M.2 to PCIe 3.0 x4 shared bandwidth with PCIe 4.0 GPU

[k0200679]

Budget (including currency): 3000€

Country: Finland

Games, programs or workloads that it will be used for: 4k/8k video editing

Other details (existing parts lists, whether any peripherals are needed, what you're upgrading from, when you're going to buy, what resolution and refresh rate you want to play at, etc): 

 

I am planning to build a PC with 7-10 years lifespan.

Graphics card and hard drives will be renewed every 3-4 years, rest will be the same.

I came to conclusion that to be able to upgrade my graphics card and SSDs for such a long period of time PCIe 4.0 is a must.

 

Motherboard: Asus ROG Crosshair VIII Hero (WI-FI)

Graphics card: Asus ROG-STRIX-RTX2070S-O8G

CPU AMD Ryzen 3950x

DDR4 64Gb

 

I have two existing 1Tb M.2 SSDs and plan to buy one more to have 3 M.2 SSDs in total.

The problem is that Asus ROG Crosshair VIII Hero has only two M.2 slots.

So, I decided to buy “M.2 SSD to PCIe Express 3.0 x4” adapter and put my third SSD to third PCIe 4.0 x16 slot.

My question is, what impact will have M.2 SSD located in third PCIe 4.0 x16 slot to my graphics card bandwidth? Isn’t it so that PCIe slots share bandwidth between each other?

What will happen in 6 years, when I will update to PCIe 4.0 compatible high end graphics card?

 

[Kilrah]

The bottom slot goes to the chipset, not straight to the CPU. So it doesn't affect the graphics card.

[k0200679]

Thanks Kilrah! Could you tell me, whare I can read about this in more detail ?

[mariushm]

A CPU gives you 24 pci-e lanes :

* 16 go to pci-e x16 slot for video card, which may be further split into 2 x8 slots

* 4 go to m.2 connector

* 4 go to the chipset

 

The chipset takes those 4 pci-e lanes and creates 8 pci-e lanes. x570 chipset creates 8 pci-e 4.0 lanes, B550 chipset creates 8 pci-e 3.0 lanes.

So most AM4 motherboards will have 2 m.2 connectors, because the chipset can allocate 4 lanes to the 2nd m.2 connector and 4 to another pci-e x16 slot (only x4 electrically). If there's a third m.2 connector, most often it's only pci-e x2 or adding a nvme ssd in it disables the extra pci-e x16 slot or converts it to pci-e x1 ... or the 3rd m.2 connector is sata only.

 

There are exceptions, like a specific Gigabyte B550 board, which splits the pci-e x16 for video cards  into 2  x8, connects x8 to the pci-e x16 slot for video cards and the other 8 lanes are connected to two m.2 connectors. If you add a SSD in either of those two m.2 connectors, the video card slot is downgraded to x8.

 

The asus motherboard is an expensive choice, it's a motherboard aimed to overclockers and not for people like you. 

You can do much better with cheaper boards which also have more features.

 

I'd suggest boards like

MSI x570 Unify  https://www.newegg.com/msi-meg-x570-unify/p/N82E16813144273

gigabyte x570 aorus ultra https://www.newegg.com/gigabyte-x570-aorus-ultra/p/N82E16813145156

gigabyte x570 master (the one with 2 m.2 connectors that take 8 pci-e lanes from video card slot) : https://www.newegg.com/gigabyte-b550-aorus-master/p/N82E16813145217

 

gigabyte b550  vision - expensive for a b550 board but packed full of features : https://www.newegg.com/gigabyte-b550-vision-d/p/N82E16813145231

 

and lots of other better choices than that asus board

[Kilrah]

19 minutes ago, k0200679 said:

Thanks Kilrah! Could you tell me, whare I can read about this in more detail ?

Your motherboard's manual.

[HenrySalayne]

You should consider getting a TRX40 mainboard. Ryzen 3rd gen offers just 20 direct PCIe lanes and additionally 4 for the chipset. So you won't be able to run a x16 graphics card and 3 x4 NVMe drives at full speed.

https://www.gamersnexus.net/guides/3482-amd-x570-vs-x470-x370-chipset-comparison

 

The Asus X570 E-Gaming is almost the same board, but 100 € cheaper.