Feature Idea: 'Host/Cluster Waiting For Maintenance' Mode

##### ISSUE TYPE

* Improvement Request

##### COMPONENT NAME

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Host? Cluster? Im not sure
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##### CLOUDSTACK VERSION

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NA
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##### CONFIGURATION

##### OS / ENVIRONMENT

##### SUMMARY

### **Current Capability**
CloudStack currently offers a 'Maintenance' Mode, which facilitates the live migration of all VMs from a host and removes the host from the cluster for maintenance.

### **Proposed Feature: "Waiting for Maintenance" Mode**
The proposed "Waiting for Maintenance" Mode introduces a preparatory state that addresses scenarios where live migration is impractical or impossible. This feature would enable gradual decommissioning or maintenance while avoiding service disruption.

### **General Idea of How It Might Work:**

**1. **Operator Responsibilities:****
- Customer communication and notification will be managed entirely by the cloud company, outside of CloudStack. This is to inform customers that they are given a time window to voluntarily restart their VMs before the cut off date.

**2. CloudStack Responsibilities:**
- Block the creation of new VMs to the host/cluster marked as 'Waiting For Maintenance'
- Ensure restarted VMs are relocated to clusters with matching host tags.

_This is actually a similar process as how AWS Cloud does it: https://aws.amazon.com/maintenance-help/_

### **Use Cases**
**Scenario 1: Decommissioning an Old Compute Cluster**

Problem:
- Legacy clusters with outdated CPU architectures cannot perform live migration due to compatibility issues
(e.g., VM freezing during migration causing downtime).
- Existing VMs must restart to migrate to a new cluster with compatible architectures.
- The old cluster remains active, risking the placement of new VMs and hindering decommissioning.

**Scenario 2: Maintenance of GPU Clusters with GPU Passthrough**

Problem:
- GPU passthrough prevents live migration, unlike vGPU setups that allow seamless migration.
- Downtime-free maintenance is not feasible, requiring customer cooperation to restart affected VMs.

##### STEPS TO REPRODUCE

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NA
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##### EXPECTED RESULTS

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Refer to Above
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##### ACTUAL RESULTS

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Not able to facilitate smooth decomissioning of servers for compute where live migration is not possible.
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Commencez par lire le comportement existant de CloudStack pour les hosts et les clusters en mode maintenance, puis suivez la manière dont sont gérées la création, le redémarrage et la migration des VMs, ainsi que leur placement en fonction des tags des hosts. Définissez l’état visible par l’opérateur et ses effets pour les hosts comme pour les clusters, notamment la manière dont les VMs redémarrées sont placées ; le travail est considéré comme terminé lorsque les nouvelles VMs évitent les ressources marquées et que les VMs redémarrées atteignent des clusters compatibles portant les tags appropriés sans perturber les workloads existants.

Rédigé par le modèle d'indexation à partir du texte de l'issue.