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To my understanding, a gen3 x4 slot is equal to a gen2 x8 slot in terms of speed.

I assume that by default a gen3 card can not get full speed, because the gen3 card is only x4 physically, and therefore will only use half of an x8 slot. Plus potential protocol differences between gen3 and gen2.
But wouldn't it theoretically be possible to have an adapter condense x8 gen2 to x4 gen3?

Seemingly all information and posts about gen3 <> gen2 are just stating the obvious - that it pcie is backwards compatible, but nothing about using double the lanes. Nor was I able to find any adapters or something.
Is there no usecase for this? I would have imagined that people other than me want to put a gen3 network or storage cards into an older server that only has pice gen2.

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As I understand it, in principle you could do this by putting a PCI express bridge chip* between the slot and the card. Bridges that can do the job certainly exist, it did not take me long to find one on broadcom's website.

In practice though I think such a product would be difficult to get off the ground for three reasons.

  1. The overall appeal is pretty niche to start with, you need someone who has an older server, wants to use a newer card, is desperate to eke out the last bit of performance and understands that such a bridge chip can potentially give them that.
  2. Physical constraints are a PITA, even if you accept that the card must be low-profile and the host regular profile (which cuts out a bunch of servers just to start with) it's still going to be a pretty tight squeeze to get everything into the space available.
  3. Bleeding edge parts tend not to be very accessible to small integrators, most IC vendors tend to make stuff available to their big customers first and the little guys much later if-ever.

* Sometimes also called a switch chip, though that name can be confusing because the term switch is also used to refer to simple signal switches used to re-route PCIe lanes on motherboards.

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  • excellent, thanks for pointing me towards bridge chips. About this being a niche, I thought this should/would be a more common usecase, but I guess I was wrong. I do have one follow up though about pcie adapters. I'll post that as a separate question and would love your input there as well. Thanks Nov 26, 2019 at 23:07

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