(SOLVED) Need Help Understanding M.2 Slots

Dazr87

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May 11, 2019
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UPDATE:

Thanks everyone for their help. I understand things now ?

-----

So I'm in the process of buying PC parts for an SFF build.
I'm limited on space so I was planning on using a couple of M.2 drives and a 7mm 2.5" either SSD or HDD.
The board im gonna use (ROG Strix X470-I) says this:

Expansion Slot:
AMD Ryzen™ 2nd Generation/ Ryzen™ 1st Generation Processors:
1 x PCIe 3.0/2.0 x16 (x16 mode)


Storage:
AMD Ryzen™ 2nd Generation/ Ryzen™ 1st Generation Processors:
1 x M.2 Socket 3, with M key, type 2242/2260/2280 storage devices support (SATA & PCIE 3.0 x 4 mode)
1 x M.2 Socket 3, with M key, type 2242/2260/2280 storage devices support (PCIE 3.0 x 4 mode)*1


*1 The M.2_2 slot shares with PCIE x16. When M.2_2 slot runs in PCIE mode, the PCIE x16 slot will run at x8 mode.

The m.2 drives i want to use are: WD_Black SN750 NVMe - Interface PCIe Gen3 8 Gb/s, up to 4 lanes.

the GPU i'll be using is the Radeon VII.

Will I face any problems/reduction in speed on the drives or GPU if I use both M.2 slots?, If so, Is there a way to use one slot as SATA so it doesnt take away the PCIE Lanes? Or if i'm using the bottom M.2_2 slot, that will happen regardless?

Thanks!
 
Last edited:

tinyitx

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If you use the bottom M.2_2 slot, the PCIe x16 slot will run at x8 speed. This is regardless whether the top M.2_1 slot is in SATA or PCIe mode. There is no way to avoid this.

The M.2 drives will not be affected if you use both M.2 slots.

Regarding the Radeon VII, I use Techpowerup's reviews as reference. This card is about 35-39% slower than a RTX 2080Ti. And, TPU did a PCIe scaling with the 2080Ti and showed that there is about 2-3% performance loss when going down from x16 to x8.
So, as an Radeon VII is weaker (~35%, say), the performance loss will be less than 2-3%. Let's say it is 1%.
If you are a benchmarker, then this might be meaningful as you will score several points fewer.
However, if you are a gamer, then this means you will get 99 fps instead of 100fps. The '1 frame per second' loss is not noticeable.
 
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TheHig

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If you run a PCIE x4 (nvme) drive as such in the primary slot you drop the GPU lanes to x8. The second slot can only do SATA 6. This is a chipset limitation. The bios on my board allows you to choose SATA or pcie in the main m.2 slot so you could just run 2 SATA drives.

I run the Ch7 with a Samsung 970 nvme at x4 primary and a 1tb Crucial m.2 at SATA 6 in the second slot. My Vega 64 is at x8 technically but I see no difference in games. Gamers Nexus and Hardware Unboxed have some videos addressing pcie bandwidth and gaming. Worth checking out.

@tinyitx techpowerup’s gpu relative comparison chart is a terrific resource. Props.
 
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Dazr87

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thanks guys.

So the main M.2 slot that is the M.2_2 yeah? on the underside of the board? and the secondary is on top? that's what it looked like in the asus X470-i manual.

and I can only use SATA M.2 in the top/M.2_1 slot right?

Also, Can I use the WD Blue M.2 SSD in the M.2 Sata slot? it is B+M, but the board says both slots are M, but there is no M key only sata drives that i can find, the M Key all seem to be NVMe

Cheers
 

Soul_Est

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Feb 12, 2016
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thanks guys.

So the main M.2 slot that is the M.2_2 yeah? on the underside of the board? and the secondary is on top? that's what it looked like in the asus X470-i manual.

and I can only use SATA M.2 in the top/M.2_1 slot right?

Also, Can I use the WD Blue M.2 SSD in the M.2 Sata slot? it is B+M, but the board says both slots are M, but there is no M key only sata drives that i can find, the M Key all seem to be NVMe

Cheers
You can only use a SATA (B + M key) m.2 drive in the top slot which the WD Blue is. Found that out when working with my B450-i board.
 
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tinyitx

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Jan 25, 2018
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thanks guys.

So the main M.2 slot that is the M.2_2 yeah? on the underside of the board? and the secondary is on top? that's what it looked like in the asus X470-i manual.

and I can only use SATA M.2 in the top/M.2_1 slot right?

Also, Can I use the WD Blue M.2 SSD in the M.2 Sata slot? it is B+M, but the board says both slots are M, but there is no M key only sata drives that i can find, the M Key all seem to be NVMe

Cheers
Pages 1-2 and 1-18 of the User Manual indicate that the top slot is M.2_1 and the bottom slot is M.2_2.
M.2_1 supports both PCIe mode and SATA mode.
M.2_2 supports only PCIe mode.

The Western Digital M.2 SSD you listed above is PCIe mode only. Si, it works in both top and bottom slot.
If you want SATA M.2 drive, WD Blue is an option. In the QVL of the motherboard, a whole bunch of compatible (at that time...so, not exhaustive) M.2 SATA SSDs are listed, including WDS500G1B0B, WDS250G1B0B, WDS100T1B0B, WDS120G1G0B120GB. These WD models are B+M. So, no need to find M-keyed only drives. And, I think (not too sure), all NVMe PCIe M.2 drives are M-keyed only.
 
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Choidebu

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Aug 16, 2017
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I've got B350-i, but I think up to x470-i the board layout remains the same.

The top slot with the heatsink is pci-e/nvme only, while the bottom can be sata.

Now about putting an nvme drive in the top slot will drop pcie x16 to pcie x8, I don't think so. If you put it in the bottom one, yes. This doesn't happen if you put a sata M.2 drive like WD blue.
 
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