My motherboard has a PCIe 3.0 x16 slot, and the platform can split it into two x8 connections. Since PCIe 3.0 x8 offers roughly the same bandwidth as PCIe 4.0 x4, I'm wondering whether a PCIe adapter could convert one PCIe 3.0 x8 connection into a PCIe 4.0 x4 connection so an NVMe SSD could reach its full rated speed. Would this require an active switch or bridge chip rather than a passive adapter?
3 Answers
Your CPU and platform still determine the speed of the connection to the motherboard. An adapter cannot magically upgrade the host controller to PCIe 4.0, and a bridge may introduce a small amount of latency. For storage use, though, a proper PCIe switch should generally be workable if you can find a compatible card.
A passive adapter cannot do this. PCIe links negotiate their generation and lane count, so simply rewiring eight older-generation lanes into four newer-generation lanes will not work. An active PCIe switch or bridge would be required to present a PCIe 4.0 x4 link to the SSD while using PCIe 3.0 x8 upstream.
In principle, a PCIe switch card based on a suitable PCIe 4.0 switch chip could connect to the system over PCIe 3.0 x8 and provide a PCIe 4.0 x4 downstream link. That could give the SSD bandwidth comparable to PCIe 4.0 x4, but these cards are specialized and may cost a lot more than the SSD itself.
They do exist, although availability can be inconsistent. The upstream PCIe 3.0 x8 link remains the overall bandwidth limit, so the SSD cannot exceed what that connection can carry.
PCIe switch chips can handle this kind of translation. They add logic between the host and device, rather than merely combining or rearranging the lanes.