diff --git a/swh/storage/buffer.py b/swh/storage/buffer.py
index 8d870c75..3ff22bdd 100644
--- a/swh/storage/buffer.py
+++ b/swh/storage/buffer.py
@@ -1,170 +1,174 @@
 # Copyright (C) 2019-2020 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
 
 from functools import partial
 from typing import Dict, Iterable, Mapping, Sequence, Tuple
 
 from typing_extensions import Literal
 
 from swh.core.utils import grouper
 from swh.model.model import BaseModel, Content, SkippedContent
 from swh.storage import get_storage
 from swh.storage.interface import StorageInterface
 
 LObjectType = Literal[
     "content", "skipped_content", "directory", "revision", "release", "snapshot"
 ]
 OBJECT_TYPES: Tuple[LObjectType, ...] = (
     "content",
     "skipped_content",
     "directory",
     "revision",
     "release",
     "snapshot",
 )
 
 DEFAULT_BUFFER_THRESHOLDS: Dict[str, int] = {
     "content": 10000,
     "content_bytes": 100 * 1024 * 1024,
     "skipped_content": 10000,
     "directory": 25000,
     "revision": 100000,
     "release": 100000,
     "snapshot": 25000,
 }
 
 
 class BufferingProxyStorage:
     """Storage implementation in charge of accumulating objects prior to
        discussing with the "main" storage.
 
     Deduplicates values based on a tuple of keys depending on the object type.
 
     Sample configuration use case for buffering storage:
 
     .. code-block:: yaml
 
         storage:
           cls: buffer
           args:
             storage:
               cls: remote
               args: http://storage.internal.staging.swh.network:5002/
             min_batch_size:
               content: 10000
               content_bytes: 100000000
               skipped_content: 10000
               directory: 5000
               revision: 1000
               release: 10000
               snapshot: 5000
 
     """
 
     def __init__(self, storage: Mapping, min_batch_size: Mapping = {}):
         self.storage: StorageInterface = get_storage(**storage)
 
         self._buffer_thresholds = {**DEFAULT_BUFFER_THRESHOLDS, **min_batch_size}
 
         self._objects: Dict[LObjectType, Dict[Tuple[str, ...], BaseModel]] = {
             k: {} for k in OBJECT_TYPES
         }
         self._contents_size: int = 0
 
     def __getattr__(self, key: str):
         if key.endswith("_add"):
             object_type = key.rsplit("_", 1)[0]
             if object_type in OBJECT_TYPES:
                 return partial(self.object_add, object_type=object_type, keys=["id"],)
         if key == "storage":
             raise AttributeError(key)
         return getattr(self.storage, key)
 
     def content_add(self, contents: Sequence[Content]) -> Dict:
         """Push contents to write to the storage in the buffer.
 
         Following policies apply:
             - if the buffer's threshold is hit, flush content to the storage.
             - otherwise, if the total size of buffered contents's threshold is hit,
             flush content to the storage.
 
         """
         stats = self.object_add(
             contents,
             object_type="content",
             keys=["sha1", "sha1_git", "sha256", "blake2s256"],
         )
         if not stats:  # We did not flush already
             self._contents_size += sum(c.length for c in contents)
             if self._contents_size >= self._buffer_thresholds["content_bytes"]:
                 return self.flush(["content"])
 
         return stats
 
     def skipped_content_add(self, contents: Sequence[SkippedContent]) -> Dict:
         return self.object_add(
             contents,
             object_type="skipped_content",
             keys=["sha1", "sha1_git", "sha256", "blake2s256"],
         )
 
     def object_add(
         self,
         objects: Sequence[BaseModel],
         *,
         object_type: LObjectType,
         keys: Iterable[str],
     ) -> Dict[str, int]:
         """Push objects to write to the storage in the buffer. Flushes the
         buffer to the storage if the threshold is hit.
 
