@Override
  public void write(DataOutput out) throws IOException {
    out.writeShort(THREE_LONGS);
    out.writeLong(pathHash);
    out.writeLong(modTime);
    out.writeLong(length);
  }
Length allows for some backward compatibility wrt field addition.
ql/src/java/org/apache/hadoop/hive/ql/io/SyntheticFileId.java