docs: add guide on editing machine configuration

This covers new configuration update modes and new commands in 0.9.

Signed-off-by: Andrey Smirnov <smirnov.andrey@gmail.com>

website/content/docs/v0.9/Guides/editing-machine-configuration.md

@@ -0,0 +1,104 @@
+---
+title: "Editing Machine Configuration"
+description: "How to edit and patch Talos machine configuration, with reboot, immediately, or stage update on reboot."
+---
+
+Talos node state is fully defined by [machine configuration](../Reference/configuration/).
+Initial configuration is delivered to the node at bootstrap time, but configuration can be updated while the node is running.
+
+> Note: Be sure that config is persisted so that configuration updates are not overwritten on reboots.
+> Configuration persistence was enabled by default since Talos 0.5 (`persist: true` in machine configuration).
+
+There are three `talosctl` commands which facilitate machine configuration updates:
+
+* `talosctl apply-config` to apply configuration from the file
+* `talosctl edit machineconfig` to launch an editor with existing node configuration, make changes and apply configuration back
+* `talosctl patch machineconfig` to apply automated machine configuration via JSON patch
+
+Each of these commands can operate in one of three modes:
+
+* apply change with a reboot (default): update configuration, reboot Talos node to apply configuration change
+* apply change immediately (`--immediate` flag): change is applied immediately without a reboot, only `.cluster` sub-tree of the machine configuration can be updated in Talos 0.9
+* apply change on next reboot (`--on-reboot`): change is staged to be applied after a reboot, but node is not rebooted
+
+> Note: applying change on next reboot (`--on-reboot`) doesn't modify current node configuration, so next call to
+> `talosctl edit machineconfig --on-reboot` will not see changes
+
+### `talosctl apply-config`
+
+This command is mostly used to submit initial machine configuration to the node (generated by `talosctl gen config`).
+It can be used to apply new configuration from the file to the running node as well, but most of the time it's not convenient, as it doesn't operate on the current node machine configuration.
+
+Example:
+
+```bash
+talosctl -n <IP> apply-config -f config.yaml
+```
+
+Command `apply-config` can also be invoked as `apply machineconfig`:
+
+```bash
+talosctl -n <IP> apply machineconfig -f config.yaml
+```
+
+Applying machine configuration immediately (without a reboot):
+
+```bash
+talosctl -n IP apply machineconfig -f config.yaml --immediate
+```
+
+### `taloctl edit machineconfig`
+
+Command `talosctl edit` loads current machine configuration from the node and launches configured editor to modify the config.
+If config hasn't been changed in the editor (or if updated config is empty), update is not applied.
+
+> Note: Talos uses environment variables `TALOS_EDITOR`, `EDITOR` to pick up the editor preference.
+> If environment variables are missing, `vi` editor is used by default.
+
+Example:
+
+```bash
+talosctl -n <IP> edit machineconfig
+```
+
+Configuration can be edited for multiple nodes if multiple IP addresses are specified:
+
+```bash
+talosctl -n <IP1>,<IP2>,... edit machineconfig
+```
+
+Applying machine configuration change immediately (without a reboot):
+
+```bash
+talosctl -n <IP> edit machineconfig --immediate
+```
+
+### `talosctl patch machineconfig`
+
+Command `talosctl patch` works similar to `talosctl edit` command - it loads current machine configuration, but instead of launching configured editor it applies [JSON patch](http://jsonpatch.com/) to the configuration and writes result back to the node.
+
+Example, updating kubelet version (with a reboot):
+
+```bash
+$ talosctl -n <IP> patch machineconfig -p '[{"op": "replace", "path": "/machine/kubelet/image", "value": "ghcr.io/talos-systems/kubelet:v1.20.4"}]'
+patched mc at the node <IP>
+```
+
+Updating kube-apiserver version in immediate mode (without a reboot):
+
+```bash
+$ talosctl -n <IP> patch machineconfig --immediate -p '[{"op": "replace", "path": "/cluster/apiServer/image", "value": "k8s.gcr.io/kube-apiserver:v1.20.4"}]'
+patched mc at the node <IP>
+```
+
+Patch might be applied to multiple nodes when multiple IPs are specified:
+
+```bash
+taloctl -n <IP1>,<IP2>,... patch machineconfig --immediate -p '[{...}]'
+```
+
+### Recovering from Node Boot Failures
+
+If a Talos node fails to boot because of wrong configuration (for example, control plane endpoint is incorrect), configuration can be updated to fix the issue.
+If the boot sequence is still running, Talos might refuse applying config in default mode.
+In that case `--on-reboot` mode can be used coupled with `talosctl reboot` command to trigger a reboot and apply configuration update.