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

def _get_vg_free_pe(self, vg_name):

    build_command = ['vgdisplay', vg_name, '--units', 'g']
    (rc, stdout, err) = self._run_command(build_command)
    if (rc != 0):
        self.failure(err=err, rc=rc, msg=('failed    to    read    vg    %s' % vg_name), command='    '.join(build_command))
    vg_info = [i.strip() for i in stdout.splitlines()][1:]
    free_pe = [i for i in vg_info if i.startswith('Free')]
    _free_pe = free_pe[0].split()
    return (float(_free_pe[(-2)]), _free_pe[(-1)])
