diff --git a/swh/storage/cli.py b/swh/storage/cli.py
--- a/swh/storage/cli.py
+++ b/swh/storage/cli.py
@@ -15,7 +15,6 @@
 from swh.core.cli import swh as swh_cli_group
 from swh.storage.replay import ModelObjectDeserializer
 
-
 try:
     from systemd.daemon import notify
 except ImportError:
@@ -192,8 +191,37 @@
 def replay(ctx, stop_after_objects, object_types):
     """Fill a Storage by reading a Journal.
 
-    There can be several 'replayers' filling a Storage as long as they use
-    the same `group-id`.
+    This is typically used for a mirror configuration, reading the Software
+    Heritage kafka journal to retrieve objects of the Software Heritage main
+    storage to feed a replication storage. There can be several 'replayers'
+    filling a Storage as long as they use the same `group-id`.
+
+    The expected configuration file should have 2 sections:
+
+    - storage: the configuration of the storage in which to add objects
+      received from the kafka journal,
+
+    - journal_client: the configuration of access to the kafka journal. See the
+      documentation of `swh.journal` for more details on the possible
+      configuration entries in this section.
+
+      https://docs.softwareheritage.org/devel/apidoc/swh.journal.client.html
+
+    In addition to these 2 mandatory config sections, a third 'replayer' may be
+    specified with a 'error_reporter' config entry allowing to specify redis
+    connection parameters that will be used to report non-recoverable mirroring,
+    eg.::
+
+      storage:
+        [...]
+      journal_client:
+        [...]
+      replayer:
+        error_reporter:
+          host: redis.local
+          port: 6379
+          db: 1
+
     """
     import functools
 
@@ -206,13 +234,16 @@
     conf = ctx.obj["config"]
     storage = get_storage(**conf.pop("storage"))
 
-    if "error_reporter" in conf:
+    client_cfg = conf.pop("journal_client")
+    replayer_cfg = conf.pop("replayer", {})
+
+    if "error_reporter" in replayer_cfg:
         from redis import Redis
 
-        reporter = Redis(**conf["error_reporter"]).set
+        reporter = Redis(**replayer_cfg.get("error_reporter")).set
     else:
         reporter = None
-    validate = conf.get("privileged", False)
+    validate = client_cfg.get("privileged", False)
 
     if not validate and reporter:
         ctx.fail(
@@ -222,7 +253,6 @@
 
     deserializer = ModelObjectDeserializer(reporter=reporter, validate=validate)
 
-    client_cfg = conf.pop("journal_client")
     client_cfg["value_deserializer"] = deserializer.convert
     if object_types:
         client_cfg["object_types"] = object_types