIe lanes are implemented via the Intel chipset, not the CPU.

And whether Dell officially support it or not is, I guess, up to them.

many thanks!

hope Dell can update the bios asap.

The dual NVMe SSD support is actually broken since BIOS version 2.2.0 (the one that introduces resizable BAR support). I had two NVMe SSDs in the case before that and works OK. But after 2.2.0 BIOS, the system will either freeze at Dell logo, or simply won't POST at all when a secondary NVMe drive is added to any other PCIe slots. Yes, all three slots (x1, x4 which comes from PCH, and even the x16 which is CPU direct) won't work. 

Even in the older BIOS versions, there's a glitch that some NVMe SSDs (like the one I'm using with Silicon Motion controller) only works in the on-board M.2 slot but not the x1 or x4 slots. 

I believe it's a firmware issue and Dell should be able to fix in a BIOS update. But if a representative says it's not supported, then it means they  and that's just disappointing. 

Okey dokey... Thanks for the " ", really apprecated. Would appreciate more if this issue can be actually resolved in a future BIOS update, but I don't give too much hope. 

There's a new BIOS update for the XPS 8940, v2.3.0.  So maybe this one fixes the problem...

I don't know whether it was due to new BIOS 2.3.0, but after some experimentation, I have two nvme ssds working, one in the dedicated m.2 slot, one on a PCIe adapter. They are both Samsung ssds. I have only been able to get this work with SATA set to RAID in BIOS. At this point, I have also had to remove the NVIDIA RTX 3060 from the x16 PCIe slot, too. Gonna try to reinstall it, now that the system at least recognizes both ssds! Will update!

Cheers,

David

Just a quick update: after reinstalling (but not connecting to a monitor) the NVIDIA gpu and updating the driver, I rebooted and connected NVIDIA gpu to the monitor, and all is working perfectly....for now!

@pstock21 Just to let you know, Samsung recommnends that SATA operation be set to AHCI in the BIOS. You may wish to change SATA operation in the BIOS if you encounter problems with the RAID setting. To change to AHCI without reinstalling Windows, you need to change the current Windows installation to AHCI by running the following procedure: http://triplescomputers.com/blog/uncategorized/solution-switch-windows-10-from-raidide-to-ahci-operation/

Note that the BIOS change is not made until step 4 of the procedure.

Some Samsung software like Samsung Magician and the Samsung NVMe driver require AHCI, but if don't use that software and are not having problems with RAID then it is up to you to decide if you want to make the change.

Thanks! But I went through all the permutations, and only in RAID was I able to get both ssds recognized. I realize I am sacrificing Samsung Magician (meh) and the Samsung driver (maybe a loss?), but this is the only way it would work. Moreover, according to CrystalMark, there is no performance difference between the Intel RAID driver and the Samsung NVMe driver. If AHCI would work, I'd prefer it--but it doesn't