@Override
  public Cacheable read(BucketEntry be) throws IOException {
    ByteBuff dst = be.allocator.allocate(be.getLength());
    bufferArray.read(be.offset(), dst);
    dst.position(0).limit(be.getLength());
    return be.wrapAsCacheable(dst);
  }
IO engine that stores data to a file on the local block device using memory mapping mechanism
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/bucket/ExclusiveMemoryMmapIOEngine.java