static long guessClusteredDirSize(File dir) {
  long n = 0;
  long clusterSize = assumedClusterSize();
  for (File f : findAllFiles_noDirs(dir))
    n += roundUpTo(clusterSize, f.length());
  ret n;
}

static long guessClusteredDirSize() {
  ret guessClusteredDirSize(programDir());
}