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).
+
+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.