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When starting a VM, /var/log/messages was spammed with the following message: xt_physdev: using --physdev-out in the OUTPUT, FORWARD and POSTROUTING chains for non-bridged traffic is not supported anymore. With each extra VM I start, the messages get amplified exponentially. This results in longer starting times every new VM, relative the the previously started VM. When I ran a test with starting 100 equal VM's, the first VM started in about 2 seconds, the 100th VM took 48 seconds to start. I'm running a vanilla 3.7.1 kernel, but I have the same issue on VM hosts with kernel 3.2.28 or 3.2.0, running libvirt 0.9.12 and 0.9.8 respectively. Looking into the warning, it seemed that iptables need an extra argument, --physdev-is-bridged, in commands like: iptables -A libvirt-out -m physdev --physdev-is-bridged --physdev-out vnet99 -g FP-vnet99 With that, the warnings in /var/log/messages are gone and running the test again proved the 100th VM started in 3.8 seconds.
LibVirt : simple API for virtualization
Libvirt is a C toolkit to interact with the virtualization capabilities
of recent versions of Linux (and other OSes). It is free software
available under the GNU Lesser General Public License. Virtualization of
the Linux Operating System means the ability to run multiple instances of
Operating Systems concurrently on a single hardware system where the basic
resources are driven by a Linux instance. The library aim at providing
long term stable C API initially for the Xen paravirtualization but
should be able to integrate other virtualization mechanisms if needed.
Daniel Veillard <veillard@redhat.com>