tiflash: add upgrade doc for 3.1

TOC.md

@@ -302,6 +302,7 @@
     - [Maintain a TiFlash Cluster](/reference/tiflash/maintain.md)
     - [Monitor TiFlash](/reference/tiflash/monitor.md)
     - [Scale TiFlash](/reference/tiflash/scale.md)
+    - [Upgrade TiFlash Nodes](/reference/tiflash/upgrade.md)
     - [Configure TiFlash](/reference/tiflash/configuration.md)
   + TiDB Binlog
     - [Overview](/reference/tidb-binlog/overview.md)

reference/tiflash/upgrade.md

@@ -0,0 +1,51 @@
+---
+title: Upgrade TiFlash Nodes
+summary: Learn how to upgrade TiFlash nodes.
+category: reference
+---
+
+# Upgrade TiFlash Nodes
+
+To upgrade TiFlash nodes, take the following steps:
+
+1. Back up the `tidb-ansible` folder:
+
+    {{< copyable "shell-regular" >}}
+
+    ```shell
+    mv tidb-ansible tidb-ansible-bak
+    ```
+
+2. Download tidb-ansible that corresponds to the tag of TiDB v3.1:
+
+    {{< copyable "shell-regular" >}}
+
+    ```shell
+    git clone -b $tag https://github.com/pingcap/tidb-ansible.git
+    ```
+
+3. Download binary:
+
+    {{< copyable "shell-regular" >}}
+
+    ```shell
+    ansible-playbook local_prepare.yml
+    ```
+
+4. Edit the `inventory.ini` file.
+
+5. Rolling upgrade TiFlash:
+
+    {{< copyable "shell-regular" >}}
+
+    ```shell
+    ansible-playbook rolling_update.yml --tags tiflash
+    ```
+
+6. Rolling upgrade the TiDB monitoring component:
+
+    {{< copyable "shell-regular" >}}
+
+    ```shell
+    ansible-playbook rolling_update_monitor.yml
+    ```