PCIE Gen 3.0 SSD on PCIE Gen 4.0 x2 M.2 Slot Banwidth?

[SonOfaMonkey]

I have a PCIE Gen 4.0 x570 mobo with a PCIE 2x M.2 slot going to the chipset. I'm wondering if I can put a Gen 3 NVME SSD that's rated for 3.5GB/s on the M.2 and get the full potential of the SSD, given that PCIE 4.0 is 2GB/s per lane? Or will the NVME SSD only be able to take advantage of 2x lanes in PCIE 3.0 mode at 1GB/s per lane, since NVME drives are set up for x4 lanes?

[are512]

Im unsure about the technical reasoning, But I also have a NVME PCIE 3 SSD going into a PCIE4 slot, and the crystal disk benchmark shows no detriment compared to expected values

[mariushm]

The m.2 slot is backwards compatible with pci-e 3.0, so it will switch to pci-e 3.0 mode if you install a pci-e 3.0 SSD (or device) in it.  

 

PCI-E 3.0 is ~ 970 MB/s per lane, and you have 4 lanes in a M.2 connector, so you have up to 3880 MB/s

PCI-E 4.0 is ~1940 MB/s per lane, and you have 4 lanes in a M.2 connector, so you have up to 7760 MB/s  

 

The number of lanes doesn't change, just the bandwith changes.

[SonOfaMonkey]

2 hours ago, mariushm said:

The m.2 slot is backwards compatible with pci-e 3.0, so it will switch to pci-e 3.0 mode if you install a pci-e 3.0 SSD (or device) in it.

Ok, so it seems I'd be limited to 2GB/s total on an x2 slot since it would revert to PCIE 3.0 mode.

 

Tangential question: Are there any downsides to putting that x2-limited M.2 in Raid 0 with an x4 U.2 mobo connected drive? Not for a boot drive, but as a storage drive and maybe I could get faster speeds than either drive alone? Or would it ruin random read/write with the mismatched PCIE lanes and connectors?

[BlenderRookie]

On 1/25/2021 at 10:14 AM, mariushm said:

The m.2 slot is backwards compatible with pci-e 3.0, so it will switch to pci-e 3.0 mode if you install a pci-e 3.0 SSD (or device) in it.  

 

PCI-E 3.0 is ~ 970 MB/s per lane, and you have 4 lanes in a M.2 connector, so you have up to 3880 MB/s

PCI-E 4.0 is ~1940 MB/s per lane, and you have 4 lanes in a M.2 connector, so you have up to 7760 MB/s  

 

The number of lanes doesn't change, just the bandwith changes.

Thanks for this information. I assumed that's how that would work but it's great to have confirmation.