docs: @Override
      public long alloc(int chunks, long prevPos, int prevChunks) {
          long ret = s.allocReturnCode(chunks);
          if (prevPos >= 0)
              s.free(prevPos, prevChunks);
          if (ret >= 0)
              return ret;
          while (true) {
              s.nextTier();
              ret = s.allocReturnCode(chunks);
              if (ret >= 0)
                  return ret;
          }
      }