C6E which M.2 slot is best for booting from NVMe drive

[R5Eandme]

Greetings. The Crosshair VI Extreme contains two M.2 slots and if you are installing only one M.2 as an NVMe boot drive, it leads to the question of which slot is most efficient for booting the OS? Here are pictures showing the two M.2 socket positions. The M.2_1socket on the right side of the image that is covered by the X370 heat sink accommodates M-key type 2240/2260/2280 drives. The M.2_2 socket at the bottom of the image that is not covered by the X370 heat sink accommodates M-key type 2240/2260/2280/22110 drives.


The M.2_1 socket can be placed in either SATA or PCIE 3.0 X4 mode, which is confusing to me because the PCIE 3.0 controller is in the Ryzen CPU while the SATA AHCI controller is usually in the mobo chipset, the X370. (What does it mean for an M.2_1 socket to be in "SATA" mode anyway?) The X370 does have a PCIE controller, but it is PCIE 2.0 and it controls the black X1 and X4 slots on the motherboard, so it cannot directly control the M.2_1 socket when that socket is in PCIE 3.0 mode. Can the M.2_1 socket be switchable between the CPU and X370 controllers, and if you switch M.2_1 to PCIE 3.0 mode, does it use the X370 chip as a "pass-through" to the CPU PCIE controller? Alternatively, the M.2_2 socket is only in PCIE 3.0 x4 mode and shares bandwidth with the PCIEX8_2 card slot which is under the control of the Ryzen 7 CPU. Additionally, the ASUS web pages indicate that you can install two NVME drives in M.2_1 and M.2_2 in RAID, so they would both be managed by the CPU PCIE controller, I would think. See this image from the specs sheet:

My inclination is that faster booting might occur using the M.2_2 socket because it directly uses the CPU PCIE controller, but I really don't know. The downside is that the M.2_2 slot can't take advantage of the X370 heat sink. Does anyone know if there would be a difference in boot efficiency between these two mounting options? It would be good for an ASUS engineer to comment. The manual does not offer enough information about this.
Thanks!

[R5Eandme]

An additional thought: The PCIE (black) slots that are controlled by the X370 chipset run at PCIE 2.0. So it is a bit confusing that the M.2_1 socket can be placed in SATA or PCIE 3.0 X4 mode, unless putting it into PCIE 3.0 X4 mode switches the M.2_1 socket to the CPU PCIE controller instead of X370 controller? If that is true then it is best to put the boot drive in the M.2_1 socket, switch it to PCIE 3.0 X4 mode, and use that big heat sink to keep it cool.

[R5Eandme]

I sent my questions to ASUS tech support and received useful answers for the case where the C6E uses a Ryzen 7 CPU with it's 24 PCIE lanes. Actually, 4 lanes become dedicated to communication with the X370 chipset, leaving 20 lanes for video cards and M.2 drives according to the following rules:

(1) Both the M.2_1 and M.2_2 sockets use the CPU PCIE controller (not the X370) and therefore they share some of the Ryzen 20 lanes with the video and other PCIE cards. NVME drives are X4 devices, using 4 lanes each.

(2) If you place an NVME drive into M.2_2 along bottom edge of mobo, then the PCIEX8_2 will be DISABLED. If you place a PCIE card into the PCIEX8_2 slot then M.2_2 will be DISABLED. They not only share bandwidth as stated in the manual, but if one of them is occupied then the other is disabled.

(3) If you have one NVME drive, it is best to place it into the M.2_1 (under the X370 heatsink) because that socket does not share bandwidth with the PCIEX8_2 slot, and therefore will leave PCIEX8_2 available for use.

(4) Placing a video card into PCIE_X16/X8_1 it will receive 16 lanes unless either PCIEX8_2 or M.2_2 are occupied, in which case the PCIE_X16/X8_1 slot will receive 8 lanes. (In the Aug 2017 issue of MAXIMUM PC, "PCI Express Explained" p 58, states that even an advanced video card driving a 4K monitor may only infrequently saturate an X8 PCIE bus).

(5) You can have two video cards in PCIE_X16/X8_1 and PCIEX8_2, but they will each receive 8 lanes, and the M.2_2 slot will be disabled. M.2_1 is still enabled, and an NVME drive there would use the remaining 4 lanes: 8 + 8 + 4 = 20 lanes.

(6) You can have two NVME drives in M.2_1 and M.2_2, maybe in RAID, but the PCIEX8_2 slot will be disabled and the video card in PCIE_X16/X8_1 will receive 8 lanes.

(7) The black PCIE slots (PCIEX1_1, PCIEX1_2, PCIEX1_3, and PCIEX4_3) all use the PCIE controller in the X370 chipset, which provides 4 PCIE lanes in addition to those provided by the Ryzen cpu.

(8) You can have up to four X1 PCIE cards in PCIEX1_1, PCIEX1_2, PCIEX1_3, and PCIEX4_3. But if you place an X2 or X4 card into PCIEX4_3, then PCIEX1_1, PCIEX1_2, and PCIEX1_3 will be disabled.

I hope this is useful.

[Jeff007245]

Very useful indeed and exactly what I was looking for.

So in other words, we cannot crossfire and install an M.2 Sata + M.2 NVME to take advantage of StoreMi/Fuze drive.

I was hoping to run said setup, but it looks like your can't have Crossfire and NVME on M.2_2 at the same time...

Well that is unfortunate.

[R5Eandme]

Jeff007245 wrote:
Very useful indeed and exactly what I was looking for.

So in other words, we cannot crossfire and install an M.2 Sata + M.2 NVME to take advantage of StoreMi/Fuze drive.

I was hoping to run said setup, but it looks like your can't have Crossfire and NVME on M.2_2 at the same time...

Well that is unfortunate.

Hello Jeff007245, yes it seems that you cannot crossfire and at same time have use of the M.2_2 socket, at least according to what the ASUS tech told me in our chat. Ideally there would be shared bandwidth between that second PCIE slot and the M.2_2 socket, but he said that one or the other would be disabled.

I am using a C6E for learning LINUX but for future builds I will pay more attention to CPUs and chipsets that offer tons of PCIE lanes, especially in this age of NVMe drives. A Threadripper CPU will do nicely as it supports 64 PCIE lanes. So glad we have a resurgent AMD. Based on Intel pricing, Intel hasn't gotten the memo yet.