@Override
      public FanOutOneBlockAsyncDFSOutput next(FileSystem fs, Path p) throws IOException {
        throw new UnsupportedOperationException();
      }
Create a {@link FanOutOneBlockAsyncDFSOutput}. The method maybe blocked so do not call it
inside an {@link EventLoop}.
hbase-asyncfs/src/main/java/org/apache/hadoop/hbase/io/asyncfs/FanOutOneBlockAsyncDFSOutputHelper.java