Sessions still written to filesystem with database driver

Hello there,

Finally switch from CI2 to CI3 but i'm now having a strange behaviour on CI3. Despite using the database driver, sessions are stored both in files and in the database.

It seems that because of this, file storage is conflicting with other applications on the server already using this same directory to store file sessions thus resulting into "random" fatal errors : session_start(): Failed to initialize storage module: user (path: /var/lib/php5/sessions)

Is there any reason why sessions are stored in both ? Here's my configuration regarding sessions :

  'sess_driver' => 'database',
  'sess_cookie_name' => 'xxxxx',
  'sess_expiration' => 10800,
  'sess_save_path' => 'users_sessions_ci3',
  'sess_match_ip' => false,
  'sess_time_to_update' => 40,
  'sess_regenerate_destroy' => false,

Thanks in advance for your help.