@VisibleForTesting
  void createRootDirectoryIfNecessary(File rootDirectory) throws IOException {
    try {
      FileUtils.mkdirs(rootDirectory);
    } catch (FileUtils.CreateDirectoryException cde) {
      mCacheErrorLogger.logError(
          CacheErrorLogger.CacheErrorCategory.WRITE_CREATE_DIR,
          TAG,
          "createRootDirectoryIfNecessary",
          cde);
      throw cde;
    }
    FLog.d(TAG, "Created cache directory %s", rootDirectory.getAbsolutePath());
  }
Gets a concrete disk-storage instance. If nothing has changed since the last call, then the
last state is returned

@return an instance of the appropriate DiskStorage class
@throws IOException
imagepipeline-base/src/main/java/com/facebook/cache/disk/DynamicDefaultDiskStorage.java