code: public static String commitPath(StorageDir dir, long blockId) {
      return PathUtils.concatPath(dir.getDirPath(), blockId);
    }
docs: Committed block is stored in BlockStore under its {@link StorageDir} as a block file named
  after its blockId. e.g. Block 100 of StorageDir "/mnt/mem/0" has path:
  <p>
  /mnt/mem/0/100
  
  @param blockId the block id
  @param dir the parent directory
  @return committed file path