@Override
  public CompletableFuture<Long> flush(boolean syncBlock) {
    CompletableFuture<Long> future = new CompletableFuture<>();
    flush0(future, syncBlock);
    return future;
  }
Flush the buffer out to datanodes.
@param syncBlock will call hsync if true, otherwise hflush.
@return A CompletableFuture that hold the acked length after flushing.
hbase-asyncfs/src/main/java/org/apache/hadoop/hbase/io/asyncfs/FanOutOneBlockAsyncDFSOutput.java