NVMe - PCIe Lanes and Slot

[Hi P]

Sorry for the long question!

 

Ryzen 2700x has 20 PCIe lanes available, so let's take this as the example:

 

2 GPUs (-16 lanes, dual x8)

1 M.2 (-4 lanes)

Lanes available: 0

 

 

1.- The Asus CH7 supports 2 M.2 by using the PCIe lanes available on the CPU, none from the chipset, so taking the setup above:

 

- What would happen if we throw an extra (second) M.2 into said build?

 

- Would both M.2 run at half speed or the second one won't be detected?

 

 

2.- The Asus x470-F supports a single M.2 x4 on the CPU, and a second M.2 x2 on the chipset.

 

- Using the build above (having 0 lanes left), connecting the second M.2 (chipset) would work perfeclty but at half speed? are there other drawbacks?

 

 

3.- The Gigabyte Gaming 7 supports a single M.2 Gen 3.0 x4 on the CPU, and a second M.2 Gen 2.0 x4 on the chipset.

 

- What happens if I connect an M.2 Gen 3.0 into the Gen 2.0 slot?

 

- Is the Gen 3.0 x4 twice as fast as the Gen 2.0 x4?

 

 

Almost forgot, does M.2 support mean they all support NVMe M.2?

 

 

Thank you very much to whoever took the time to help

 

 

[Jurrunio]

9 minutes ago, Hi P said:

2 M.2 by using the PCIe lanes available on the CPU, none from the chipset,

where does it say that?

[Hi P]

3 minutes ago, Jurrunio said:

where does it say that?

Please feel free to correct me if I'm wrong, I just saw both M.2 listed below the Ryzen gen 2, and under chipset there wasn't any.

 

Is that not how it works? how does it work?

[Jurrunio]

9 minutes ago, Hi P said:

Please feel free to correct me if I'm wrong, I just saw both M.2 listed below the Ryzen gen 2, and under chipset there wasn't any.

 

Is that not how it works? how does it work?

As far as I can see. that only means Ryzen can use 2 M.2 PCIe x4 drives, whlle Bulldozer-based CPUs can only use 1 SATA drive

[Taf the Ghost]

You have to check the motherboard manual for details on specific layouts, just as a general note.

 

But, for X370/X470 generally, the boards with Two M.2 slots, one will run PCIe x4 while the other runs as SATA. In the Hero's case, one of the slots appears to only do PCIe, if I'm reading the manual correctly.  https://dlcdnets.asus.com/pub/ASUS/mb/SocketAM4/ROG_CROSSHAIR-VII-HERO_WI-FI/E13834_ROG_CROSSHAIR_VII_HERO_WI-FI_UM_WEB.pdf On page 24, you *can* run two NVMe PCIe SSDs in x4 mode at the same time, but the GPU slot drops to x8.

 

Ryzen CPUs have 24 PCIe lanes. x16 to the GPU slots; x4 to the NVMe slot; x2 to the Chipset features. It's the other x2 that seem to be the shared bandwidth for the SoC features and the x1 slots, I think. I've yet to find clarity on that point.

[Hi P]

4 minutes ago, VegetableStu said:

The second slot shares bandwidth with the second x16 slot. if you only have one GPU it'll most probably switch it to x8/x8 mode anyway to get to the second m.2 slot, regardless of the second x16 slot being populated or not.

Thank you for all the info!

 

Regarding the quoted part, placing the second M.2 on a mobo with two GPus along the first M.2 being used, would it drop an x8 GPU to x4?