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+# Support Kubernetes Cluster Autoscaler
+
+Longhorn should support Kubernetes Cluster Autoscaler.
+
+## Summary
+
+Currently, Longhorn pods are [blocking CA from removing a node](https://github.com/kubernetes/autoscaler/blob/master/cluster-autoscaler/FAQ.md#what-types-of-pods-can-prevent-ca-from-removing-a-node). This proposes to introduce a new global setting `kubernetes-cluster-autoscaler-enabled` that will annotate Longhorn components and also add logic for instance-manager PodDisruptionBudget management.
+
+### Related Issues
+
+https://github.com/longhorn/longhorn/issues/2203
+
+## Motivation
+
+### Goals
+
+- Longhorn should block CA from scaling down if a node met ANY condition:
+    - Any volume attached
+    - Contains a backing image manager pod
+    - Contains a share manager pod
+- Longhorn should not block CA from scaling down if a node met ALL conditions:
+    - All volume detached and there is another schedulable node with volume replica and replica IM PDB.
+    - Not contain a backing image manager pod
+    - Not contain a share manager pod
+
+### Non-goals [optional]
+
+- CA setup.
+- CA blocked by kube-system components.
+- CA blocked by backing image manager pod. (TODO)
+- CA blocked by share manager pod. (TODO)
+
+## Proposal
+Set `kubernetes-cluster-autoscaler-enabled` adds `cluster-autoscaler.kubernetes.io/safe-to-evict` annotation to Longhorn pods that are not backed by a controller, or with local storage volume mounts. To avoid data loss, Longhorn does not annotate the backing image manager and share manager pods.
+
+Currently, Longhorn creates instance-manager PDBs for replica/engine regardless of the volume state.
+During scale down, CA tries to find a removable node but failed by those instance-manager PDBs.
+
+We can add IM PDB handling to create and retained when the PDB is required:
+
+- There are volumes/engines running on the node. We need to guarantee that the volumes won't crash.
+- The only available/valid replica of a volume is on the node. Here we need to prevent the volume data from being lost.
+
+### User Stories
+
+#### CA scaling
+Before the enhancement, CA will be blocked by
+- Pods that are not backed by a controller (engine/replica instance manager).
+- Pods with [local storage volume mounts](https://github.com/kubernetes/autoscaler/blob/master/cluster-autoscaler/utils/drain/drain.go#L222) (longhorn-ui, longhorn-csi-plugin, csi-attacher, csi-provisioner, csi-resizer, csi-snapshotter).
+
+After enhancement, instance manager PDB will be actively managed by Longhorn:
+- Creates all engine/replica instance manager PDB when the volume is attached.
+- Delete engine instance manager PDB when the volume is detached.
+- Delete but keep 1 replica instance manager PDB when the volume is detached.
+
+the user can set a new global setting `kubernetes-cluster-autoscaler-enabled` to unblock CA scaling. This allows Longhorn to annotate Longhorn-managed deployments and engine/replica instance manager pods with `cluster-autoscaler.kubernetes.io/safe-to-evict`.
+
+
+### User Experience In Detail
+
+- Configure the setting via Longhorn UI or kubectl.
+- Ensure all volume replica count is set to more than 1.
+- CA is not blocked by Longhorn components when the node doesn't contain volume replica, backing image manager pod, and share manager pod.
+    - Engine/Replica instance-manager PDB will block the node if the volume is attached.
+    - Replica instance-manager PDB will block the node when CA tries to delete the last node with the volume replica.
+
+### API changes
+
+`None`
+
+## Design
+
+### Implementation Overview
+
+#### Global setting
+- Add new global setting `Kubernetes Cluster Autoscaler Enabled (Experimental)`.
+  - The setting is `boolean`.
+  - The default value is `false`.
+
+#### Annotations
+
+When setting `kubernetes-cluster-autoscaler-enabled` is `true`, Longhorn will add annotation `cluster-autoscaler.kubernetes.io/safe-to-evict` for the following pods:
+- The engine and replica instance-manager pods because those are not backed by a controller and use local storage mounts.
