I bought a CFExpress to M.2 reader, and I'm thinking I am going to get a M.2 to USB 3 card (which of course would limit the CFExpress reader to USB3 speeds).A CFExpress card uses PCIexpress to communicate with the host (either single or four lane, type dependent).
How do you connect such card to your RPI? If it's a USB3 reader then that bandwith is your limit. I case you have an addon-on card which uses a PCIe gen 2 switch you should be able to run it from PCIe bus .
Make use of a gen3 switch and you can even go faster (but AFAIk no RPi compatible board available so far).
Test the performance of your theoretical 300MBps uSD and report back what you get in real life.
That's better than the alternative: using a PCIe switch and cutting bandwidth for both the NVMe drive and the card reader in half.
With regards to SD cards, I am not talking about uSD cards. I'm talking about full-size SD cards used in modern digital cameras,
Which "modern digital cameras" are you talking about?
specifially SDXC UHS-II cards, which would connect via USB.
That really doesn't matter. "Slowest link in the chain", remember. The fastest device will spend time waiting on the slowest. Regardless of which direction data is flowing. How much time depends on their relative speeds.
FWIW, on the Pi5 and CM5, there is only one (1) available PCIe lane. The USB controller is part of the RP1 southbridge which has its own PCIex4 link to the SoC. You get more bandwidth per drive with NVMe on PCIe and card reader on USB than you do with both on PCIe. Or with one on each USB3 port.
Statistics: Posted by thagrol — Sun Jan 05, 2025 11:40 pm