code: public File getFile(int namespaceId, Block b) {
      lock.readLock().lock();
      try {
        DatanodeBlockInfo info = volumeMap.get(namespaceId, b);
        if (info != null) {
          return info.getDataFileToRead();
        }
        return null;
      } finally {
        lock.readLock().unlock();
      }
    }
docs: Turn the block identifier into a filename.