Impact of lane sharing on USB 4 ports

[alderman]

I was debating purchasing a Samsung 990 EVO Plus SSD to place in Slot M2_2 on a MAG X870E TOMAHAWK WIFI.
The EVO Plus supports both PCIe 4.0 x4 and PCIe 5.0 x2 interfaces.

So if I put the EVO Plus into Slot M2_2, I can set it to PCIe 5.0 x2. Can anyone clarify the impact on the USB 4 ports? What's the real world change in performance and functionality of these ports?

According to the datasheet:

USB 40Gbps Type-C ports on the back panel and M2_2 slot share PCIe 5.0 x4 bandwidth. Both run at PCIe 5.0 x2 when a device is installed in the M2_2 slot. You can switch M2_2 to PCIe 5.0 x4 in the BIOS, but this will disable the USB 40Gbps Type-C ports. The USB4 host controller supports up to PCIe 4.0 x4.

Also given the choice between a Samsung 990 EVO Plus SSD running PCIe 5.0 x2 and a Samsung 990 Pro running at PCIe 4.0 x4, anyone have any advice as to which to choose if they're similarly priced and it's a desktop?

 

[badboy2k]

I was debating purchasing a Samsung 990 EVO Plus SSD to place in Slot M2_2 on a MAG X870E TOMAHAWK WIFI.
The EVO Plus supports both PCIe 4.0 x4 and PCIe 5.0 x2 interfaces.

So if I put the EVO Plus into Slot M2_2, I can set it to PCIe 5.0 x2. Can anyone clarify the impact on the USB 4 ports? What's the real world change in performance and functionality of these ports?

According to the datasheet:


Also given the choice between a Samsung 990 EVO Plus SSD running PCIe 5.0 x2 and a Samsung 990 Pro running at PCIe 4.0 x4, anyone have any advice as to which to choose if they're similarly priced and it's a desktop?

at x2 5.0 it will top out at 8GB/s, x4 4.0 is also 8GB/s and the real world difference with the USB dropping to x2 lanes is 0% as its still got 80Gb/s so could run 2 40Gb/s Ports just fine

 

[zotta.vla15dc02ea]

What happens when you install the ssd in that slot is pretty clear. "Both run at PCIe 5.0 x2 when a device is installed in the M2_2 slot. You can switch M2_2 to PCIe 5.0 x4 in the BIOS, but this will disable the USB 40Gbps Type-C ports." I have a B850 but for the X870E Tomahawk I found this on reddit for you (this is in case you install the 990 Pro there) "You can disable two of the back USB c ports to get full x4 on m2 slot 2. If you want those two slots but still full speed Samsung 990 Evo plus is the only option as it can do 5.0x2 for full speed. Other 4.0 drives will go half speed at 4.0x2. since it's the E model the rest of the slots will have full 4 lanes."

Bandwidth should theoretically be the same between 5x2 and 4x4 BUT the real difference is that 990 Pro has DRAM cache which is better for large file transfers and sustained speeds so it performs a bit better (at 4x4 of course). Depends what you are doing on your desktop. If only gaming, either one is good but if also similarly priced, then 990 Pro is a better choice overall. You have a comparison here. It also depends if you really need those USB4 ports at their max speed or not. Lane sharing and not needing USB4 made me choose the B850 Tomahawk instead (which still shares M2_3 with the seconds PCIE slot but I'm not using either one of them for now).

 

[DarkOmenX]

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