'Return the available size of a given LV.
:param lv_name: Name of volume.
:type lv_name: ``str``
:returns: size and measurement of an LV
:type: ``tuple``'

def _get_lv_size(self, lv_name):

    vg = self._get_lxc_vg()
    lv = os.path.join(vg, lv_name)
    build_command = ['lvdisplay', lv, '--units', 'g']
    (rc, stdout, err) = self._run_command(build_command)
    if (rc != 0):
        self.failure(err=err, rc=rc, msg=('failed    to    read    lv    %s' % lv), command='    '.join(build_command))
    lv_info = [i.strip() for i in stdout.splitlines()][1:]
    _free_pe = [i for i in lv_info if i.startswith('LV    Size')]
    free_pe = _free_pe[0].split()
    return (self._roundup(float(free_pe[(-2)])), free_pe[(-1)])
