diff --git a/CHANGES.md b/CHANGES.md
index fb07650c2c..c9b6a13d7f 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,3 +1,9 @@
+Unreleased
+==========
+
+Note that this release includes a change in Synapse to use Redis as a cache ─ as well as a pub/sub mechanism ─ if Redis support is enabled. No action is needed by server administrators, and we do not expect resource usage of the Redis instance to change dramatically.
+
+
 Synapse 1.26.0rc2 (2021-01-25)
 ==============================
 
diff --git a/changelog.d/9227.misc b/changelog.d/9227.misc
new file mode 100644
index 0000000000..a6cb77fbb2
--- /dev/null
+++ b/changelog.d/9227.misc
@@ -0,0 +1 @@
+Precompute joined hosts and store in Redis.
diff --git a/docs/workers.md b/docs/workers.md
index d01683681f..d2927d95a6 100644
--- a/docs/workers.md
+++ b/docs/workers.md
@@ -40,6 +40,9 @@ which relays replication commands between processes. This can give a significant
 cpu saving on the main process and will be a prerequisite for upcoming
 performance improvements.
 
+If Redis support is enabled Synapse will use it as a shared cache, as well as a
+pub/sub mechanism.
+
 See the [Architectural diagram](#architectural-diagram) section at the end for
 a visualisation of what this looks like.
 
@@ -271,7 +274,7 @@ using):
 Note that a HTTP listener with `client` and `federation` resources must be
 configured in the `worker_listeners` option in the worker config.
 
-Ensure that all SSO logins go to a single process (usually the main process). 
+Ensure that all SSO logins go to a single process (usually the main process).
 For multiple workers not handling the SSO endpoints properly, see
 [#7530](https://github.com/matrix-org/synapse/issues/7530).