From 8de3283ebe520e945ccfb66242f4795ecdd0d1f4 Mon Sep 17 00:00:00 2001
From: Twilight Sparkle <19155609+Twi1ightSparkle@users.noreply.github.com>
Date: Thu, 22 Feb 2024 17:36:41 +0000
Subject: [PATCH] Add docs on upgrading from a very old version (#16951)

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>
---
 changelog.d/16951.doc                  |  1 +
 docs/upgrade.md                        | 20 ++++++++++++++++++++
 docs/usage/administration/admin_faq.md |  5 +++++
 3 files changed, 26 insertions(+)
 create mode 100644 changelog.d/16951.doc

diff --git a/changelog.d/16951.doc b/changelog.d/16951.doc
new file mode 100644
index 0000000000..a7fb33979d
--- /dev/null
+++ b/changelog.d/16951.doc
@@ -0,0 +1 @@
+Add docs on upgrading from a very old version.
\ No newline at end of file
diff --git a/docs/upgrade.md b/docs/upgrade.md
index 7f67ef8806..640fed3ae3 100644
--- a/docs/upgrade.md
+++ b/docs/upgrade.md
@@ -97,6 +97,26 @@ v1.61.0.
 
 <!-- REPLACE_WITH_SCHEMA_VERSIONS -->
 
+## Upgrading from a very old version
+
+You need to read all of the upgrade notes for each version between your current
+version and the latest so that you can update your dependencies, environment,
+config files, etc. if necessary. But you do not need to perform an
+upgrade to each individual version that was missed.
+
+We do not have a list of which versions must be installed. Instead, we recommend
+that you upgrade through each incompatible database schema version, which would
+give you the ability to roll back the maximum number of versions should anything
+go wrong. See [Rolling back to older versions](#rolling-back-to-older-versions)
+above.
+
+Additionally, new versions of Synapse will occasionally run database migrations
+and background updates to update the database. Synapse will not start until
+database migrations are complete. You should wait until background updates from
+each upgrade are complete before moving on to the next upgrade, to avoid
+stacking them up. You can monitor the currently running background updates with
+[the Admin API](usage/administration/admin_api/background_updates.html#status).
+
 # Upgrading to v1.100.0
 
 ## Minimum supported Rust version
diff --git a/docs/usage/administration/admin_faq.md b/docs/usage/administration/admin_faq.md
index 5c9ee7d0aa..092dcc1c84 100644
--- a/docs/usage/administration/admin_faq.md
+++ b/docs/usage/administration/admin_faq.md
@@ -120,6 +120,11 @@ for file in $source_directory/*; do
 done
 ```
 
+How do I upgrade from a very old version of Synapse to the latest?
+---
+See [this](../../upgrade.html#upgrading-from-a-very-old-version) section in the
+upgrade docs.
+
 Manually resetting passwords
 ---
 Users can reset their password through their client. Alternatively, a server admin
-- 
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