USB Hub questions

I have a couple questions about usb hubs:

If I only have one device connected to it, is the speed as fast as if I plugged it in directly to computer?

If I have 2 flash drives plugged into and copy data simultaniously to my computer does the data alternate or do they both get transfered at the same time? (my usb flash drives max out at a read of 5 mb/s and i know usb can go much faster (20 mb/s or so))

If i copy data from 1 flash drive to another (both are plugged into the hub), does the data go to the computer or does the hub just redirect the data?

agz

Posted 2013-03-18T02:08:09.130

Reputation: 6 820

Answers

If you have one device plugged into a USB hub, assuming it and the USB device have the same USB version, then the device will run at the same speed. Each USB hub does add a little latency, but you would not notice it.

When copying from 2 devices simultaneously, if they are on the same hub, the flow will alternate from device to device. But this happens so quickly, its as if they were simultaneous. You will always get better performance by spreading your USB devices across different hubs - like the connecting a device to the front USB ports and another to the back. USB has different versions, supporting different speeds. Your hub might be version 2, which is up to 35 MB/s, and your slower flash drive might be version 1, which transfers at 12 Mbit/s.

As for copying data from one flash drive to another through the same hub, the data will always flow through the computer. USB hubs do not understand filesystems and/or devices connected to it, they just provide connectivity - and possibly power.

Keltari

Posted 2013-03-18T02:08:09.130

Reputation: 57 019

So for question 2, would both flash drives be transferring at 12mbit to the computer because the 2 together doesnt excede the 35 mb/s transfer? – agz – 2013-03-18T03:22:29.770

transfers will flow at the lowest speed of the chain. So a USB v3 device's speed would be wasted on a USB 1 hub. And if you have a fast v3 hub, plugging in v1 device, the device will run at v1 speeds. – Keltari – 2013-03-18T03:23:06.177

Ok thats good to know, but my question was if 2 devices run at 12 mbit, will the speed in the cable between the hub and the computer run at 24 mbits? – agz – 2013-03-18T19:33:29.973

no, the hub can only sustain 12 mb/s – Keltari – 2013-03-18T22:37:01.307

If I only have one device connected to it, is the speed as fast as if I plugged it in directly to computer?

It depends on the hub. But assuming the hub has its own source of power and operates at the fastest transfer rate the device and port support, then yes. Otherwise, it could force the device into a lower power mode or a lower transfer speed.

If I have 2 flash drives plugged into and copy data simultaniously to my computer does the data alternate or do they both get transfered at the same time? (my usb flash drives max out at a read of 5 mb/s and i know usb can go much faster (20 mb/s or so))

The flash drives transfer data internally at the same time. The USB bus alternates. This is one of the reasons devices have buffers. While the total transfer rate from both devices can't exceed the port's speed, you will see both devices transferring at their full internal speed thanks to buffering.

If i copy data from 1 flash drive to another (both are plugged into the hub), does the data go to the computer or does the hub just redirect the data?

It goes to the computer. The data can't be redirected because it isn't the same on both devices. Filesystems don't just store data, they store metadata as well. They have journalling rules, buffer flushing rules, block sizes, and so on. There just isn't enough smarts on either device for it to talk directly to the other -- neither device understands its own filesystem let alone the other device's filesystem.

David Schwartz

Posted 2013-03-18T02:08:09.130

Reputation: 58 310