'Check once per class, whether fallocate(1) is available,
and that the instances directory supports fallocate(2).'

def _can_fallocate(self):

    can_fallocate = getattr(self.__class__, 'can_fallocate', None)
    if (can_fallocate is None):
        test_path = (self.path + '.fallocate_test')
        (_out, err) = utils.trycmd('fallocate', '-l', '1', test_path)
        fileutils.delete_if_exists(test_path)
        can_fallocate = (not err)
        self.__class__.can_fallocate = can_fallocate
        if (not can_fallocate):
            LOG.warning('Unable    to    preallocate    image    at    path:    %(path)s', {'path': self.path})
    return can_fallocate
