Running my NVMe's in PCI 4.0 mode on an AMD B550 board?
Posted by el_jbase@reddit | buildapc | View on Reddit | 9 comments
I currently own an Asus B550 TUF Gaming Plus motherboard with RTX 3090 and recently upgraded my CPU from 3900X to 5950X. I also added 64GB of RAM to a total of 96GB and purchased two Samsung 990 Pro drives (2TB and 4TB). I never play games, I edit video in Adobe Premiere under Windows 10, I also experiment with AI and neural networks (hence the 3090). The 2TB is now my system disk and the 4TB is used to keep video files.
When I installed my new NVMe's (which I love btw) I learned, that there are only two 4.0 PCIE ports on my system, provided by the CPU. The ones from the chipset are 3.0 only, so my 4TB mounted in M.2_2 runs in PCIE 3.0 mode, which effectively halves its performance. I also have an older NVMe (Kingtson 2TB) which I installed into the Asus Hyper 4xM.2 PCIEX16 Gen5 splitter adapter (purchased with future upgrades in mind).
Here's what I'd like. I want to have my GPU in a real X16 slot and also have all my NVMe's in the splitter (cooling is excellent there). All of these devices must run in PCIE 4.0 mode. But Asus manual tells me I must install the M.2 splitter into PCIEX16_1 to get all M.2 slots working. That means the GPU will go into PCIEX16_2, which is PCIE 3.0 and is actually a 4x. To have PCIE 4.0 on all slots I gather I must upgrade to the X570 chipset board. However, only used X570 boards are available locally (while you can easily find a B550 new!). Also, I'm not quite sure how to use the splitter if the X570 boards also have a "trimmed to X4" PCIEX16_2 (where would the GPU go?).
So, I've already upgraded my RAM and CPU, if I have to also upgrade the motherboard, should I have rather upgraded to the AM5 system then? Does buying an X570 board even make sense in 2025? But I already got the DDR4 RAM and the 5950x, Imma have to sell 'em then lol! What do I do? :)
kaje@reddit
CPUs only have 16 lanes for CPU connected PCIe slots. That can run one slot at x16 or two slots at x8 each on some mobos. Additional slots connect to the chipset and won't support more than x4.
majoroutage@reddit
Not true anymore AM4, for example, has 20. x16 for the GPU and x4 for an M.2 slot.
kaje@reddit
I was only talking about the PCIe slots, I wasn't talking about the M.2 slot.
majoroutage@reddit
That wasn't how I read it at first, but okay.
Technically there wouldn't be anything stopping a board maker for using them for a second PCIe slot though.
kaje@reddit
Sure, even if a board did though, it's still not going to support 4 drives on a Hyper M.2 card.
majoroutage@reddit
Probably true.
Although I've always wondered with those things, if there were only 4 lanes, does that limit you to a single slot at x4, or do all 4 slots get limited to x1.
kaje@reddit
It would just support a single drive. The mobo itself needs to support bifurcation on the slot to provides lanes to each drive. Need a mobo that supports 4x4x4x4 on the PCIe x16 slot for 4 drives.
el_jbase@reddit (OP)
It looks like my only option here is use the motherboard's M.2 slots, like for example, X570 Aorus Pro that has 3 of them. So, if I install 3 NVMe drives and a 3090 RTX, will the GPU run at PCIE 16x full speed?
majoroutage@reddit
There is nothing you can do to get two full-speed, non-shared X16 slots on AM4. Doesn't matter what board you buy, it just doesn't support enough lanes.