Pcie4 NVME on Pcie3 board

Rand__

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Mar 6, 2014
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Originally I had planned to skip Pcie4 in favour of the upcoming PCIe5, but just been told IcyDock are actually going to ship out my replacement MB699VP-B V2 today (trade up due to incompatibility issues) so I find myself with a PCIE 4 capable NVME backplane all of a sudden;)

1. Are there any advantages on running Pcie4 NVMEs on a PCIE3 bus (eg via onboard Occulink port)?
O/C it should max out the x4 link (opposed to PCIe3 NVME drives), but are there protocol advantages too (like SAS3 > SAS2) ?

2. I assume if there are PCI4 to 3 adapters in existence (x4->x8, x8->x16) they are slow as hell, hugely expensive and not intended for the mass market?

Bonus Question - are the Supermicro BPN XXX- A-N4 Backplanes PCIE4 capable, i.e are they just passing through whatever they get similar for SAS/SATA or are there active parts on it?

Thanks :)
 

i386

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Bonus Question - are the Supermicro BPN XXX- A-N4 Backplanes PCIE4 capable, i.e are they just passing through whatever they get similar for SAS/SATA or are there active parts on it?
The backplanes with 2-8 nvme slots are all passive and should work with pcie 4.0.
Should because of cable quality it could fall back to pcie 3.0 speeds.
 
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NateS

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Apr 19, 2021
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1. Are there any advantages on running Pcie4 NVMEs on a PCIE3 bus (eg via onboard Occulink port)?
O/C it should max out the x4 link (opposed to PCIe3 NVME drives), but are there protocol advantages too (like SAS3 > SAS2) ?
Not really. The Gen4 drive will just fall back to Gen3 speeds, and will function exactly as a Gen3 drive would. Possibly since the Gen4 drive has better signal integrity specs, you'd be able to get away with longer-than-spec cables for your Gen3 signal, but that's about it.

Unlike with SAS/SATA, the protocol spec (NVMe) is separate from the electrical spec (PCIe). You can find both Gen3 and Gen4 drives with all different NVMe versions.

2. I assume if there are PCI4 to 3 adapters in existence (x4->x8, x8->x16) they are slow as hell, hugely expensive and not intended for the mass market?
Well, there's a possibility that a Gen4 switch chip might be able to do it, such as the ones found in some HBAs. Theoretically, since the switch chip has independent links to both the host and the drive, it could link to the host at Gen3x16 for example, and the drive at Gen4x4, and forward the data between them at full bandwidth. I'm not sure if all (or any) switches would support this though, so you might want to confirm with the manufacturers that this would work before spending money on it.

Bonus Question - are the Supermicro BPN XXX- A-N4 Backplanes PCIE4 capable, i.e are they just passing through whatever they get similar for SAS/SATA or are there active parts on it?
They're likely passive, but that doesn't necessarily mean they'll work with Gen4. Often Gen4 backplanes will need retimer chips to drive cables long enough to reach the motherboard, but if you're able to physically position it so that you can use short cables, it might work.
 
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skorpioskorpio

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The only issues I've ever read about involve intermediate passive components in the middle that do not adhere to the negotiated specs of the end points. So connecting a PCIe v4 slot to a PCIe v4 capable card with a PCIe v3 rated riser cable. So in your case, I guess that would be either the SFF cable or maybe the Occulink board, though if that has active component that should negotiate the connection.

The only place I've seen this come up as a reoccurring issue is with people building small form factor gaming machines where the motherboard and the graphics card both do PCIeV4 but the case came packaged with a PCIeV3 riser. So I have to assume that there is either some keying contact of some sort that is different between the 2 -or- these PCIe riser cables are just built without enough engineering OCD to tolerate the speed difference.