         """
         buffer_ = self._objects[object_type]
         for obj in objects:
             obj_key = tuple(getattr(obj, key) for key in keys)
             buffer_[obj_key] = obj
         if len(buffer_) >= self._buffer_thresholds[object_type]:
             return self.flush()
 
         return {}
 
     def flush(
         self, object_types: Sequence[LObjectType] = OBJECT_TYPES
     ) -> Dict[str, int]:
-        summary: Dict[str, int] = self.storage.flush(object_types)
+        summary: Dict[str, int] = {}
 
         def update_summary(stats):
             for k, v in stats.items():
                 summary[k] = v + summary.get(k, 0)
 
         for object_type in object_types:
             buffer_ = self._objects[object_type]
             batches = grouper(buffer_.values(), n=self._buffer_thresholds[object_type])
             for batch in batches:
                 add_fn = getattr(self.storage, "%s_add" % object_type)
                 stats = add_fn(list(batch))
                 update_summary(stats)
 
+        # Flush underlying storage
+        stats = self.storage.flush(object_types)
+        update_summary(stats)
+
         self.clear_buffers(object_types)
 
         return summary
 
     def clear_buffers(self, object_types: Sequence[LObjectType] = OBJECT_TYPES) -> None:
         """Clear objects from current buffer.
 
         WARNING:
 
             data that has not been flushed to storage will be lost when this
             method is called. This should only be called when `flush` fails and
             you want to continue your processing.
 
         """
         for object_type in object_types:
             buffer_ = self._objects[object_type]
             buffer_.clear()
             if object_type == "content":
                 self._contents_size = 0
 
         self.storage.clear_buffers(object_types)
diff --git a/swh/storage/tests/test_buffer.py b/swh/storage/tests/test_buffer.py
index d3b61f44..ee359ee7 100644
--- a/swh/storage/tests/test_buffer.py
+++ b/swh/storage/tests/test_buffer.py
@@ -1,510 +1,558 @@
 # Copyright (C) 2019-2021 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
 
+from typing import Optional
+from unittest.mock import Mock
+
 from swh.storage import get_storage
 from swh.storage.buffer import BufferingProxyStorage
 
 
 def get_storage_with_buffer_config(**buffer_config) -> BufferingProxyStorage:
     steps = [
         {"cls": "buffer", **buffer_config},
         {"cls": "memory"},
     ]
 
     ret = get_storage("pipeline", steps=steps)
     assert isinstance(ret, BufferingProxyStorage)
     return ret
 
 
 def test_buffering_proxy_storage_content_threshold_not_hit(sample_data) -> None:
     contents = sample_data.contents[:2]
     contents_dict = [c.to_dict() for c in contents]
 
     storage = get_storage_with_buffer_config(min_batch_size={"content": 10,})
     s = storage.content_add(contents)
     assert s == {}
 
     # contents have not been written to storage
     missing_contents = storage.content_missing(contents_dict)
     assert set(missing_contents) == set([contents[0].sha1, contents[1].sha1])
 
     s = storage.flush()
     assert s == {
         "content:add": 1 + 1,
         "content:add:bytes": contents[0].length + contents[1].length,
     }
 
     missing_contents = storage.content_missing(contents_dict)
     assert list(missing_contents) == []
 
 
 def test_buffering_proxy_storage_content_threshold_nb_hit(sample_data) -> None:
     content = sample_data.content
     content_dict = content.to_dict()
 
     storage = get_storage_with_buffer_config(min_batch_size={"content": 1,})
 
     s = storage.content_add([content])
     assert s == {
         "content:add": 1,
         "content:add:bytes": content.length,
     }
 
     missing_contents = storage.content_missing([content_dict])
     assert list(missing_contents) == []
 
     s = storage.flush()
     assert s == {}
 
 
 def test_buffering_proxy_storage_content_deduplicate(sample_data) -> None:
     contents = sample_data.contents[:2]
     storage = get_storage_with_buffer_config(min_batch_size={"content": 2,})
 
