diff --git a/changelog.d/8419.feature b/changelog.d/8419.feature
new file mode 100644
index 000000000..b363e929e
--- /dev/null
+++ b/changelog.d/8419.feature
@@ -0,0 +1 @@
+Add experimental support for sharding event persister.
diff --git a/synapse/storage/util/id_generators.py b/synapse/storage/util/id_generators.py
index 4fd7573e2..02fbb656e 100644
--- a/synapse/storage/util/id_generators.py
+++ b/synapse/storage/util/id_generators.py
@@ -273,6 +273,19 @@ class MultiWriterIdGenerator:
 
         # Load the current positions of all writers for the stream.
         if self._writers:
+            # We delete any stale entries in the positions table. This is
+            # important if we add back a writer after a long time; we want to
+            # consider that a "new" writer, rather than using the old stale
+            # entry here.
+            sql = """
+                DELETE FROM stream_positions
+                WHERE
+                    stream_name = ?
+                    AND instance_name != ALL(?)
+            """
+            sql = self._db.engine.convert_param_style(sql)
+            cur.execute(sql, (self._stream_name, self._writers))
+
             sql = """
                 SELECT instance_name, stream_id FROM stream_positions
                 WHERE stream_name = ?
@@ -453,11 +466,22 @@ class MultiWriterIdGenerator:
         """Returns the position of the given writer.
         """
 
+        # If we don't have an entry for the given instance name, we assume it's a
+        # new writer.
+        #
+        # For new writers we assume their initial position to be the current
+        # persisted up to position. This stops Synapse from doing a full table
+        # scan when a new writer announces itself over replication.
         with self._lock:
-            return self._return_factor * self._current_positions.get(instance_name, 0)
+            return self._return_factor * self._current_positions.get(
+                instance_name, self._persisted_upto_position
+            )
 
     def get_positions(self) -> Dict[str, int]:
         """Get a copy of the current positon map.
+
+        Note that this won't necessarily include all configured writers if some
+        writers haven't written anything yet.
         """
 
         with self._lock:
diff --git a/tests/storage/test_id_generators.py b/tests/storage/test_id_generators.py
index 4558bee7b..392b08832 100644
--- a/tests/storage/test_id_generators.py
+++ b/tests/storage/test_id_generators.py
@@ -390,17 +390,28 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
         # Initial config has two writers
         id_gen = self._create_id_generator("first", writers=["first", "second"])
         self.assertEqual(id_gen.get_persisted_upto_position(), 3)
+        self.assertEqual(id_gen.get_current_token_for_writer("first"), 3)
+        self.assertEqual(id_gen.get_current_token_for_writer("second"), 5)
 
         # New config removes one of the configs. Note that if the writer is
         # removed from config we assume that it has been shut down and has
         # finished persisting, hence why the persisted upto position is 5.
         id_gen_2 = self._create_id_generator("second", writers=["second"])
         self.assertEqual(id_gen_2.get_persisted_upto_position(), 5)
+        self.assertEqual(id_gen_2.get_current_token_for_writer("second"), 5)
 
         # This config points to a single, previously unused writer.
         id_gen_3 = self._create_id_generator("third", writers=["third"])
         self.assertEqual(id_gen_3.get_persisted_upto_position(), 5)
 
+        # For new writers we assume their initial position to be the current
+        # persisted up to position. This stops Synapse from doing a full table
+        # scan when a new writer comes along.
+        self.assertEqual(id_gen_3.get_current_token_for_writer("third"), 5)
+
+        id_gen_4 = self._create_id_generator("fourth", writers=["third"])
+        self.assertEqual(id_gen_4.get_current_token_for_writer("third"), 5)
+
         # Check that we get a sane next stream ID with this new config.
 
         async def _get_next_async():
@@ -410,6 +421,13 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
         self.get_success(_get_next_async())
         self.assertEqual(id_gen_3.get_persisted_upto_position(), 6)
 
+        # If we add back the old "first" then we shouldn't see the persisted up
+        # to position revert back to 3.
+        id_gen_5 = self._create_id_generator("five", writers=["first", "third"])
+        self.assertEqual(id_gen_5.get_persisted_upto_position(), 6)
+        self.assertEqual(id_gen_5.get_current_token_for_writer("first"), 6)
+        self.assertEqual(id_gen_5.get_current_token_for_writer("third"), 6)
+
     def test_sequence_consistency(self):
         """Test that we error out if the table and sequence diverges.
         """