Super User is a question and answer site for computer enthusiasts and power users. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

i'm using qemu-kvm(1.1.1) to run a Windows 2008 R2 Virtual Machine. The host machine is running CentOs 6.3. I'm exporting a USB 2.0 Hdd device to the VM using USB-redirection with spice channel. So far, everything is working fine, except that the speed is very slow. I'm getting 5MB/s max, against 25-30 MB/s when the hdd is mounted on hte host directly. Do you have any idea of what could cause such a limitation. Is there any tweak to increase the speed. Any help would be very much appreciated.


share|improve this question

This is a limitation with how the USB is redirected over the network. Its not just an issue with network performance but Qmenu/kvm implementation it self perhaps some thing they will improve on on later releases.

Try using Using USB pass-through instead of spice over the network. I think the spice is good when you are trying to mount USB from the thin client. But if you are plugging up the USB right into the server USB pass-through will give you the performance you need. Make sure that the USB is not mounted on the host system.

you can get the vendor id using /sbin/lsusb look for "ID :" of the desired USB device

Edit your vm config file.

    <hostdev mode='subsystem' type='usb'>
        <vendor id='0x0a5c'/>
        <product id='0x2110'/>
share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.