Network stuttering


Hi all,

I have the following problem:

I have a server with sufficient ressources (12 GB RAM, Quad-Core XEON 5600, Supermicro board with Intel XEON chipset) ond one big drive (a raid array residing on a 3ware / amcc 9650SE).

I have installed Ubuntu natty including qemu 0.13.0. I am not using libvirt since it did not work for me.

On the drive, there are 3 partitions: One for the main system, one for a guest (Windows Server 2003 R2), one for another guest (Windows Server 2008 R8). This setup is working like a charm except for the following thing:

When I start only the Windows 2003 guest, its network is as fast as usual. But as soon as I additionally start the Windows 2008 guest, the network of the 2003 guest begins to stutter. It seems to be nearly as fast as before, but the connection stalls in non-regular intervals. For example, when streaming music from the Windows 2003 server to some clients which really is a low bandwidth task, the music is interrupted from time to time at the client. This even happens when only one client is connected, and it happens only if the Windows 2008 guest is running also.

I am starting the guests using the following commands:

/usr/bin/qemu-system-x86_64 -M pc -cpu host -drive file=/dev/sda6,if=virtio,cache=none,index=0 -pidfile ./ -boot c -k de -m 3072 -smp 1 -daemonize -usb -usbdevice “tablet” -name garak -net nic,vlan=0,model=virtio -net tap,vlan=0,ifname=virtnet1,script=/etc/qemu-ifup,downscript=/etc/qemu-ifup -vnc :1

/usr/bin/qemu-system-x86_64 -M pc -cpu host -drive file=/dev/sda7,if=virtio,cache=none,index=0 -boot c -pidfile ./ -m 6144 -smp 1 -k de -daemonize -usb -usbdevice “tablet” -name dax -net nic,vlan=0,model=virtio -net tap,vlan=0,ifname=virtnet0,script=/etc/qemu-ifup,downscript=/etc/qemu-ifdown -vnc :2

Please note that I already have tested to use e1000 instead of virtio for the network and using ide instead or virtio for the block devices. As you can see, the guests are residing on an own partition each since this seems to be much faster than image files.

Does anyone have any idea regarding the low-bandwidth network stuttering?

Thank you very much,