[libvirt] problems starting several qemu VMS simultaneously

[Serge Hallyn]

Quoting Serge Hallyn (serge.hallyn at canonical.com):
> Quoting Serge Hallyn (serge.hallyn at canonical.com):
> > Quoting Wen Congyang (wency at cn.fujitsu.com):
> > > At 03/22/2012 06:54 AM, Serge Hallyn Wrote:
> > > > Hi,
> > > > 
> > > > I grabbed today's git head of libvirt.  Created a VM (clean install of
> > > > ubuntu oneiric, installed through virt-manager), and cloned it 3 times.
> > > > Then I did
> > > > 
> > > > serge at ubuntu:~$ for i in `seq 1 4`; do virsh start o$i  > /tmp/o$i 2>&1 & done
> > > > [1] 12184
> > > > [2] 12185
> > > > [3] 12186
> > > > [4] 12187
> > > > serge at ubuntu:~$ virsh list
> > > > error: Failed to list active domains
> > > > error: End of file while reading data: Input/output error
> 
> (Note that most of the time, virsh list actually succeeds, only one of
> the virsh start's fails)
> 
> > > I cannot reproduce this problem on RHEL6.

At last, found the trigger.  I can now reproduce this on uptodate fedora
16, using http://people.canonical.com/~serge/breaklibvirt.sh (run as root
from /home/$user).

The trigger is the qemu hook.  If I don't have a qemu hook, then I can do
100 runs of the parallel starts and lists with no failures.  But introduce
a slow hook, and the very first run always fails.

-serge