[libvirt] Problems with <filesystem type='block'>

[Lars Kellogg-Stedman]

Using libvirt 1.0.1, I'm trying to start an LXC container using the
'<filesytem type="block">' syntax, like this:

    <filesystem type="block" accessmode="passthrough">
      <source dev="/dev/vg_files/vm-foobar-root" />
      <target dir="/" />
    </filesystem>
 
The specified block device exists:

    # ls -lL /dev/vg_files/vm-foobar-root
    brw-rw---- 1 root disk 253, 19 Dec 21 22:23 /dev/vg_files/vm-foobar-root

If I start the domain, it appears to start without any errors...

    # virsh start foobar
    Domain foobar started

...but it's not actually running.  The log files (with loglevel=2)
don't seem to be very interesting; this is everything from the
instance log file:

2012-12-22 04:10:57.862+0000: starting up
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin LIBVIRT_DEBUG=2 LIBVIRT_LOG_OUTPUTS=2:stderr /usr/lib/libvirt/libvirt_lxc --name foobar --console 22 --security=none --handshake 25 --background --veth veth1
2012-12-22 04:10:57.967+0000: 1468: info : libvirt version: 1.0.1
2012-12-22 04:10:57.967+0000: 1468: info : lxcCapsInit:151 : No driver, not initializing security driver
PATH=/bin:/sbin TERM=linux container=lxc-libvirt container_uuid=9041e32e-1df2-00c2-4660-dfa5b41510b7 LIBVIRT_LXC_UUID=9041e32e-1df2-00c2-4660-dfa5b41510b7 LIBVIRT_LXC_NAME=foobar /sbin/init
2012-12-22 04:10:58.198+0000: 1: warning : lxcContainerDropCapabilities:1788 : libcap-ng support not compiled in, unable to clear capabilities
2012-12-22 04:10:58.198+0000: 1493: warning : lxcControllerClearCapabilities:679 : libcap-ng support not compiled in, unable to clear capabilities

Running an "strace" on the libvirtd process (strace -p <libvirtd_pid>
-f ...), it doesn't look like libvirt is ever trying to mount the
referenced filesystem.

Is this supposed to work?  It seems like the support for having
libvirt mount the block device is relatively recent, and I haven't had
much luck finding examples of other folks using this capability.

Thanks,

-- Lars