     s = storage.content_add([contents[0], contents[0]])
     assert s == {}
 
     s = storage.content_add([contents[0]])
     assert s == {}
 
     s = storage.content_add([contents[1]])
     assert s == {
         "content:add": 1 + 1,
         "content:add:bytes": contents[0].length + contents[1].length,
     }
 
     missing_contents = storage.content_missing([c.to_dict() for c in contents])
     assert list(missing_contents) == []
 
     s = storage.flush()
     assert s == {}
 
 
 def test_buffering_proxy_storage_content_threshold_bytes_hit(sample_data) -> None:
     contents = sample_data.contents[:2]
     content_bytes_min_batch_size = 2
     storage = get_storage_with_buffer_config(
         min_batch_size={"content": 10, "content_bytes": content_bytes_min_batch_size,}
     )
 
     assert contents[0].length > content_bytes_min_batch_size
 
     s = storage.content_add([contents[0]])
     assert s == {
         "content:add": 1,
         "content:add:bytes": contents[0].length,
     }
 
     missing_contents = storage.content_missing([contents[0].to_dict()])
     assert list(missing_contents) == []
 
     s = storage.flush()
     assert s == {}
 
 
 def test_buffering_proxy_storage_skipped_content_threshold_not_hit(sample_data) -> None:
     contents = sample_data.skipped_contents
     contents_dict = [c.to_dict() for c in contents]
     storage = get_storage_with_buffer_config(min_batch_size={"skipped_content": 10,})
     s = storage.skipped_content_add([contents[0], contents[1]])
     assert s == {}
 
     # contents have not been written to storage
     missing_contents = storage.skipped_content_missing(contents_dict)
     assert {c["sha1"] for c in missing_contents} == {c.sha1 for c in contents}
 
     s = storage.flush()
     assert s == {"skipped_content:add": 1 + 1}
 
     missing_contents = storage.skipped_content_missing(contents_dict)
     assert list(missing_contents) == []
 
 
 def test_buffering_proxy_storage_skipped_content_threshold_nb_hit(sample_data) -> None:
     contents = sample_data.skipped_contents
     storage = get_storage_with_buffer_config(min_batch_size={"skipped_content": 1,})
 
     s = storage.skipped_content_add([contents[0]])
     assert s == {"skipped_content:add": 1}
 
     missing_contents = storage.skipped_content_missing([contents[0].to_dict()])
     assert list(missing_contents) == []
 
     s = storage.flush()
     assert s == {}
 
 
 def test_buffering_proxy_storage_skipped_content_deduplicate(sample_data):
     contents = sample_data.skipped_contents[:2]
     storage = get_storage_with_buffer_config(min_batch_size={"skipped_content": 2,})
 
     s = storage.skipped_content_add([contents[0], contents[0]])
     assert s == {}
 
     s = storage.skipped_content_add([contents[0]])
     assert s == {}
 
     s = storage.skipped_content_add([contents[1]])
     assert s == {
         "skipped_content:add": 1 + 1,
     }
 
     missing_contents = storage.skipped_content_missing([c.to_dict() for c in contents])
     assert list(missing_contents) == []
 
     s = storage.flush()
     assert s == {}
 
 
 def test_buffering_proxy_storage_directory_threshold_not_hit(sample_data) -> None:
     directory = sample_data.directory
     storage = get_storage_with_buffer_config(min_batch_size={"directory": 10,})
     s = storage.directory_add([directory])
     assert s == {}
 
     missing_directories = storage.directory_missing([directory.id])
     assert list(missing_directories) == [directory.id]
 
     s = storage.flush()
     assert s == {
         "directory:add": 1,
     }
 
     missing_directories = storage.directory_missing([directory.id])
     assert list(missing_directories) == []
 
 
 def test_buffering_proxy_storage_directory_threshold_hit(sample_data) -> None:
     directory = sample_data.directory
     storage = get_storage_with_buffer_config(min_batch_size={"directory": 1,})
     s = storage.directory_add([directory])
     assert s == {
         "directory:add": 1,
     }
 
