void closeFileChannels() {
    for (FileChannel fileChannel : fileChannels) {
      try {
        fileChannel.close();
      } catch (IOException e) {
        LOG.warn("Failed to close FileChannel", e);
      }
    }
  }
Transfers data from file to the given byte buffer
@param be an {@link BucketEntry} which maintains an (offset, len, refCnt)
@return the {@link Cacheable} with block data inside.
@throws IOException if any IO error happen.
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/bucket/FileIOEngine.java