28 or 14 usb mass storage devices on raspberry pi4

[Terror404]

Hello!

I'm sorry, if this is in the wrong section, but I don't really know, what is causing my problem.

My application: I want to connect 28 mass storage devices to my rpi4. I've built a system with four 7-port-usb-hubs. These hubs are powered with 20 amps in total. On my rpi3 just 14 devices showed up in /dev. On my rpi4 21-24 devices are showing up. At first I thought, that it is a problem caused by too low current. So I tried everything with Y-cables in addition. Same result. So now am I at the point where I think, that this is not caused by too low current. Raspberry has a 5.1V 3A source too. In /boot/config.txt I set max_usb_current=1. Still the same result.

Now I decided to just use 14 devices per raspberry and use two of them connected via ethernet. But sometimes 7 devices disconnect and reconnect after 2-3 seconds (this happens every 2-5mins). This is not caused by the hubs, as I used different hubs from different manufacturers. 7 devices on their own are stable everytime. But if I just plug one device into the second hub, it sometimes is reconnecting.

I also tried to connect this "hub tree" with all my 28 devices to an expensive laptop (~2000 euros) with windows 10. All 28 are recognised.
Cheap desktop pc with windows 10 (~200 euros): 21
Rpi4: 21-24
Rpi3: 14

Is it possbile that the usb hardware is just not "fast" enough to handle that many devices?

At first I want it to work with two rpi4 each connected to 14 devices without reconnecting to have a backup-solution.
But in the best case it should work with only one rpi4.

Does anyone know, what is causing this problem?

Sorry for my bad english,
terror404


Pi: Raspberry Pi 4 Model B
OS: Raspbian Buster

[jamesh]

There is an upper limit (different on Pi3 vs Pi4) on how many devices can be attached.

You are also going to be limited by bandwidth, especially with USB2 devices.

I'd be inclined to use more Pi4 - perhaps 5 devices per Pi. You will get better performance.

lsusb -t might give more information.

[LTolledo]

are those "mass storage device" USB thumb drives or are those of another type? (you have not mentioned that)