[Bug 571408] [NEW] libvirt-bin on latest lucid: heavy packet loss

[Fionn]

Public bug reported:


I have to report a very strange bug here which I came upon today:

I am running a quad core server with 4 KVM guests. Host and all guests are all lucid latest. Guests run in a bridged setup for outside communication as well as in the default KVM network for guest-to-guest communications.
This setup is complete and worked already while I setup the guests, until yesterday the Host and all guests started to show heavy packet loss on their network interfaces. The host computer did not expose any packet loss if libvirt was not started on boot and when libvirt was started, packets began to fall into oblivion. Overall, the loss rate was about 75-95%.
I first thought that one of the latest libvirt or kernels updates might be the the culprit but downgrading all libvirt and kvm packages as well as going back one kernel version did not change the situation.

Then I began to experiment with iptables and network settings and after a couple of hours found out:
When the packet loss occurs, doing 
sysctl -w net.ipv4.ip_forward=0

stops the packet loss problem immediately. But even more interestingly,
setting ip_forward back to 1 does NOT cause the packet loss problem to
reappear! The system does now run as expected again and I made a little
checker script that will execute the two sysctls with some seconds in
between, should packet loss be detected again.

Nevertheless, I thought this might be worth reporting and maybe one of
you developers wants to have additional information.

Package information:
linux-image-2.6.32-21-server    2.6.32-21.32
qemu-kvm                        0.12.3+noroms-0ubuntu9
libvirt-bin                     0.7.5-5ubuntu27 
libvirt0                        0.7.5-5ubuntu27 
python-libvirt                  0.7.5-5ubuntu27
ubuntu-virt-server              1.2 
qemu-common                     0.12.3+noroms-0ubuntu9

** Affects: libvirt (Ubuntu)
     Importance: Undecided
         Status: New

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libvirt-bin on latest lucid: heavy packet loss
https://bugs.launchpad.net/bugs/571408
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