Thunderbolt Transfer Bottleneck from PXIe-8881 to External PCIe 4.0 Disk

Issue Details

Is there a downstream bottleneck when transferring data through Thunderbolt from a PXIe-8881 controller to an external PCIe 4.0 NVMe disk? I am expecting at least 3000MBps of transfer rate, but only get 900MBps.

Solution

The theoretical maximum PCIe bandwidth that can be tunneled through Thunderbolt is around 2.4GB/s.

The total throughput of Thunderbolt is 5GB/s but not all of this is available for PCIe tunneling because some bandwidth was reserved for display and USB.

It’s important not to rely solely on marketing numbers for actual throughput. You need to consider the entire data path. For example, if you are using a benchmarking tool to check SSD throughput, the data will be generated by the CPU and then copied to the SSD or vice versa.

However, if you are copying data from another source, such as an SSD on a PXIe-8881, the data path would be: PXIe-8881 SSD -> Platform Controller Hub (PCH) -> CPU -> PCH -> Alpine Ridge Thunderbolt Controller -> Thunderbolt external SSD.

There might be a bottleneck at the DMI link between the PCH and CPU or other components along the data path.