Cheated out of PCIe Lanes

I have an Asus ROG Crosshair VII Hero (WiFi) motherboard and Ryzen 7 2700X CPU, 1 NVMe SSD, 1 M.2 SATA SSD, and an EVGA GTX 1080 Ti graphics card in the PCIe 3.0 X16 slot. Here's my dilemma:

M.2_1 slot will accept SATA or NVMe SSD and will not share any PCIe lanes.
M.2_2 slot will accept only NMVe SSD and if it is occupied, it take 8 PCIe lanes from the PCIe 3.0 X16, making it an 8 lane slot.

According to a couple of websites, including this one:
PCIe 3.0 x8 vs. x16: Does It Impact GPU Performance? | GamersNexus - Gaming PC Builds & Hardware Benchmarks

Running the 1080 Ti in 8 lanes v. 16 lanes really does not affect performance. But, it just really bugs me that the SATA M.2 SSD has to go in slot 1, leaving only slot 2 for the NVMe SSD which is going to take 8 lanes away from the GPU.

So...should I (1) leave well enough alone. or (2) put the SATA M.2 SSD in an adapter that goes into a PCIe X4 slot with a SATA cable to a SATA port, move the NVMe SSD to M.2 slot 1, and restore the full 16 lanes to the 1080 Ti graphics card?

I wouldn't bet my life on this, but I believe that M.2_2 steals PCI-E lanes from the second PCI-E X16 slot, which is electrically X8/X4 to begin with.

From the manual:

The PCIE_x8/x4_2 slot shares bandwidth with M.2_2 slot. PCIE_x8/x4_2
slot will run in x4 mode if M.2_2 is enabled in PCIe mode.

That slot is the PCI-E X16 slot 2, which is X8/X4 electrically. Also, an NVME M.2 card is PCI-E X4; it should only consume 4 lanes.

If you keep the graphics card in PCI-E X16 Slot 1, it will see the full 16 lanes if you have a 2700X CPU (2nd generation Ryzen).

So: keep the graphics card in Slot 1, and use the M.2 socket for you second NVME drive. The graphics card will get the full 16 lanes, and the M.2 drive its full 4.

NavyLCDR said:

Unfortunately, @bobkn, the people who wrote the manual obviously did not communicate with the people who wrote the UEFI firmware, (also look at the table on page 1-8 of the manual):

You must be right. The Ryzen 7 X2700 only provides 20 PCI-E lanes. As per the table you reference, that could not allow both M.2 slots to have 4 lanes. (8+4+4 = 16. 16+4+4 =24, no go, even if PCI-E X16_2 was disabled.) The text in the manual must be in error. (But not the table.) Sad.

I wonder whether switching to a Ryzen 7 3700X (24 PCI-E lanes) would create enable 16 lanes for X16_1, X16_2 disabled, 4 lanes on M.2_1, and 4 on M.2_2? I know that the Crosshair VII Hero WiFi supports the x3700 (BIOS 3004 or later). That'd be a mere $350US upgrade (NewEgg).

I voted for the adapter because a) cheap adapters are available and b) I'm compulsive.

(I recently upgraded the M.2 drive in my primary system to get one that does PCI-E 4.0 sequential reads at 7000 MB/s. Mostly irrelevant, outside of benchmarks.)

Ugh. I was thinking about the upgrade anyway....

Also, I notice that BIOS version 4007 says it supports new CPUs, so I wonder if that also expanded the number of PCIe lanes assignable by the motherboard.

You talked me into, @bobkn. I'm upgrading to the Ryzen 7 3800XT.

A usual Ryzen should have the PCI-E slot as x16 with a single NVME SSD.
If you have a Ryzen xxxxG or a Ryzen 3 3xxxG, then you will be limited to PCI-E x8!

RJARRRPCGP said:

A usual Ryzen should have the PCI-E slot as x16 with a single NVME SSD.
If you have a Ryzen xxxxG or a Ryzen 3 3xxxG, then you will be limited to PCI-E x8!

Already covered.