Troubleshooting Azure Virtual Machine Scale Sets

This guide provides common troubleshooting steps and solutions for issues you might encounter with Azure Virtual Machine Scale Sets (VMSS).

Common Issues and Solutions

1. Instance Failures During Creation or Update

Symptom: Virtual machines within the scale set are failing to provision or update, often with error messages related to image deployment, network configuration, or disk issues.

• Check Instance Status: Navigate to the scale set in the Azure portal, go to "Instances", and check the status of failed instances. Click on an instance to see detailed error messages.
• Review Activity Log: The scale set's Activity Log can provide higher-level insights into deployment failures.
• Verify Image and Extensions: Ensure the OS image is valid and accessible. Check the status and logs of any VM extensions being deployed. Common extension issues include incorrect configuration or failures during their execution.
• Network Configuration: Validate that the Network Security Group (NSG) rules, Azure Firewall policies, or User Defined Routes (UDRs) are not blocking necessary traffic for the VMs to reach Azure services or perform health checks.
• Disk Provisioning: Ensure sufficient disk capacity and that disk types are compatible. Check for quota limits on managed disks.

2. Application Unresponsiveness or Crashes

Symptom: Instances are running, but the application hosted on them is not responding or crashing. This can impact the scale set's health probes.

• Check Application Logs: Access the application logs directly on the VM instances (via RDP, SSH, or log forwarding solutions like Azure Monitor Logs) to diagnose application-specific errors.
• Health Probes: If using custom health probes, ensure they are configured correctly and that the application is responding as expected to the probe requests. Verify the protocol, port, and path.
• Resource Exhaustion: Monitor CPU, memory, disk I/O, and network utilization on the instances. High utilization can lead to application unresponsiveness. Consider scaling up or out.
• Dependencies: Check if the application relies on external services (databases, APIs, etc.) and ensure those dependencies are healthy and accessible from the scale set instances.

3. Health Probe Failures

Symptom: The load balancer or application gateway reports health probe failures for instances, leading to traffic being diverted away from them.

• Probe Configuration: Double-check the probe's protocol (HTTP, HTTPS, TCP), port, and path. Ensure the path is valid and returns a successful status code (e.g., 200 OK for HTTP/S).
• Network Connectivity: Verify that the load balancer/application gateway can reach the probe endpoint on the VM instances. This involves checking NSGs, firewalls, and routing.
• Application Responsiveness: As mentioned above, if the application is not responding to the probe, the probe will fail.
• Instance State: Ensure the VM instances are running and that the application service is started and healthy within the OS.

4. Scale-Out/Scale-In Issues

Symptom: The scale set is not scaling out when load increases or not scaling in when load decreases, or it's scaling inconsistently.

• Autoscaling Metrics: Review the metrics used for autoscaling (CPU, memory, custom metrics). Ensure they are reporting accurate data and that the thresholds are configured correctly.
• Autoscaling Rules: Verify the min, max, and default instance counts, as well as the scale-out/scale-in cooldown periods.
• Metric Aggregation: Understand how the autoscaling metric is aggregated across the scale set instances.
• Scale Set State: Ensure the scale set itself is healthy and not in a constrained state due to other issues.

5. Networking Connectivity Problems Between Instances

Symptom: Instances within the scale set cannot communicate with each other.

• VNet and Subnet Configuration: Ensure instances are in the same or properly peered virtual networks and subnets.
• NSG Rules: Verify that Network Security Groups applied to the subnet or network interface do not block intra-VNet traffic. By default, VNet-to-VNet traffic is allowed unless explicitly denied by an NSG.
• Application-Layer Firewalls: If you have host-based firewalls enabled on the VMs, ensure they permit inter-instance communication on the required ports.

6. Instance Reimaging or Disk Corruption

Symptom: Instances become inaccessible, report disk errors, or require a complete rebuild.

• Backup and Restore: For critical data, ensure regular backups are in place. Use Azure Backup or application-level backup solutions.
• Snapshotting: Take snapshots of the OS and data disks of problematic instances before attempting any major troubleshooting steps that involve disk manipulation.
• Reimaging: If an instance's OS disk is suspected to be corrupted, you can often "reimage" the instance. This provisions a new VM using the scale set's model definition and attached data disks. Be aware this can cause data loss if not properly managed.

Advanced Troubleshooting Tools

• Azure Resource Graph: Query your scale set resources to identify misconfigurations or trends.
• Azure Monitor Logs (Log Analytics): Collect logs from your scale set instances for in-depth analysis of application and system events.
• Azure Advisor: Provides recommendations for performance, security, cost, and operational excellence for your Azure resources.
• Azure CLI / PowerShell: Script complex troubleshooting tasks and retrieve detailed instance information.