code: public TempBlockMeta createTempBlockMeta(long sessionId, long blockId, long initialBlockSize) {
      return new TempBlockMeta(sessionId, blockId, initialBlockSize, mDir);
    }
docs: Creates a {@link TempBlockMeta} given sessionId, blockId, and initialBlockSize.
  
  @param sessionId of the owning session
  @param blockId of the new block
  @param initialBlockSize of the new block
  @return a new {@link TempBlockMeta} under the underlying directory