diff --git a/swh/storage/buffer.py b/swh/storage/buffer.py
--- a/swh/storage/buffer.py
+++ b/swh/storage/buffer.py
@@ -14,7 +14,13 @@
 from swh.storage.interface import StorageInterface
 
 LObjectType = Literal[
-    "content", "skipped_content", "directory", "revision", "release", "snapshot"
+    "content",
+    "skipped_content",
+    "directory",
+    "revision",
+    "release",
+    "snapshot",
+    "extid",
 ]
 OBJECT_TYPES: Tuple[LObjectType, ...] = (
     "content",
@@ -23,6 +29,7 @@
     "revision",
     "release",
     "snapshot",
+    "extid",
 )
 
 DEFAULT_BUFFER_THRESHOLDS: Dict[str, int] = {
@@ -33,6 +40,7 @@
     "revision": 100000,
     "release": 100000,
     "snapshot": 25000,
+    "extid": 10000,
 }
 
 
diff --git a/swh/storage/tests/test_buffer.py b/swh/storage/tests/test_buffer.py
--- a/swh/storage/tests/test_buffer.py
+++ b/swh/storage/tests/test_buffer.py
@@ -161,6 +161,70 @@
     assert s == {}
 
 
+def test_buffering_proxy_storage_extid_threshold_not_hit(sample_data) -> None:
+    extid = sample_data.extid1
+    storage = get_storage_with_buffer_config(min_batch_size={"extid": 10,})
+    s = storage.extid_add([extid])
+    assert s == {}
+
+    present_extids = storage.extid_get_from_target(
+        extid.target.object_type, [extid.target.object_id]
+    )
+    assert list(present_extids) == []
+
+    s = storage.flush()
+    assert s == {
+        "extid:add": 1,
+    }
+
+    present_extids = storage.extid_get_from_target(
+        extid.target.object_type, [extid.target.object_id]
+    )
+    assert list(present_extids) == [extid]
+
+
+def test_buffering_proxy_storage_extid_threshold_hit(sample_data) -> None:
+    extid = sample_data.extid1
+    storage = get_storage_with_buffer_config(min_batch_size={"extid": 1,})
+    s = storage.extid_add([extid])
+    assert s == {
+        "extid:add": 1,
+    }
+
+    present_extids = storage.extid_get_from_target(
+        extid.target.object_type, [extid.target.object_id]
+    )
+    assert list(present_extids) == [extid]
+
+    s = storage.flush()
+    assert s == {}
+
+
+def test_buffering_proxy_storage_extid_deduplicate(sample_data) -> None:
+    extids = sample_data.extids[:2]
+    storage = get_storage_with_buffer_config(min_batch_size={"extid": 2,})
+
+    s = storage.extid_add([extids[0], extids[0]])
+    assert s == {}
+
+    s = storage.extid_add([extids[0]])
+    assert s == {}
+
+    s = storage.extid_add([extids[1]])
+    assert s == {
+        "extid:add": 1 + 1,
+    }
+
+    for extid in extids:
+        present_extids = storage.extid_get_from_target(
+            extid.target.object_type, [extid.target.object_id]
+        )
+        assert list(present_extids) == [extid]
+
+    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,})