Proposal for adding Additional Disks to node

Author: anshuman-agarwala

docs/proposal/additional-disks.md

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+# Adding additional volumes to Nodes
+
+[Github Issue](https://github.com/kubernetes-sigs/cluster-api-provider-ibmcloud/issues/1920)
+
+## Summary
+We want the ability to add additional Volumes to our Machine nodes.
+
+## Proposal
+We will need to make changes to both the API and the controller logic.
+With regards to the API, we will need to add a new `AdditionalVolumes` field to the Machine Spec to contain all the different volumes that are added to the node, the field will be a slice of type `VPCVolume`. As an initial design we can keep the field [append-only](https://kubernetes.io/blog/2022/09/29/enforce-immutability-using-cel/#append-only-list-of-containers).
+
+### API Changes
+The Additional Volumes field will be a part of the `IBMVPCMachineSpec` struct and will be defined as follows:
+```go
+    // AdditionalVolumes is the list of additional volumes attached to the disk
+	// +kubebuilder:validation:Optional
+	// +kubebuilder:validation:MaxItems=10
+	// +kubebuilder:validation:XValidation:rule="oldSelf.all(x, x in self)",message="Values may only be added"
+	AdditionalVolumes []*VPCVolume `json:"additionalVolumes,omitempty"`
+```
+
+The `VPCVolume` struct is [already present](https://github.com/kubernetes-sigs/cluster-api-provider-ibmcloud/blob/3bf33db00c0a46dfc2b78861e8519072fa51b441/api/v1beta2/ibmvpcmachine_types.go#L99) in the API spec and is currently being used to describe the boot disks.
+
+A sample IBMVPCMachineSpec instance with AdditionalDisks would look like the following:
+```yaml
+# apiVersion, kind, metadata...
+spec:
+  additionalVolumes:
+  # Additional Volume 1
+  - name: GeneralVolume
+    profile: "general-purpose" # Using general-purpose as the volume profile here allows us to use the defaults associated with this template
+  # Additional Volume 2
+  - iops: 10000
+    name: CustomVolume
+    profile: "custom" # Using a custom profile allows us more freedom when choosing the volume spec
+    sizeGiB: 120
+  # Standard Boot Volume, this is already a part of the spec
+  bootVolume:
+    name: BootVolume
+    profile: "general-purpose"
+    sizeGiB: 100
+  # Other IBMVPCMachine.Spec fields
+```
+
+### Controller flow
+The flow for creating Additional Volumes will be similar to creating the Boot Volume.
+We will need to add checks in the admission and mutation webhooks to validate any changes to the field like we do for `VPCVolume`.
+We can allow the field be unset initially so if a user wants to add Volumes after provisioning a machine, they will be able to.
+
+The Machine create flow will be as follows:
+If a user does not add Additional Volumes while creating the Machine, the standard create flow will be followed.
+If they add Additional Volumes when creating the Machine, the Machine reconciler will also create the Additional Volumes and attach them to the machine.
+
+The Machine update flow will be as follows:
+The `AdditionalVolumes` field will be append-only so a user will not be able to remove already created volumes. If a user adds more volumes to the slice, those Volumes will be provisioned and then attached to the machine.
+
+The Delete flow will not change much, the Additional Volumes' deletion flow will be similar to the Boot Volume's. 
+
+### Limitation
+The biggest limitation of the current approach will be that the `AdditionalVolumes` field will be append-only and therefore there won't be a way to delete any Additional Volumes that are provisioned without deleting the entire machine.
+This limitation can be addressed later by enhancing the controller flow to manage update scenarios where Volumes are removed from the `AdditionalVolumes` slice. This would also require the append-only nature of the `AdditionalVolumes` field to change.