Azure Documentation

Scaling Azure Virtual Machines

Scaling your Azure Virtual Machines (VMs) is crucial for ensuring your applications remain available and performant under varying loads. Azure offers several mechanisms to scale your VMs, broadly categorized into vertical scaling (changing the size of a VM) and horizontal scaling (adding or removing instances of a VM).

Vertical Scaling (Scale Up/Down)

Vertical scaling involves changing the size of a single VM to provide more or less compute resources (CPU, memory, disk I/O). This is often the simplest way to adjust performance for a specific workload.

When to Use Vertical Scaling:

• Your application has a limited number of components that can't be easily distributed.
• You need to increase resources for a single, high-demand VM.
• You want to reduce costs by downscaling when demand is low.

How to Scale Vertically:

You can scale a VM vertically through the Azure portal, Azure CLI, or PowerShell.

1. Navigate to your Virtual Machine resource in the Azure portal.
2. Under the Settings section, select Size.
3. Choose a new VM size from the available options. Azure will display compatible sizes based on your VM's current configuration and available regions.
4. Click Resize. Note that resizing often requires the VM to be stopped (deallocated).

Example using Azure CLI:

az vm resize --resource-group MyResourceGroup --name MyVM --size Standard_DS4_v2
            

Horizontal Scaling (Scale Out/In)

Horizontal scaling involves adding or removing instances of your application running on VMs. This is typically achieved using Azure Virtual Machine Scale Sets (VMSS).

When to Use Horizontal Scaling:

• Your application is designed to be stateless or can manage distributed state effectively.
• You need to handle significant fluctuations in traffic or workload.
• You require high availability by distributing your application across multiple instances.

Azure Virtual Machine Scale Sets (VMSS):

VMSS allows you to deploy and manage a set of identical, load-balanced VMs. They provide automatic scaling based on predefined metrics.

• Automatic Scaling: Configure rules to automatically increase or decrease the number of VM instances in your scale set based on CPU usage, network traffic, or custom metrics.
• Orchestration Modes: VMSS offers different orchestration modes (uniform and flexible) to suit various deployment scenarios.
• Load Balancing: Easily integrate with Azure Load Balancer or Application Gateway to distribute traffic across VM instances.

How to Configure Automatic Scaling for VMSS:

1. Navigate to your Virtual Machine Scale Set resource in the Azure portal.
2. Under the Settings section, select Scale (auto-scale).
3. Choose a scaling mode: Manual, Auto, or Custom. For automatic scaling, select Auto.
4. Configure the instance limits (minimum, maximum, default).
5. Define scale-out and scale-in rules based on metrics like CPU percentage, disk I/O, or network in/out.
6. Set the cool-down period to prevent rapid scaling fluctuations.

Example configuration for an auto-scale rule (conceptual):

{
              "name": "ScaleOutRule",
              "description": "Scale out when CPU is high",
              "action": {
                "direction": "Increase",
                "type": "Count",
                "value": "2",
                "minInstanceCount": 2
              },
              "condition": {
                "dataSource": {
                  "metricName": "Percentage CPU",
                  "resourceId": "/subscriptions/.../resourceGroups/.../providers/Microsoft.Compute/virtualMachineScaleSets/..."
                },
                "operator": "GreaterThan",
                "threshold": 70,
                "timeAggregation": "Average",
                "category": "Metric"
              },
              "scaleInRules": [],
              "scaleOutRules": []
            }
            

Choosing the Right Scaling Strategy

The best scaling strategy depends on your application's architecture, performance requirements, and budget.

• Simple Workloads: Vertical scaling might be sufficient.
• Web Applications, APIs, Microservices: Horizontal scaling with VMSS is generally preferred for elasticity and resilience.
• Hybrid Approach: Sometimes, a combination of scaling strategies is optimal. For instance, you might scale out a VMSS and, within each instance, use vertical scaling to adjust the resources of individual VMs if needed.

Always monitor your application's performance and resource utilization to fine-tune your scaling configurations.