private void writeTrailer(Path trailerPath, FixedFileTrailer t, byte[] useBytesInstead)
    throws IOException {
    assert (t == null) != (useBytesInstead == null); // Expect one non-null.

    FSDataOutputStream fsdos = fs.create(trailerPath);
    fsdos.write(135); // to make deserializer's job less trivial
    if (useBytesInstead != null) {
      fsdos.write(useBytesInstead);
    } else {
      t.serialize(fsdos);
    }
    fsdos.close();
  }
The number of used fields by version. Indexed by version minus two. Min version that we support
is V2
hbase-server/src/test/java/org/apache/hadoop/hbase/io/hfile/TestFixedFileTrailer.java