@Override
      public ByteBuffer allocate(long size) throws IOException {
        ByteBuffer buffer = fileChannel.map(java.nio.channels.FileChannel.MapMode.READ_WRITE,
          pos.getAndIncrement() * size, size);
        return buffer;
      }
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