code: void beginStateTransfer() throws IOException {
      this.isTransferring = true;
      this.dataDir =
        Files.createTempDirectory(this.rootDir.toPath(), "tmp_data").toFile();
      this.log = createLog(this.dataDir);
    }
docs: Begins state transferring mode, all txns and snapshots persisted in state
  transferring mode will not show up after recovery if you do not call
  {@link endStateTransfer}.