Question Ryzen 3000: NVMe on Mobo or expansion cards?

Tritous

Honorable
Apr 3, 2013
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10,510
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I'm looking at a new build based on a Ryzen 3800X and am trying to choose a mobo. One problem I have is cooling the NVMe slots and I'm looking at the option of using a proper PCIe expansion card rather than the build in slots.

I expect using a vertical slot will make cooling far easier, but I have a feeling that two of the M.2 slots are links to dedicated NVMe lanes supported by the processor rather than the chipset - and there may be a performance difference between these and using an expansion slot.

Does anyone know how this is managed in the new Ryzens? Are we better off ignoring the M.2 slots and getting an expansion card or do we integrate it all into the mobo slots?
 

popatim

Titan
Moderator
Well unless you plan on using your GPU slot then you’ll be going thru the chipset anyways. If your board has 2 gpu slots then installing an m2 card into the 2nd slot will drop the first slot to x8 mode. This isn’t really a problem though.
 

Tritous

Honorable
Apr 3, 2013
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10,510
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Well unless you plan on using your GPU slot then you’ll be going thru the chipset anyways. If your board has 2 gpu slots then installing an m2 card into the 2nd slot will drop the first slot to x8 mode. This isn’t really a problem though.
To my knowledge this is not true for the ryzen 3000s. It is usually more true of the 3rd m2, although some mobos seem to share it with a set of SATA ports.

It also doesn't cover the question :)
 

popatim

Titan
Moderator
Sorry to be obtuse, what i was referring to was that using the slot adapter in a non-GPU slot will having you going thru the chipset, which only has a pcie4 x4 connection back to the CPU for all devices connected to the chipset to share (Sata, USB, and non-gpu pcie slots) so unless you want to give up your x16 mode on your GPU then you might want to be researching cooling your M2 slots or which Motherbd's have integrated M2 cooling . I can't help with cooling M2 slots much.
 
Nov 12, 2019
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Sorry if this might be too late of a post OP,
Just get a heat sink for your NVMe M.2 SSD

You can use the first NVMe without bottle-neck as it has 4X dedicated lanes from the CPU. Second and third NVMe might sometimes be bottle-necked at half their speed or round-robbin(Not sure which the chipset uses) only if both of them are sending or receiving data from the cpu at the same time and only if they reach the maximum speeds for X4 pcie 4.0 as they are connected to the chipset which is limited to the X4 speed between the CPU and Chipset. The second or third one should run at their full speed if one of them isn't sending or receiving data from the CPU for example doing nothing.

As for your X16 slots, If your motherboard has two then only one will be on the CPU at X16 which is 32 GB/s, and the second one on the chipset at X4 or 8 GB/s which is through the chipset and will share the 8 GB/s on the X4 lane that is between the CPU and chipset. If however you have 3 PCIE X16 slots, then one X16 will run at 32 GB/s or both run at 16 GB/s if two X16 slots are used as they share the same 16 lanes to the CPU. The third one will be connected to the chipset and only run at X4 speed, or about 8GB/s.


TLDR:
AMD CPU lanes: 24 PCIE 4.0 = about 2 GB/s per lane
X16 for X16 slot or if you have three PCIE X16 slots and two are used then they run at X8 + X8 speeds
X4 for Nvme or X2 for 1 Nvme + X2 for 2 SATA 3.0 (not all motherboards use sata here)
X4 for chipset that has 16 lanes that that motherboards makers can use for various configuration but bottle-necked through the X4 to CPU

If you have two PCIE X16 slots then the last will always be on the chipset at X4 speed which goes through a x4 lane bottle-neck that all chipset lanes must pass through.
If you have three PCIE X16 slots then the last will be on the chipset and be at X4 speed which goes through a x4 lane bottle-neck that all chipset lanes must pass through.
 
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