diff --git a/swh/objstorage/multiplexer/multiplexer_objstorage.py b/swh/objstorage/multiplexer/multiplexer_objstorage.py
--- a/swh/objstorage/multiplexer/multiplexer_objstorage.py
+++ b/swh/objstorage/multiplexer/multiplexer_objstorage.py
@@ -245,10 +245,21 @@
 
         """
         write_threads = list(self.get_write_threads())
-        return sum(self.wrap_call(
+        results = self.wrap_call(
             write_threads, 'add_batch', contents,
             check_presence=check_presence,
-        )) // len(write_threads)
+        )
+        object_add = 0
+        object_add_bytes = 0
+
+        for result in results:
+            object_add += result['object:add']
+            object_add_bytes += result['object:add:bytes']
+
+        return {
+            'object:add': object_add // len(result),
+            'object:add:bytes': object_add_bytes // len(result),
+        }
 
     def restore(self, content, obj_id=None):
         return self.wrap_call(
diff --git a/swh/objstorage/multiplexer/striping_objstorage.py b/swh/objstorage/multiplexer/striping_objstorage.py
--- a/swh/objstorage/multiplexer/striping_objstorage.py
+++ b/swh/objstorage/multiplexer/striping_objstorage.py
@@ -64,8 +64,15 @@
                 check_presence=check_presence,
                 mailbox=mailbox,
             )
-        return sum(
-            ObjStorageThread.collect_results(
-                mailbox, len(content_by_storage_index)
-            )
+
+        results = ObjStorageThread.collect_results(
+            mailbox, len(content_by_storage_index)
         )
+        summary = {
+            'object:add': 0,
+            'object:add:bytes': 0,
+        }
+        for result in results:
+            summary['object:add'] += result['object:add']
+            summary['object:add:bytes'] += result['object:add:bytes']
+        return summary
diff --git a/swh/objstorage/objstorage.py b/swh/objstorage/objstorage.py
--- a/swh/objstorage/objstorage.py
+++ b/swh/objstorage/objstorage.py
@@ -1,4 +1,4 @@
-# Copyright (C) 2015-2018  The Software Heritage developers
+# Copyright (C) 2015-2020  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
@@ -11,6 +11,8 @@
 
 from swh.model import hashutil
 
+from typing import Dict
+
 from .exc import ObjNotFoundError
 
 
@@ -149,19 +151,31 @@
         """
         pass
 
-    def add_batch(self, contents, check_presence=True):
+    def add_batch(self, contents, check_presence=True) -> Dict:
         """Add a batch of new objects to the object storage.
 
         Args:
-          contents (dict): mapping from obj_id to object contents
+            contents: mapping from obj_id to object contents
+
         Returns:
-          the number of objects added to the storage
+            the summary of objects added to the storage (count of object,
+            count of bytes object)
+
         """
-        ctr = 0
+        object_add = 0
+        object_add_bytes = 0
+
         for obj_id, content in contents.items():
-            self.add(content, obj_id, check_presence=check_presence)
-            ctr += 1
-        return ctr
+            if check_presence and obj_id in self:
+                continue
+            self.add(content, obj_id, check_presence=False)
+            object_add += 1
+            object_add_bytes += len(content)
+
+        return {
+            'object:add': object_add,
+            'object:add:bytes': object_add_bytes,
+        }
 
     def restore(self, content, obj_id=None, *args, **kwargs):
         """Restore a content that have been corrupted.
diff --git a/swh/objstorage/tests/objstorage_testing.py b/swh/objstorage/tests/objstorage_testing.py
--- a/swh/objstorage/tests/objstorage_testing.py
+++ b/swh/objstorage/tests/objstorage_testing.py
@@ -160,13 +160,21 @@
 
     def test_add_batch(self):
         contents = {}
+        expected_content_add = 0
+        expected_content_add_bytes = 0
         for i in range(50):
             content = b'Test content %02d' % i
             content, obj_id = self.hash_content(content)
             contents[obj_id] = content
+            expected_content_add_bytes += len(content)
+            expected_content_add += 1
 
         ret = self.storage.add_batch(contents)
-        self.assertEqual(len(contents), ret)
+
+        self.assertEqual(ret, {
+            'object:add': expected_content_add,
+            'object:add:bytes': expected_content_add_bytes,
+        })
         for obj_id in contents:
             self.assertIn(obj_id, self.storage)
 
diff --git a/tox.ini b/tox.ini
--- a/tox.ini
+++ b/tox.ini
@@ -6,6 +6,7 @@
   testing
 deps =
   pytest-cov
+  dev: pdbpp
 commands =
   pytest --cov={envsitepackagesdir}/swh/objstorage \
          {envsitepackagesdir}/swh/objstorage \