Correct statement about schema changes for 3.4 release
Also add Zero-downtime upgrade and Downgrade sections.
Change-Id: I52ac3e41332c055888154a08bb48bdc88942af0c
diff --git a/pages/site/releases/3.4.md b/pages/site/releases/3.4.md
index 4e552ed..eac3975 100644
--- a/pages/site/releases/3.4.md
+++ b/pages/site/releases/3.4.md
@@ -33,12 +33,55 @@
### Schema changes
-This release contains schema changes. To upgrade:
+This release doesn't contain schema changes.
-``` sh
- java -jar gerrit.war init -d site_path
+The changes index version has been increased to version 61, new change search
+operators were added: `mergedafter` and `mergedbefore`.
+
+To run off-line reindexing of the changes (optional):
+
+```sh
+ java -jar gerrit.war reindex --index changes -d site_path
```
+By default the changes index is automatically rebuilt upon the Gerrit startup
+after the upgrade.
+
+### Zero-downtime upgrade
+
+Gerrit supports zero-downtime upgrade from Gerrit v3.3 when configured using
+a [high-availability configuration](https://gerrit.googlesource.com/plugins/high-availability/+/refs/heads/master/README.md),
+when the Git repositories are stored in a shared filesystem such as NFS or similar.
+
+For upgrading with zero-downtime, you should enable the rolling upgrade migration
+in `gerrit.config` on both Gerrit servers by setting the
+[`gerrit.experimentalRollingUpgrade`](http://gerrit-documentation.storage.googleapis.com/Documentation/3.3.3/config-gerrit.html#gerrit.experimentalRollingUpgrade)
+to `true`.
+
+During the zero-downtime upgrade, Gerrit end-users would not notice any outage
+or service disruption. They will be able to perform any read/write Gerrit operation
+on the GUI or using the Git protocol.
+
+The zero-downtime upgrade consists of the following steps:
+
+1. Have Gerrit servers upgraded to v3.3 in high-availability configuration,
+ healthy and able to handle the incoming traffic properly.
+2. Set `gerrit.experimentalRollingUpgrade` to `true` in `gerrit.config` on both Gerrit servers.
+3. Set the first Gerrit server unhealthy.
+4. Shutdown the first Gerrit server, upgrade to v3.4 and start Gerrit again.
+5. Verify that the first Gerrit server is working properly and then make it healthy again.
+6. Wait for the first Gerrit server to start serving traffic normally.
+7. Repeat steps 3. to 6. for the second Gerrit server.
+8. Remove `gerrit.experimentalRollingUpgrade` from `gerrit.config` on both Gerrit servers.
+
+### Downgrade
+
+Downgrade to any Gerrit v3.3.x release is possible, but requires the following manual steps:
+
+1. Shutdown a migrated Gerrit v3.4.x server
+2. Run Gerrit v3.3.x init
+3. Startup Gerrit v3.3.x server
+
### Breaking changes
HTML plugin support is removed. HTML plugins should be migrated to JavaScript plugins.
