docs: Use FLUO to update nodes

docs/how-to-guides/auto-update-flatcar.md

@@ -0,0 +1,83 @@
+---
+title: Auto-updating Flatcar Container Linux
+weight: 10
+---
+
+## Introduction
+
+At the moment, Lokomotive supports only [Flatcar Container Linux](https://www.flatcar-linux.org/) as
+the underlying operating system for nodes. While Flatcar can keep itself up to date, when running
+Kubernetes on top of it, we recommend using the [Flatcar Linux Update Operator
+(FLUO)](https://github.com/kinvolk/flatcar-linux-update-operator) to perform updates to avoid
+rebooting too many nodes at a time which could cause a service outage.
+
+FLUO is a supported Lokomotive component.
+
+This guide will show how to install it and disable its
+auto-update feature for specific nodes, which might be useful when you run services, which require
+special care before shutting down (e.g. storage clusters).
+
+> **NOTE:** If you want to use the FLUO component on a non-Lokomotive cluster, make sure your
+> Flatcar nodes have `locksmithd.service` systemd unit disabled to avoid nodes rebooting on their
+> own. On Lokomotive, it is disabled by default.
+
+## Prerequisites
+
+- A Lokomotive cluster accessible via `kubectl`.
+
+## Steps
+
+### Step 1: Disable auto-update for sensitive nodes
+
+If you want to update the particular nodes manually in a controlled fashion, there is a way to
+disable automatic updates. Disabling updates can come in handy when the workloads run by these
+machines are storage or ingress network related, where applications cannot tolerate the abrupt
+reboot of node.
+
+Please annotate the nodes as follows:
+
+```bash
+kubectl annotate node <node name> "flatcar-linux-update.v1.flatcar-linux.net/reboot-paused=true"
+```
+
+### Step 2: Configure FLUO
+
+Add the following content to your cluster configuration (e.g. in `fluo.lokocfg` file):
+
+```tf
+component "flatcar-linux-update-operator" {}
+```
+
+### Step 3: Install FLUO
+
+Execute the following command to deploy the `flatcar-linux-update-operator` component:
+
+```bash
+lokoctl component apply flatcar-linux-update-operator
+```
+
+Verify that pods in the `reboot-coordinator` namespace are running (this may take a
+few minutes):
+
+```bash
+kubectl -n reboot-coordinator get pods
+```
+
+Now that you have installed FLUO, nodes without annotation
+`flatcar-linux-update.v1.flatcar-linux.net/reboot-paused=true` will be updated automatically when
+a new version of Flatcar is available. One-by-one, the selected node is first drained before the
+reboot for an update.
+
+### Step 4: Test installation (optional)
+
+You can annotate a node to trigger an automatic reboot:
+
+```bash
+export NODE="<node name>"
+kubectl annotate node $NODE --overwrite \
+    flatcar-linux-update.v1.flatcar-linux.net/reboot-needed="true"
+```
+
+You can also SSH into a node and trigger an update check by running
+`update_engine_client -check_for_update` or simulate a reboot is needed by running
+`locksmithctl send-need-reboot`.