$storageDir does not have proper permissions?

Hello,

We're getting a warning that function.mkdir cannot execute because of improper permissions in $storageDir (based off line 149 in /custom/plugins/appointments/includes/external/google/Cache/File.php).

private function getCacheDir($file, $forWrite)
  {
    // use the first 2 characters of the hash as a directory prefix
    // this should prevent slowdowns due to huge directory listings
    // and thus give some basic amount of scalability
    $storageDir = $this->path . '/' . substr(md5($file), 0, 2);
    if ($forWrite && ! is_dir($storageDir)) {
      if (! mkdir($storageDir, 0755, true)) {
        $this->client->getLogger()->error(
            'File cache creation failed',
            array('dir' => $storageDir)
        );
        throw new App_Google_Cache_Exception("Could not create storage directory: $storageDir");
      }
    }
    return $storageDir;
  }

Where would this $storageDir be located?

Any help would be much appreciated, thanks!