Set<String> getBackupFiles() {
      return Collections.unmodifiableSet(backupFiles);
    }
Return the container usage in map from given {@code filename}.
@param filename The filename to load container usage.
@return A map that represents container usage.
@throws IOException
ambry-quota/src/main/java/com/github/ambry/quota/storage/MySqlStorageUsageRefresher.java