/**
   * A marker file that is created on each root directory if a rolling upgrade
   * is in progress. The NN does not inform the DN when a rolling upgrade is
   * finalized. All the DN can infer is whether or not a rolling upgrade is
   * currently in progress. When the rolling upgrade is not in progress:
   *   1. If the marker file is present, then a rolling upgrade just completed.
   *      If a 'previous' directory exists, it can be deleted now.
   *   2. If the marker file is absent, then a regular upgrade may be in
   *      progress. Do not delete the 'previous' directory.
   */
/**
   * A marker file that is created on each root directory if a rolling upgrade
   * is in progress. The NN does not inform the DN when a rolling upgrade is
   * finalized. All the DN can infer is whether or not a rolling upgrade is
   * currently in progress. When the rolling upgrade is not in progress:
   *   1. If the marker file is present, then a rolling upgrade just completed.
   *      If a 'previous' directory exists, it can be deleted now.
   *   2. If the marker file is absent, then a regular upgrade may be in
   *      progress. Do not delete the 'previous' directory.
   */ | BlockPoolSliceStorage.java