Comment 3 for bug 1046313

Note nbd isn't supported on RHEL, so I presume you manually added that module to your kernel build too.
It might be that you're hitting old qemu bugs. Though looking a bit more at your logs I don't actually
see the umount command being run. That suggests an issue with the nova lxc umount logic :(

If there really is no mention of "Running cmd.*umount" in your logs for the problematic runs,
then could you change the _device_for_path() function in /usr/local/nova/nova/virt/disk/api.py
to add this before the return:

    if not device:
        LOG.debug("Didn't find %s in %s" % (path, open("/proc/mounts").read()))

If that outputs something it should help determine why umount is not being called.

thanks,
Pádraig.