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5.3 Compute - Virtual machines (VMs) - VM Scale Sets (VMMS).md

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VM Scale Sets (VMSS)

  • Group that holds identically configured VMs
  • Used for
    • Need to create and manage multiple VMs
      • Centrally create and manage multple VMs (Windows Server or Linux)
    • Need for high availability and app resiliency
      • Horizontal scaling, scaling up and down based on spikes
    • Need for large (1000) scale
      • E.g. Azure Batch uses scale sets under the hood
    • Need for IaaS autoscale
      • Scale out and in based on metrics based autoscale

PaaS Scaling vs IaaS Scaling

  • Azure App Service
    • High agility at the expense of administrative power
    • The underlying Hyper-V Vms are almost totally abstracted from you
    • Easy manual, scheduled, or automatic scale out and scale back
  • Virtual Machine Scale Set (VMSS)
    • Maximum administrative power at the expense of agility
    • VMSS represents Azure's approach to IaaS horizontal scaling

Deploying a VM Scale Set

  • Create virtual machine scale set
    • Availability zone
      • Scale scale sets across one and more availability zones
      • ❗ All regions do not support availability zone
    • Instance count & instance set
    • Low priority
      • Take advantage of unutilized capacity
        • Compute power that customers/Microsoft is not using
        • Save costs
      • Good for workloads that can handle interruption
        • Stateless workloads
        • VMs in the scale set may be evicted at any time
        • You set eviction policy:
          • Stop / Deallocate
          • Delete
    • Use manage/unmanaged disks
      • ❗ Managed disks are not supported with availability zones
    • Networking
      • Application Gateway
        • 💡 Useful if your scale sets are web servers
        • ❗ Do not support RDP
      • Load Balancer
        • Supports RDP
        • You set public IP address name and domain name label (domain-name.region.cloudapp.azure.com)
  • You can also use ARM template e.g. Deploy a Windows VM Scale Set with a Custom Script Extension that deploys VMs, load balancer and a powershell script to be executed after deployment.

Connecting to VMs

  • In portal: Choose VM → Settings → Instances you can see all the instances
  • To connect to individual instances you need load balancer and NAT (network address translation)
    • You can't RDP/SSH into individual instances directly
    • You can connect to load balancer IPs
      • In portal: Load Balancer → Inbound NAT rules
    • NAT maps different VMs on different ports.

Configuring Autoscale

  • Manual: Through Portal/SDK/CLI/PowerShell
  • Autoscale
  • Scheduled: If you know when the load will be high you can plan for that and scale with time triggers
  • Metrics: Use various metrics from various sources to determine when to scale in/out
  • Manage in VMSS → Scaling →
    • Enable auto-scaling
    • Select scale-mode
    • Scale based on metric
      • Add rule
      • E.g. increase instance count by 1 when CPU percentage above 70%
      • 💡 You should also create scale mode that bring down the scale count
      • Properties
        • Duration: Good to not be confused when scaling out/in, so set a duration to e.g. 10 minutes
        • Cooldown: Waits after scale operation before new scale operation
    • Scale to specific instance count
      • Time-based scaling
      • Set start and end date