Description of problem:

Inbound data transfers to a Fedora 17 qemu-kvm VM on a Fedora 17 virtualization host from a different physical machine over local gigabit ethernet start reasonably fast, but slow down to very slow speeds (<100KB/s) within 30 seconds of starting.


Version-Release number of selected component (if applicable):

Fedora 17 current live-cd
kernel 3.3.4-5.fc17.x86_64
qemu-kvm-1.0.1-1.fc17.x86_64


How reproducible:

The problem happens for every VM that I start


Steps to Reproduce:
1. Have two physical machines on the same local gigabit ethernet network. The tester machine and the virtualization host.
2. Install Fedora 17, qemu-kvm and virt-manager on the virtualization host
3. Start a new virtual machine in Virtual Machine Manager, specifying the virtual network as "Host device p3p1: macvtap" and "Source mode: Bridge"
4. Test a large inbound data transfer (e.g. a large scp) into the VM from the other physical machine over the local gigabit ethernet network.

[I include a complete list of steps to duplicate using only a Fedora 17 live CD below]


Actual results:

Inbound data transfer starts reasonably fast, but rapidly slows to <100KB/s.


Expected results:

Inbound data transfer should be similar speed to the VM as to the virtualization host.


Additional info:

Here are the complete list of steps to duplicate using only a Fedora 17 live CD:

0. Assumes a virtualization host and a second physical test machine connected on a gigabit ethernet network with a DHCP server allocating IPs.

1. Boot the Fedora 17 live CD on the virtualization host and click on "Try Fedora"

2. At a root prompt run:

yum install virt-manager libvirt qemu-kvm gtk-vnc-devel openssh-server
service libvirtd start
service sshd start
iptables -F
passwd

3. Run virt-manager and create a new virtual machine:

Name: test
Local install media
Use CDROM or DVD: Fedora-17-x86_64-Live-Desktop.iso (/dev/sr0)
OS type: Linux
Version: Fedora 17
Memory: 1024MB
CPUs: 1
Disable storage for this virtual machine (will use Live CD only)
Checked: Customize before install
Advanced options: no changes

In the settings, click on NIC, select "Host device p3p1: macvtap" with "Source mode: Bridge" and click "Apply"

Click "Begin installation"

4. The Fedora 17 live CD will boot in the VM. Click on "Try Fedora"

5. At a root prompt inside the VM, run:

yum install openssh-server
service sshd start
iptables -F
passwd

6. From the external physical test machine test network transfer speed to and from the virtualization host and to and from the VM (e.g. with scp).

You will find that the following commands are fast run on the test machine:

scp ~/1GB-file root@virtualization-host:1GB-file
scp root@virtualization-host:1GB-file ~/1GB-file
scp root@vm:1GB-file ~/1GB-file

But the following command run on the test machine starts fast but rapidly slows down to <100KB/s:

scp ~/1GB-file root@vm:1GB-file

The same holds for other large network transfers - I am simply testing with scp since it gives a visible speed indication.