     missing_directories = storage.directory_missing([directory.id])
     assert list(missing_directories) == []
 
     s = storage.flush()
     assert s == {}
 
 
 def test_buffering_proxy_storage_directory_deduplicate(sample_data) -> None:
     directories = sample_data.directories[:2]
     storage = get_storage_with_buffer_config(min_batch_size={"directory": 2,})
 
     s = storage.directory_add([directories[0], directories[0]])
     assert s == {}
 
     s = storage.directory_add([directories[0]])
     assert s == {}
 
     s = storage.directory_add([directories[1]])
     assert s == {
         "directory:add": 1 + 1,
     }
 
     missing_directories = storage.directory_missing([d.id for d in directories])
     assert list(missing_directories) == []
 
     s = storage.flush()
     assert s == {}
 
 
 def test_buffering_proxy_storage_revision_threshold_not_hit(sample_data) -> None:
     revision = sample_data.revision
     storage = get_storage_with_buffer_config(min_batch_size={"revision": 10,})
     s = storage.revision_add([revision])
     assert s == {}
 
     missing_revisions = storage.revision_missing([revision.id])
     assert list(missing_revisions) == [revision.id]
 
     s = storage.flush()
     assert s == {
         "revision:add": 1,
     }
 
     missing_revisions = storage.revision_missing([revision.id])
     assert list(missing_revisions) == []
 
 
 def test_buffering_proxy_storage_revision_threshold_hit(sample_data) -> None:
     revision = sample_data.revision
     storage = get_storage_with_buffer_config(min_batch_size={"revision": 1,})
     s = storage.revision_add([revision])
     assert s == {
         "revision:add": 1,
     }
 
     missing_revisions = storage.revision_missing([revision.id])
     assert list(missing_revisions) == []
 
     s = storage.flush()
     assert s == {}
 
 
 def test_buffering_proxy_storage_revision_deduplicate(sample_data) -> None:
     revisions = sample_data.revisions[:2]
     storage = get_storage_with_buffer_config(min_batch_size={"revision": 2,})
 
     s = storage.revision_add([revisions[0], revisions[0]])
     assert s == {}
 
     s = storage.revision_add([revisions[0]])
     assert s == {}
 
     s = storage.revision_add([revisions[1]])
     assert s == {
         "revision:add": 1 + 1,
     }
 
     missing_revisions = storage.revision_missing([r.id for r in revisions])
     assert list(missing_revisions) == []
 
     s = storage.flush()
     assert s == {}
 
 
 def test_buffering_proxy_storage_release_threshold_not_hit(sample_data) -> None:
     releases = sample_data.releases
     threshold = 10
 
     assert len(releases) < threshold
     storage = get_storage_with_buffer_config(
         min_batch_size={"release": threshold,}  # configuration set
     )
     s = storage.release_add(releases)
     assert s == {}
 
     release_ids = [r.id for r in releases]
     missing_releases = storage.release_missing(release_ids)
     assert list(missing_releases) == release_ids
 
     s = storage.flush()
     assert s == {
         "release:add": len(releases),
     }
 
     missing_releases = storage.release_missing(release_ids)
     assert list(missing_releases) == []
 
 
 def test_buffering_proxy_storage_release_threshold_hit(sample_data) -> None:
     releases = sample_data.releases
     threshold = 2
     assert len(releases) > threshold
 
     storage = get_storage_with_buffer_config(
         min_batch_size={"release": threshold,}  # configuration set
     )
 
     s = storage.release_add(releases)
     assert s == {
         "release:add": len(releases),
     }
 
     release_ids = [r.id for r in releases]
     missing_releases = storage.release_missing(release_ids)
     assert list(missing_releases) == []
 