+- The deployment workloads are managed by the longhorn manager and using any local storage mount. The managed components are labeled with `longhorn.io/managed-by: longhorn-manager`.
+
+#### PodDisruptionBudget
+
+- No change to the logic to cleanup PDB if instance-manager doesn't exist.
+
+- Engine IM PDB:
+    - Delete PDB if volumes are detached;
+        - There is no instance process in IM (im.Status.Instance).
+        - The same logic applies when a node is un-schedulable. Node is un-schedulable when marked in spec or with CA tainted `ToBeDeletedByClusterAutoscaler`;
+    - Create PDB if volumes are attached; there are instance processes in IM (im.Status.Instance).
+
+- Replica IM PDB:
+    - Delete PDB if setting `allow-node-drain-with-last-healthy-replica` is enabled.
+    - Delete PDB if volumes are detached;
+        - There is no instance process in IM (im.Status.Instance)
+        - There are other schedulable nodes with healthy volume replica and have replica IM PDB.
+    - Delete PDB when a node is un-schedulable. Node is un-schedulable when marked in spec or with CA tainted `ToBeDeletedByClusterAutoscaler`;
+        - Check if the condition is met to delete PDB (same check as to when volumes are detached).
+        - Enqueue the replica instance-manager of another schedulable node with the volume replica.
+        - Delete PDB.
+    - Create PDB if volumes are attached:
+        - There are instance processes in IM (im.Status.Instance).
+    - Create PDB when volumes are detached;
+        - There is no instance process in IM (im.Status.Instance)
+        - The replica has been started. There are no other schedulable nodes with healthy volume replica and have replica IM PDB.
+
+### Test plan
+
+#### Scenario: test CA
+
+    Given Cluster with Kubernetes cluster-autoscaler.
+    And Longhorn installed.
+    And Set `kubernetes-cluster-autoscaler-enabled` to `true`.
+    And Create deployment with cpu request.
+    ```
+    resources:
+      limits:
+        cpu: 300m
+        memory: 30Mi
+      requests:
+        cpu: 150m
+        memory: 15Mi
+    ```
+
+    When Trigger CA to scale-up by increase deployment replicas.
+         (double the node number, not including host node)
+    ```
+    10 * math.ceil(allocatable_millicpu/cpu_request*node_number/10)
+    ```
+    Then Cluster should have double the node number.
+
+    When Trigger CA to scale-down by decrease deployment replicas.
+         (original node number)
+    Then Cluster should have original node number.
+
+#### Scenario: test CA scale down all nodes containing volume replicas
+
+    Given Cluster with Kubernetes cluster-autoscaler.
+    And Longhorn installed.
+    And Set `kubernetes-cluster-autoscaler-enabled` to `true`.
+    And Create volume.
+    And Attach the volume.
+    And Write some data to volume.
+    And Detach the volume.
+    And Create deployment with cpu request.
+
+    When Trigger CA to scale-up by increase deployment replicas.
+         (double the node number, not including host node)
+    Then Cluster should have double the node number.
+
+    When Annotate new nodes with `cluster-autoscaler.kubernetes.io/scale-down-disabled`.
+         (this ensures scale-down only the old nodes)
+    And Trigger CA to scale-down by decrease deployment replicas.
+        (original node number)
+    Then Cluster should have original node number + 1 blocked node.
+
+    When Attach the volume to a new node. This triggers replica rebuild.
+    And Volume data should be the same.
+    And Detach the volume.
+    Then Cluster should have original node number.
+    And Volume data should be the same.
+
+#### Scenario: test CA should block scale down of node running backing image manager pod
+
+Similar to `Scenario: test CA scale down all nodes containing volume replicas`.
+
+### Upgrade strategy
+
+`N/A`
+
+## Note [optional]
+
+`N/A`