The 4-Drive M.2 NVMe SSD to PCIe 3.0 x4 Adapter Card is nice, but it matches 16 PCIe 3.0 downstream lanes to 4 PCIe upstream lanes. This would be useful for an array of SSDs (or other M.2 form-factor add-in cards) that are on PCIe 2.0 or use less than half of a lane of PCIe 3.0 bandwidth, but the vast majority of SSDs are already capable of saturating PCIe 3.0 × 4, and the motherboards they are attached to are capable of PCIe 4.0 or 5.0. Thus, the adapter card’s design is a bottleneck in what could otherwise be a much more efficient utilization of the underutilized PCIe bandwidth on tap.
Working out the theoretical efficiencies of each hypothetical adapter pair with different sets of SSDs:
| 4 × PCIe 4.0 Upstream | 8 × PCIe 4.0 Upstream | 4 × PCIe 5.0 Upstream | 8 × PCIe 5.0 Upstream | |
| 4 × PCIe 3.0 SSDs | 50% efficiency | 100% efficiency | 100% efficiency | 50% efficiency |
| 8 × PCIe 3.0 SSDs | 25% efficiency | 50% efficiency | 50% efficiency | 100% efficiency |
| 4 × PCIe 4.0 SSDs | 25% efficiency | 50% efficiency | 50% efficiency | 100% efficiency |
| 8 × PCIe 4.0 SSDs | 12.5% efficiency | 25% efficiency | 25% efficiency | 50% efficiency |
| 4 × PCIe 5.0 SSDs | 12.5% efficiency | 25% efficiency | 25% efficiency | 50% efficiency |
Most SSDs barely saturate the PCIe lanes they operate over, and most only ever use a little more than a half of their possible bandwidth, so 50% efficiency could be taken as a sweet spot goal to target.
Motherboards with 4 × PCIe 5.0 slots hardly exist. I believe there is only a handful of such motherboards, so a 4 × PCIe 5.0 upstream card would be a mismatch of capabilities for most users. If not 8 × PCIe 4.0, an 8-lane upstream/16-lane downstream configuration would be the most favored choice since there aren’t any PCIe 5.0 switches on the market with less than 24 lanes. An 8 × PCIe 5.0 adapter card that could accommodate 4 PCIe 5.0 SSDs would also be able to target a possibly burgeoning market of 2-lane PCIe 5.0 SSDs such as the SAMSUNG 990 EVO 5.0.
I’d understand if a PCIe 5.0 variant poses insurmountable challenges to bring to market. After all, the likes of Microchip and Broadcom haven’t even brought such adapters to market despite being the designers of the very PCIe 5.0 switch chips on the market that would be needed to make them possible!
