code: private void formatImage(NamespaceInfo nsInfo) throws IOException {
      Preconditions.checkState(nsInfo.getNamespaceID() != 0,
          "can't format with uninitialized namespace info: %s",
          nsInfo.toColonSeparatedString());
      LOG.info("Formatting image " + this.getJournalId() + " with namespace info: (" +
          nsInfo.toColonSeparatedString() + ")");
      imageStorage.backupDirs();
      imageStorage.format(nsInfo);
      // clear obsolete image digests
      checkpointImageDigests.clear();
    }
docs: Format the local storage with the given namespace.