Status NewAppendableFile(const std::string& fname, TransactionToken* token,
                           std::unique_ptr<WritableFile>* result) override;
  Status NewReadOnlyMemoryRegionFromFile(
      const std::string& fname, TransactionToken* token,
      std::unique_ptr<ReadOnlyMemoryRegion>* result) override;
  Status FileExists(const std::string& fname, TransactionToken* token) override;