Update instructions for GitLab 9.0 with Geo

doc/gitlab-geo/updating_the_geo_nodes.md

@@ -22,3 +22,56 @@ sudo yum install gitlab-ee
 
 For installations from source, [follow the instructions for your GitLab version]
 (https://gitlab.com/gitlab-org/gitlab-ee/tree/master/doc/update).
+
+# Upgrade to 9.0.x
+
+> **IMPORTANT**: 9.0 requires manual steps in the secondary node,
+because we are upgrading PostgreSQL to 9.6 on the primary and Postgres
+doesn't support upgrading secondary nodes while keeping the
+Streaming Replication working.
+
+Before starting the upgrade to 9.0 on both **primary** and **secondary** nodes,
+stop all service in the secondary node: `gitlab-ctl stop` and make a backup of the `recovery.conf`
+located at: `/var/opt/gitlab/postgresql/data/recovery.conf`
+
+Follow regular upgrade instruction for 9.0 on the primary node.
+
+At the end of the upgrade procedures your primary node will be running with
+PostgreSQL on 9.6.x branch. To prevent a desynchronization of the repository
+replication, stop all services but the `postgresql` as we will use it to
+re-initialize the secondary node's database:
+
+
+**Run in the primary node:**
+
+```
+sudo gitlab-ctl stop
+sudo gitlab-ctl start postgresql
+```
+
+**Run in the secondary node:**
+
+Follow the instructions [here](https://docs.gitlab.com/ee/gitlab-geo/database.html#step-3-initiate-the-replication-process)
+to Create the `replica.sh` script and execute the instructions below:
+
+```
+gitlab-ctl stop
+
+sudo cp /var/opt/gitlab/postgresql/data/recovery.conf ~/
+
+# prevent running database migrations on the secondary node:
+touch /etc/gitlab/skip-automigrations
+
+# we need to remove the old database:
+rm -rf /var/opt/gitlab/postgresql
+
+gitlab-ctl reconfigure
+gitlab-ctl pg-upgrade
+
+# see the stored credentials for the database, that you will need to re-initialize replication:
+grep -s primary_conninfo ~/recovery.conf
+
+# run the recovery script with the credentials above
+bash /tmp/replica.sh
+```
+