refine docs

Author: bertiethorpe

docs/experimental/compute-init.md

@@ -4,18 +4,15 @@ See the role README.md
 
 # Changes to image / tofu state
 
-When a compute group has ignore_image_changes set as true in a compute group
-partition within tofu/main.tf, and the image is updated:
-
-Subsequent changes to the tf cluster_image variable for that compute group don’t
-actually result in a change via tofu plan/apply. This is done with the
-lifecycle meta-argument "ignore_changes" in the compute resource.
-
-As part of compute-init, the image_id is templated out to hostvars so that
-ansible will have image_id for each compute node.
-
-WIP:    Attempts to change the cluster image from tofu then act as a target
-        for compute-init to read and rebuild to via slurm control.
+When a compute group has the `ignore_image_changes` parameter set to true,
+changes to the `image_id` parameter (which defaults to `cluster_image_id`) are
+ignored by OpenTofu.
+
+Regardless of whether `ignore_image_changes` is set, OpenTofu templates out the
+`image_id` into the Ansible inventory for each compute node. The `compute_init`
+role templates out hostvars to the control node, which means the "target" image
+ID is then available on the control node. Subsequent work will use this to
+rebuild the node via slurm.
 
 # CI workflow
 

environments/skeleton/{{cookiecutter.environment}}/tofu/compute/nodes.tf

@@ -93,7 +93,7 @@ resource "openstack_compute_instance_v2" "compute_fixed_image" {
 
   lifecycle {
     ignore_changes = [
-    image_id,
+      image_id,
     ]
   }
 

environments/skeleton/{{cookiecutter.environment}}/tofu/compute/variables.tf

@@ -97,6 +97,6 @@ variable "compute_init_enable" {
 
 variable "ignore_image_changes" {
     type = bool
-    description = "Whether to prevent tofu from applying changes to the cluster image"
+    description = "Whether to ignore changes to the image_id parameter"
     default = false
 }

environments/skeleton/{{cookiecutter.environment}}/tofu/variables.tf

@@ -53,6 +53,7 @@ variable "compute" {
             vnic_type: Overrides variable vnic_type
             vnic_profile: Overrides variable vnic_profile
             compute_init_enable: Toggles compute-init rebuild (see compute-init role docs)
+            ignore_image_changes: ignore changes to the image_id parameter (see docs/experimental/compute-init.md)
             volume_backed_instances: Overrides variable volume_backed_instances
             root_volume_size: Overrides variable root_volume_size
             extra_volumes: Mapping defining additional volumes to create and attach