Question I want to understand how PCIe lanes are shared on my motherboard and in which lanes should I be installing my GPU and m.2 nvme drives.

[shadybk]

I am building a new PC with a 7900xt, a 980 pro nvme ssd for my boot drive and this motherboard:

B650 AORUS ELITE AX (rev. 1.x) Key Features | Motherboard - GIGABYTE Global

Lasting Quality from GIGABYTE.GIGABYTE Ultra Durable™ motherboards bring together a unique blend of features and technologies that offer users the absolute ...

www.gigabyte.com

I will probably add more nvme storage in the future.

From this part of the specs it looks like it has three m.2 slots, one gen 5 and two gen 2, is this correct?
What is the difference between the CPU m.2 and the chipset m.2?

CPU:

1. 1 x M.2 connector (Socket 3, M key, type 25110/2280 PCIe 5.0* x4/x2 SSD support) (M2A_CPU)
   * Actual support may vary by CPU.
2. 1 x M.2 connector (Socket 3, M key, type 25110/2280 PCIe 4.0 x4/x2 SSD support) (M2B_CPU)

Chipset:

1. 1 x M.2 connector (Socket 3, M key, type 25110/2280 PCIe 4.0 x4/x2 SSD support) (M2C_SB)
2. 4 x SATA 6Gb/s connectors

The motherboard also has 3 expansion PCIe slots, one gen 4 and the other two gen 3, all with 16 lanes?
I should install my gpu in the gen 4 PCIe slot correct?
CPU:

1. 1 x PCI Express x16 slot, supporting PCIe 4.0 and running at x16 (PCIEX16)
   * For optimum performance, if only one PCI Express graphics card is to be installed, be sure to install it in the PCIEX16 slot.

Chipset:

1. 2 x PCI Express x16 slots, supporting PCIe 3.0 and running at x1 (PCIEX1_1/2)

I have heard of PCIe slots sharing lanes with m.2 slots and possibly slowing each other down if you have multiple things connected to them, can someone elaborate a little?


Basically if I just have one gpu and 3 nvme m.2 SSDs, would it cause one of the SSDs or the GPU to operate slower?

 

[Lafong]

I'd expect you'd want to put your OS NVMe in one of the two "CPU" ports. One of those is 5.0, the other is 4.0.

The 980 Pro is a 4.0 drive, so I don't think there'd be any advantage to putting it in the 5.0 port.....but no harm either.

What make and model on your other drives?

 

[shadybk]

I'd expect you'd want to put your OS NVMe in one of the two "CPU" ports. One of those is 5.0, the other is 4.0.

The 980 Pro is a 4.0 drive, so I don't think there'd be any advantage to putting it in the 5.0 port.....but no harm either.

What make and model on your other drives?

For now I will only have a 980 pro 2tb where windows will be installed on and games and an 860 evo 1tb for all my other files/documents and some games once the 980 fills up.

Once I start to run out of room I will purchase another nvme drive because I fond it weird that good nvme drives are priced either extremely competitively or even cheaper than sata SSDs.

 

[Math Geek]

on the lower to mid end of the prices, you can expect some sharing to happen.

for instance my b450 aorus pro has 2 m.2 slots but each one shares lanes with the sata ports. each one used disables 2 sata ports. in the end, despite having 6 sata ports only 2 are actually usable since i have 2 m.2 drives installed.

you will pretty much always have the first pcie slot at the full 16 lanes. that's just pretty much standard. but again at the lower end if you use a second slot, it often drops both slots to x8, disables ports or some combination of compromises.

the board you picked however, is at the top end of the spectrum and does not have the compromises.

you get 3 m.2 slots, 1 at 5.0 x4 and the other 2 at 4.0 x4.

your OS should go in the first and fastest slot since its speed matters most. of course a 4.0 drive in a 5.0 slot negates this but still best practice to use the first slot for the OS. you also get all 4 sata ports no matter what you hook up.

your gpu goes in the first pcie slot as well of course. it's the only x16 slot you have. the other slots are only 3.0 x1 since the lanes that would go to them are spread out over the m.2 and sata ports.

to get multiple x16 pcie slots you have to step up to the x670 boards at the top end that have an extra chip giving those extra lanes., those are super expensive and only worth it if you truly need the lanes (which gpu's don't actually need anyway 4.0 x8 is good enough for most cards. )

hopefully this helps break it down for you just a little bit.

 

[hotaru.hino]

From this part of the specs it looks like it has three m.2 slots, one gen 5 and two gen 2, is this correct?
What is the difference between the CPU m.2 and the chipset m.2?

There is one Gen 5 and two Gen 4s. The difference between CPU and chipset is simply where the lanes are coming from. Also note that the chipset M.2 only has 2 lanes, while the CPU ones have 4. The chipset is connected to the CPU using its own set of 4 PCIe 4.0 lanes, so there may be some bandwidth issues, but this is only a problem if you've fully populated what the chipset is capable of and they're all trying to talk to the CPU at once.

Generally speaking, if you install an M.2 SSD in the chipset M.2 slot, you're going to see it use 2 lanes worth of bandwidth most of the time.

The motherboard also has 3 expansion PCIe slots, one gen 4 and the other two gen 3, all with 16 lanes?

The other two only have 1 lane each

I should install my gpu in the gen 4 PCIe slot correct?

Yes

I have heard of PCIe slots sharing lanes with m.2 slots and possibly slowing each other down if you have multiple things connected to them, can someone elaborate a little?

They don't share lanes. PCIe is a point-to-point system, meaning there's a direct connection between the device and the controller. However, the chipset interfaces to the CPU over effectively 4 lanes of PCIe 4.0. So it can be a problem enough devices connected to the chipset are trying to shove things into RAM since the chipset itself can provide more than 4 lanes of bandwidth to devices. But in practice this scenario is really rare.

Basically if I just have one gpu and 3 nvme m.2 SSDs, would it cause one of the SSDs or the GPU to operate slower?

No.

 

[shadybk]

Thank you for all the answers, I have a much better understanding of how it works now.