[libvirt] qemu-kvm networking speed

Daniel P. Berrange berrange at redhat.com
Thu Mar 12 13:42:04 UTC 2009

On Thu, Mar 12, 2009 at 09:28:56AM -0400, Bryan_Coleman at dart.biz wrote:
> How do I change the networking speed provided to my qemu-kvm guest?  I 
> have four gigabit ports bonded; however, my guest is showing just 100Mb/s. 
>  I have used iperf to confirm.

The reported 'speed' setting for emulated NICs is basically irrelevant
since this is emulated hardware. The original real hardware may have
been specced as 100mb/s, but that has no bearing on its speed in a
virtual environment. The QEMU emulated devices will send/recv data until
the hit a bottleneck, typically the CPU usage of the QEMU process is
hit first. So you want a NIC that entails a low CPU overhead in QEMU.
In order of prefernce you'll want to try virtio, e1000, rtl8139, ne2k_pci
This can be set in libvirt xml with <model type='virtio'/> in the guest
XML <interface> section. Obviously you need to adjust your guest OS to
load correct drivers to match.

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