docs: public long approximateDataSize() {
          long result = 0;
          for (Map.Entry<String, DataNode> entry : nodes.entrySet()) {
              DataNode value = entry.getValue();
              synchronized (value) {
                  result += entry.getKey().length();
                  result += (value.data == null ? 0
                          : value.data.length);
              }
          }
          return result;
      }