diff --git a/bin/diff-db-mirror.py b/bin/diff-db-mirror.py
index fc261d3..ed56a88 100755
--- a/bin/diff-db-mirror.py
+++ b/bin/diff-db-mirror.py
@@ -1,119 +1,120 @@
 #!/usr/bin/env python3
 
 # Copyright (C) 2015  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
 import argparse
 import os
 import psycopg2
 import sys
 
 from swh.loader.tar import tarball
 
 
 def connect(db_url):
     """Open db connection.
     """
     return psycopg2.connect(db_url)
 
 
 def execute(cur, query_params):
     """Execute the query_params.
     query_params is expected to be either:
     - a sql query (string)
     - a tuple (sql query, params)
     """
     if isinstance(query_params, str):
         cur.execute(query_params)
     else:
         cur.execute(*query_params)
 
 
 def query_fetch(db_conn, query_params):
     """Execute sql query which returns results.
     query_params is expected to be either:
     - a sql query (string)
     - a tuple (sql query, params)
     """
     with db_conn.cursor() as cur:
         execute(cur, query_params)
         yield from cursor_to_bytes(cur)
 
 
 def entry_to_bytes(entry):
     """Convert an entry coming from the database to bytes"""
     if isinstance(entry, memoryview):
         return entry.tobytes()
     return entry
 
 
 def line_to_bytes(line):
     """Convert a line coming from the database to bytes"""
     return line.__class__(entry_to_bytes(entry) for entry in line)
 
 
 def cursor_to_bytes(cursor):
     """Yield all the data from a cursor as bytes"""
     yield from (line_to_bytes(line) for line in cursor)
 
 
 def list_branches(db_url):
     """Return the distinct list of branches present in occurrence_history.
 
     """
     with connect(db_url) as db_conn:
         for branch in query_fetch(db_conn,
                                   'select distinct branch from '
                                   'occurrence_history'):
             yield branch[0]
 
 
 def diff_branch(root_dir, existing_set):
     """Walk the root_dir and for every tarball not in existing_set,
     yield its absolute path.
 
     """
     for dirpath, _, filenames in os.walk(root_dir):
         for filename in filenames:
             filepath = os.path.join(dirpath, filename)
             if not os.path.exists(filepath):
                 continue
             if not tarball.is_tarball(filepath):
                 continue
             if filename in existing_set:
                 continue
 
             yield filepath
 
 
 def parse_args():
     """Parse the configuration from the cli.
 
     """
     cli = argparse.ArgumentParser(
-        description='Diff between db and local fs mirror tarballs directory.')
+        description='Diff between db and local fs mirror tarballs directory.'
+                    'Output what\'s on disk and not on db.')
     cli.add_argument('--db-url', '-d',
                      help='db-url string')
     cli.add_argument('--mirror-root-directory', '-m',
                      help='mirror root directory')
 
     args = cli.parse_args()
     if not args.db_url or not args.mirror_root_directory:
         print('Bad usage, cf. --help')
         sys.exit(1)
 
     return args
 
 
 if __name__ == '__main__':
     args = parse_args()
 
     db_url = args.db_url
 
     already_present = set(list_branches(db_url))
 
     root_dir = args.mirror_root_directory
     for filepath in diff_branch(root_dir, already_present):
         print(filepath)