Merge pull request #216 from percona/pxc-4611

PXC-4611 [DOCS] - 8.0 --> 8.4 upgrade path

Author: patrickbirch

Diff for: docs/upgrade-backup.md

@@ -0,0 +1,39 @@
+# Restore 8.0 backup to 8.4 cluster
+
+Use Percona XtraBackup to back up the source server data and restore the data to a target server, and then upgrade the server to a different version of Percona XtraDB Cluster.
+
+## Restore a database with a different server version
+
+Review [Upgrade Percona XtraDB cluster](upgrade-guide.md).
+
+Upgrade the nodes one at a time. The primary node should be the last node to be upgraded. The following steps are required on each node.
+
+## Upgrade first node
+
+1. Install the new database server version on the target server.
+
+2. Start the new database server version on the restored data directory.
+
+3. Perform any other upgrade steps as necessary.
+
+To ensure the upgrade was successful, verify the data by performing the following checks:
+
+| Areas to check          | Details                                                                 |
+|-------------------------|-------------------------------------------------------------------------|
+| Data integrity          | Compare the data before and after the upgrade to ensure no data loss or corruption occurred. |
+| Application functionality | Test the application to confirm it interacts correctly with the upgraded database. |
+| Performance metrics     | Monitor performance metrics to ensure the upgrade did not negatively impact database performance. |
+| Logs and errors         | Review logs for any errors or warnings that might indicate issues during the upgrade process. |
+
+
+## Upgrade other nodes
+
+To create the 2nd and subsequent nodes, follow these steps:
+
+1. Install Percona XtraDB Cluster (PXC) 8.4.
+
+2. Join the cluster using State Snapshot Transfer (SST).
+
+Starting from version 8.0.34, this procedure can also be used to downgrade within the same LTS version.
+
+