Where'd ya get this "~5 GB/s max total" figure from?
Testing of RAID 0 using two Thunderbolt NVMe enclosures has been done with dual Titan Ridge discrete Thunderbolt controllers of Mac mini (2018) and MacBook Pro (16-inch, 2019) and integrated Ice Lake Thunderbolt controllers of MacBook Pro.
Thunderbolt 3 add-in cards are PCIe 3.0 x4. PCIe 3.0 x4 is 31.5 Gbps (3938 MB/s) max but usually allows up to 3500 MB/s (28 Gbps) after considering PCIe protocol overhead (look for benchmarks of the fastest NVMe drives or the Host To Device or Device To Host speeds of graphics cards)...
forums.macrumors.com
Thanks to @joevt for the suggestion of raiding two high-speed NVMe drives through Thunderbolt 3 ports, I was able to confirm the on-die TB3 controller...
egpu.io
- One port of Ice Lake: 2485 MB/s (20 Gbps).
- Two ports of Ice Lake: 4668 MB/s (37 Gbps). Three ports: 4778 MB/s (38 Gbps).
- Two ports of dual Titan Ridge : 4774 MB/s (38 Gbps)
- Two ports of single Titan Ridge: 2948 MB/s (24 Gbps)
It's weird that it has that limit eh. Do you know if TB4 rectified this and bumps it up to the full 32 Gbps?
Coz I know TB4 mandates that 32 Gbps has to be available to the controller, so you'd think that artificial restriction has been sorted, otherwise they would've said TB4 must have 22 Gbps at least.
32 Gbps is for the connection (PCIe 3.0 x4) between the Thunderbolt controller and the computer. It is actually 8 GT/s/lane * 4 lanes * 128b/130T = 31.5 Gbps but that doesn't include PCIe overhead or Thunderbolt overhead which gets you the 22 Gbps that Intel stated in the original marketing material (in the discrete Alpine Ridge days).
https://thunderbolttechnology.net/sites/default/files/Thunderbolt3_TechBrief_FINAL.pdf
If you search for NVMe benchmarks, you might come up with a number like 3500 MB/s which is 28 Gbps. Thunderbolt overhead will reduce that.
Thunderbolt 4 doesn't improve things by much. It is integrated in Tiger Lake CPUs like in Ice Lake CPUs. Integrated means it can be more efficient (it's inside the CPU so it doesn't use real PCIe and the distances are much shorter). One YouTube video showed 3000 MB/s (24 Gbps). It was using a different benchmark though so I don't think it can be directly compared.
The USB4 spec says about bandwidth for PCIe tunnelling (like in Thunderbolt):
"The amount of buffering at the PCIe Adapter is implementation specific as it balances the tradeoff between PCIe tunneling performance and PCIe link latency. It is recommended that implementations make the amount of buffers configurable."
It says the same thing for USB tunneling (new to Thunderbolt 4 and USB4), just replace PCIe in the above quote with USB3.
If the amount of buffers were configurable, it is not configurable by the user.
