Ignoring the limitations of the SD card or SSD (via USB) what is the limitation on read and write speed in MB/s provided by the RPi 4B itself on the SD card and USB connection.

I don't know much about I/O but I imagine there is some maximum that the bus that goes from the SD card slot to RAM, or the USB slot to RAM actually supports (although perhaps this is not a function of random vs sequential as it would be with the disk).

So if my SD card or SSD supported infinite reads/s and infinite writes/s how much would I actually get on a RPi 4B?

  • I'm not sure whether this information is available, but in practical terms you might as well just look at one of the umpteen tests online done with various cards etc. Generally, the speed with a fast card is well below the card's max, or a parallel test done on an x86-64 box. Note that there are still differences, however, which means even if a card can work much faster than than it does on a Pi, there is not a universal cutoff speed whereby card A with a normal max of 100 and card B 150 will both test exactly the same on a Pi because they "top out" at 50. One may still be faster.
    – goldilocks
    Jun 6, 2023 at 15:39
  • a good SSD attached with a goof USB 3.0 to SSD adapter/cable will always be faster than even the best SD card - I can't give you speeds, because it's irrelevant Jun 7, 2023 at 1:29

1 Answer 1


lspci -vv list a VIA Technologies, Inc. VL805 USB 3.0 Host Controller

On Via Labs website (https://www.via-labs.com/product_show.php?id=48) you can find the following description:

The VIA Lab’s VL805 (4-Port)/VL806 (2-Port) is a USB 3.0 Host controller, which enables a PCI Express equipped platform to interface with USB Super-Speed (5 Gbps), High-Speed (480 Mbps), Full-Speed (12 Mbps), and Low-Speed (1.5 Mbps) devices. The root hub consists of four downstream facing ports enabling the simultaneous operation of multiple peripheral devices.

Please keep in mind that the 5 Gbps is a theoretical value. The actual speed depends on used drivers and in case of a SATA SSD on the SATA to USB adapter.

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