code: long getLogDiskFreeSpace() throws IOException {
      String logDir = fConf.getLogDir();
      // If the log disk is not specified we assume it is usable.
      if (logDir == null) {
        return Long.MAX_VALUE;
      }
      DF df = localDirsDf.get(logDir);
      if (df == null) {
        df = new DF(new File(logDir), fConf);
        localDirsDf.put(logDir, df);
      }
      return df.getAvailable();
    }
docs: Obtain the free space on the log disk. If the log disk is not configured,
  returns Long.MAX_VALUE
  @return The free space available.
  @throws IOException