     s = storage.flush()
     assert s == {}
 
 
 def test_buffering_proxy_storage_release_deduplicate(sample_data) -> None:
     releases = sample_data.releases[:2]
     storage = get_storage_with_buffer_config(min_batch_size={"release": 2,})
 
     s = storage.release_add([releases[0], releases[0]])
     assert s == {}
 
     s = storage.release_add([releases[0]])
     assert s == {}
 
     s = storage.release_add([releases[1]])
     assert s == {
         "release:add": 1 + 1,
     }
 
     missing_releases = storage.release_missing([r.id for r in releases])
     assert list(missing_releases) == []
 
     s = storage.flush()
     assert s == {}
 
 
 def test_buffering_proxy_storage_snapshot_threshold_not_hit(sample_data) -> None:
     snapshots = sample_data.snapshots
     threshold = 10
 
     assert len(snapshots) < threshold
     storage = get_storage_with_buffer_config(
         min_batch_size={"snapshot": threshold,}  # configuration set
     )
     s = storage.snapshot_add(snapshots)
     assert s == {}
 
     snapshot_ids = [r.id for r in snapshots]
     missing_snapshots = storage.snapshot_missing(snapshot_ids)
     assert list(missing_snapshots) == snapshot_ids
 
     s = storage.flush()
     assert s == {
         "snapshot:add": len(snapshots),
     }
 
     missing_snapshots = storage.snapshot_missing(snapshot_ids)
     assert list(missing_snapshots) == []
 
 
 def test_buffering_proxy_storage_snapshot_threshold_hit(sample_data) -> None:
     snapshots = sample_data.snapshots
     threshold = 2
     assert len(snapshots) > threshold
 
     storage = get_storage_with_buffer_config(
         min_batch_size={"snapshot": threshold,}  # configuration set
     )
 
     s = storage.snapshot_add(snapshots)
     assert s == {
         "snapshot:add": len(snapshots),
     }
 
     snapshot_ids = [r.id for r in snapshots]
     missing_snapshots = storage.snapshot_missing(snapshot_ids)
     assert list(missing_snapshots) == []
 
     s = storage.flush()
     assert s == {}
 
 
 def test_buffering_proxy_storage_snapshot_deduplicate(sample_data) -> None:
     snapshots = sample_data.snapshots[:2]
     storage = get_storage_with_buffer_config(min_batch_size={"snapshot": 2,})
 
     s = storage.snapshot_add([snapshots[0], snapshots[0]])
     assert s == {}
 
     s = storage.snapshot_add([snapshots[0]])
     assert s == {}
 
     s = storage.snapshot_add([snapshots[1]])
     assert s == {
         "snapshot:add": 1 + 1,
     }
 
     missing_snapshots = storage.snapshot_missing([r.id for r in snapshots])
     assert list(missing_snapshots) == []
 
     s = storage.flush()
     assert s == {}
 
 
 def test_buffering_proxy_storage_clear(sample_data) -> None:
     """Clear operation on buffer
 
     """
     threshold = 10
     contents = sample_data.contents
     assert 0 < len(contents) < threshold
     skipped_contents = sample_data.skipped_contents
     assert 0 < len(skipped_contents) < threshold
     directories = sample_data.directories
     assert 0 < len(directories) < threshold
     revisions = sample_data.revisions
     assert 0 < len(revisions) < threshold
     releases = sample_data.releases
     assert 0 < len(releases) < threshold
     snapshots = sample_data.snapshots
     assert 0 < len(snapshots) < threshold
 
     storage = get_storage_with_buffer_config(
         min_batch_size={
             "content": threshold,
             "skipped_content": threshold,
             "directory": threshold,
             "revision": threshold,
             "release": threshold,
         }
     )
 
     s = storage.content_add(contents)
     assert s == {}
     s = storage.skipped_content_add(skipped_contents)
     assert s == {}
     s = storage.directory_add(directories)
     assert s == {}
     s = storage.revision_add(revisions)
     assert s == {}
     s = storage.release_add(releases)
     assert s == {}
     s = storage.snapshot_add(snapshots)
     assert s == {}
 
     assert len(storage._objects["content"]) == len(contents)
     assert len(storage._objects["skipped_content"]) == len(skipped_contents)
     assert len(storage._objects["directory"]) == len(directories)
     assert len(storage._objects["revision"]) == len(revisions)
     assert len(storage._objects["release"]) == len(releases)
     assert len(storage._objects["snapshot"]) == len(snapshots)
 
