Hey folks, I have a question which I have never got answered on the internet.

I have a motherboard with Z790 chipset. It has the following M.2 support:

Intel® Z790 Chipset:

M.2_2 slot (Key M), type 2242/2260/2280 (supports PCIe 4.0 x4 mode)

M.2_3 slot (Key M), type 2242/2260/2280/22110 (supports PCIe 4.0 x4 mode)

M.2_4 slot (Key M), type 2242/2260/2280 (supports PCIe 4.0 x4 & SATA modes)

Z790 and i5-13600K are connected via DMI 4.0 x8.

An NVMe SSD has/needs x4 connection.

If I populate all the M.2 slots, 3 SSD need 12 PCIe lanes. But Z790 chipset only have 8.

What happens in this case? All SSD switch to X2 mode and I lose half the bandwith? Or 2 SSD have the full bandwith and the 3rd one is disabled?

Please help me to understand it.

Typically, one of the m.2 slots uses CPU lanes, the other two use the PCH, look for the block diagram for your board, this is mine:

I am only talking of PCH connected M.2 slots. My MOBO has 1 CPU and 3 PCH attached M.2 slots.

In that case they can still all be running at x4, but if you are reading or writing to all three at the same time, the DMI will be the bottleneck.

The chipset has a PCIE switch included in it, which behaves similarly to a network switch for PCIE data, so they just share bandwidth as needed and only bottleneck when you try to hit them all at once while doing a lot of other things (other pcie slots, sata ports, usb networking etc) but the chipset is pretty smart in how it shares the bandwidth so it really isn't an issue in normal use.

Your DMI4.0 x8 link from CPU to chipset has around ~16GigaBytes/s of throughput, ~128gigabit/s.
so you would have to put 3x of the fastest PCIE 4.0 x4 SSDs in there and run them all simultaneously at nearly full load for it to throttle, and even then they will be more than fast enough. if you used a perfectly good mid spec SSD like a WD Blue which do around 5GB/s, you could run all three without noticeable bottlenecks as long as the chipset isnt doing much else at the time.

Thank you for your answer!