@Override
  protected OutputStream getOutputStreamForCellEncoder() {
    return asyncOutputWrapper;
  }
We have to call {@link AsyncFSOutput#getSyncedLength()} after {@link AsyncFSOutput#close()}
to get the final length synced to underlying filesystem because {@link AsyncFSOutput#close()}
may also flush some data to underlying filesystem.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/AsyncProtobufLogWriter.java