     # clear only content from the buffer
     s = storage.clear_buffers(["content"])  # type: ignore
     assert s is None
 
     # specific clear operation on specific object type content only touched
     # them
     assert len(storage._objects["content"]) == 0
     assert len(storage._objects["skipped_content"]) == len(skipped_contents)
     assert len(storage._objects["directory"]) == len(directories)
     assert len(storage._objects["revision"]) == len(revisions)
     assert len(storage._objects["release"]) == len(releases)
     assert len(storage._objects["snapshot"]) == len(snapshots)
 
     # clear current buffer from all object types
     s = storage.clear_buffers()  # type: ignore
     assert s is None
 
     assert len(storage._objects["content"]) == 0
     assert len(storage._objects["skipped_content"]) == 0
     assert len(storage._objects["directory"]) == 0
     assert len(storage._objects["revision"]) == 0
     assert len(storage._objects["release"]) == 0
     assert len(storage._objects["snapshot"]) == 0
 
 
 def test_buffer_proxy_with_default_args() -> None:
     storage = get_storage_with_buffer_config()
 
     assert storage is not None
 
 
 def test_buffer_flush_stats(sample_data) -> None:
     storage = get_storage_with_buffer_config()
 
     s = storage.content_add(sample_data.contents)
     assert s == {}
     s = storage.skipped_content_add(sample_data.skipped_contents)
     assert s == {}
     s = storage.directory_add(sample_data.directories)
     assert s == {}
     s = storage.revision_add(sample_data.revisions)
     assert s == {}
     s = storage.release_add(sample_data.releases)
     assert s == {}
     s = storage.snapshot_add(sample_data.snapshots)
     assert s == {}
 
     # Flush all the things
     s = storage.flush()
     assert s["content:add"] > 0
     assert s["content:add:bytes"] > 0
     assert s["skipped_content:add"] > 0
     assert s["directory:add"] > 0
     assert s["revision:add"] > 0
     assert s["release:add"] > 0
     assert s["snapshot:add"] > 0
+
+
+def test_buffer_operation_order(sample_data) -> None:
+    storage = get_storage_with_buffer_config()
+
+    # Wrap the inner storage in a mock to track all method calls.
+    storage.storage = mocked_storage = Mock(wraps=storage.storage)
+
+    # Simulate a loader: add contents, directories, revisions, releases, then
+    # snapshots.
+    storage.content_add(sample_data.contents)
+    storage.skipped_content_add(sample_data.skipped_contents)
+    storage.directory_add(sample_data.directories)
+    storage.revision_add(sample_data.revisions)
+    storage.release_add(sample_data.releases)
+    storage.snapshot_add(sample_data.snapshots)
+
+    # Check that nothing has been flushed yet
+    assert mocked_storage.method_calls == []
+
+    # Flush all the things
+    storage.flush()
+
+    methods_called = [c[0] for c in mocked_storage.method_calls]
+    prev = -1
+    for method in [
+        "content_add",
+        "skipped_content_add",
+        "directory_add",
+        "revision_add",
+        "release_add",
+        "snapshot_add",
+        "flush",
+    ]:
+        try:
+            cur: Optional[int] = methods_called.index(method)
+        except ValueError:
+            cur = None
+
+        assert cur is not None, "Method %s not called" % method
+        assert cur > prev, "Method %s called out of order; all calls were: %s" % (
+            method,
+            methods_called,
+        )
+        prev = cur