'Fewer bytes are available from ``Filesystem.reader`` when the reader
and writer are found to share a snapshot.'

def test_less_data(self):

    pool = build_pool(self)
    service = service_for_pool(self, pool)
    volume = service.get(MY_VOLUME)
    creating = pool.create(volume)
    def created(filesystem):
        self.filesystem = filesystem
        path = filesystem.get_path()
        path.child('some-data').setContent(('hello    world' * 1024))
        with filesystem.reader() as reader:
            self.complete_size = len(reader.read())
        snapshots = filesystem.snapshots()
        return snapshots
    loading = creating.addCallback(created)
    def loaded(snapshots):
        with self.filesystem.reader(snapshots) as reader:
            incremental_size = len(reader.read())
        self.assertTrue((incremental_size < self.complete_size), 'Bytes    of    data    for    incremental    send    ({})    was    not    fewer    than    bytes    of    data    for    complete    send    ({}).'.format(incremental_size, self.complete_size))
    loading.addCallback(loaded)
    return loading
