@Override
  public String toString() {
    return "ioengine=" + this.getClass().getSimpleName() + ", path=" + this.path + ", size="
      + String.format("%,d", this.size);
  }
IO engine that stores data to a file on the specified file system using memory mapping mechanism
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/bucket/FileMmapIOEngine.java