user-guide: Add Pre-Provisioning NetworkData details

Add information to the instance customization guide to explain
Pre-Provisioning NetworkData and how it differs to the regular
networkData

Signed-off-by: Steven Hardy <[email protected]>

docs/user-guide/src/SUMMARY.md

@@ -23,6 +23,7 @@
       - [Reboot Annotation](bmo/reboot_annotation.md)
       - [Root Device Hints](bmo/root_device_hints.md)
    - [Advanced Features](bmo/features.md)
+      - [Advanced Instance Customization](bmo/advanced_instance_customization.md)
       - [Detached annotation](bmo/detached_annotation.md)
       - [External Inspection](bmo/external_inspection.md)
       - [Live ISO](bmo/live-iso.md)

docs/user-guide/src/bmo/advanced_instance_customization.md

@@ -0,0 +1,61 @@
+# Instance Customization
+
+Below we cover more advanced instance customization, more complex use-cases
+and/or where customization of the metal3 deployment may be required.
+
+For more general guidance around instance customization refer to the
+[instance customization](./instance_customization.md) section.
+
+## Pre-Provisioning NetworkData
+
+*Pre-provisioning network data* describes the desired networking configuration for the
+deploy ramdisk (IPA).
+
+Usage of this API requires an IPA ramdisk image with a tool capable of interpreting and
+applying the data such as *cloud-init*, *Glean* or alternative. The default community
+supported ramdisk does not currently contain such a tool, but it is possible to build
+a custom image, for example using [ironic-python-agent-builder][ipa_builder] with the
+[simple-init][simple_init] element enabled.
+
+Specifying pre-provisioning network data is useful in DHCP-less scenarios, where we
+cannot rely on DHCP to provide network configuration for the IPA ramdisk during the
+inspection and provisioning phases. In this situation we can use redfish virtualmedia
+to boot the IPA ramdisk, and the generated virtualmedia ISO will also serve as a
+configuration drive to provide the network configuration.
+
+The data is specified in the [OpenStack network_data.json][network_data] format
+as described for *Network data* in the [instance customization](./instance_customization.md) section.
+
+Usually, one pre-provisioning network data secret is created per host and should be
+linked to it like *Network data*. If you require the same configuration for
+pre-provisioning and the deployed OS, it is only necessary to specify pre-provisioning
+network data - the pre-provisioning secret is automatically applied to networkData if
+no alternative secret is specified.
+
+For example, given a local file `host-0-network.json`, you can create a secret:
+
+```bash
+kubectl create secret generic host-0-preprov-networkdata --from-file=networkData=host-0-network.json
+```
+
+Then you can attach it to the host during its enrollment:
+
+```yaml
+apiVersion: metal3.io/v1alpha1
+kind: BareMetalHost
+metadata:
+  name: host-0
+  namespace: my-cluster
+spec:
+  online: true
+  bootMACAddress: 80:c1:6e:7a:e8:10
+  bmc:
+    address: redfish-virtualmedia://192.168.1.13
+    credentialsName: host-0-bmc
+  preprovisioningNetworkDataName:
+    name: host-0-preprov-networkdata
+```
+
+[network_data]: https://docs.openstack.org/nova/latest/user/metadata.html#openstack-format-metadata
+[ipa_builder]: https://docs.openstack.org/ironic-python-agent-builder/
+[simple_init]: https://docs.openstack.org/diskimage-builder/latest/elements/simple-init/README.html

docs/user-guide/src/bmo/features.md

@@ -15,6 +15,7 @@
 
 ## Advanced features
 
+- [Advanced Instance Customization](./advanced_instance_customization.md)
 - [Detached Annotation](./detached_annotation.md)
 - [External Inspection](./external_inspection.md)
 - [Live ISO](./live-iso.md)

docs/user-guide/src/bmo/instance_customization.md

@@ -38,10 +38,14 @@ instead when possible.
 
 ## NetworkData
 
-*Network data* describes the desired networking configuration in the [OpenStack
-network_data.json][network_data] format supported by *cloud-init* and *Glean*.
-The format is not very well documented, but you can consult the [network_data
-JSON schema][network_data schema] shipped with OpenStack.
+*Network data* describes the desired networking configuration for the deployed
+operating system, and is typically applied on first-boot via tools such as
+*cloud-init*.
+
+The data is specified in the [OpenStack network_data.json][network_data] format
+supported by *cloud-init* and *Glean*.  The format is not very well documented,
+but you can consult the [network_data JSON schema][network_data schema]
+shipped with OpenStack.
 
 Usually, one network data secret is created per host and should be linked to
 it. For example, given a local file `host-0-network.json`, you can create a