Stage 5 · Platform

Compute & Containers

Azure Virtual Machines

VM sizes, managed disks, availability sets, scale sets, and VM extensions.

7 min readAzure Cloud EngineeringPlatform

Virtual Machines Overview

Azure Virtual Machines are Infrastructure-as-a-Service (IaaS) compute resources. You get full control over the operating system, networking, and storage. VMs are the foundation for workloads that cannot run on PaaS or containers.

Create a basic VM


az vm create \
  --resource-group rg-webapp \
  --name vm-web-01 \
  --image Ubuntu2204 \
  --size Standard_D2s_v3 \
  --admin-username azureuser \
  --ssh-key-values ~/.ssh/id_rsa.pub \
  --tags Environment=production Team=web

The --ssh-key-values flag uploads your public key for SSH access. Never use --password for production VMs.

VM Sizes and Families

Azure offers hundreds of VM sizes organized into families, each optimized for different workloads. Choosing the right family can save 50% or more compared to a generic size.

Family	Optimized For	Example Sizes
Dv5/Ev5	General purpose workloads	D2s_v5, D4s_v5, E4s_v5
Fsv2	Batch processing, gaming	F2s_v2, F4s_v2, F8s_v2
Mv2	In-memory databases	M8ms, M16ms, M32ms
Nv4/Npv4	GPU workloads, ML inference	NC4as_T4_v3, ND96asr_v4
B-series	Dev/test, burstable	B1s, B2ms, B4ms

List available VM sizes


# List all available sizes in a region
az vm list-sizes --location eastus --query "[].{name:name, vcpus:numberOfCores, memoryMB:memoryInMb}" --output table

# List sizes that support premium storage
az vm list-sizes --location eastus --query "[?premiumIo==`true`].{name:name, vcpus:numberOfCores}" --output table

Not all VM sizes are available in all regions. Always verify availability before designing around a specific size.

Use B-series for dev/test

B-series VMs use burstable CPU credits — they run at baseline and burst when needed. They cost 50-70% less than equivalent D-series VMs and are ideal for dev/test environments.

Managed Disks

Managed Disks are block-level storage volumes attached to VMs. Azure manages the underlying storage account — you just specify the size and type. Premium SSD v2 and Ultra Disks offer the lowest latency for I/O-intensive workloads.

Attach a data disk to a VM


# Create and attach a Premium SSD data disk
az vm disk attach \
  --resource-group rg-webapp \
  --vm-name vm-web-01 \
  --name disk-web-data \
  --size-gb 128 \
  --sku Premium_LRS \
  --lun 0

# List disks attached to a VM
az vm show \
  --resource-group rg-webapp \
  --name vm-web-01 \
  --query "storageProfile.dataDisks[].{name:name, sizeGB:diskSizeGb, caching:caching}" \
  --output table

Managed Disks are billed per GB/month plus per-transaction costs. Premium SSDs provide single-digit millisecond latency for production workloads.

Availability Sets and Zones

Availability distributes VMs across failure domains (racks) and update domains. Availability Sets protect against hardware failures within a datacenter. Availability Zones distribute VMs across physically separate datacenters within a region.

Feature	Availability Set	Availability Zone
SLA	99.95%	99.99%
Failure domain	Rack within a datacenter	Separate datacenter
Network latency	< 1ms (same DC)	1-2ms (same region)
Cost	Free	Free

Create VMs in an Availability Zone


# Create VMs across two zones for high availability
az vm create \
  --resource-group rg-webapp \
  --name vm-web-az1 \
  --image Ubuntu2204 \
  --size Standard_D2s_v3 \
  --zone 1 \
  --admin-username azureuser \
  --ssh-key-values ~/.ssh/id_rsa.pub

az vm create \
  --resource-group rg-webapp \
  --name vm-web-az2 \
  --image Ubuntu2204 \
  --size Standard_D2s_v3 \
  --zone 2 \
  --admin-username azureuser \
  --ssh-key-values ~/.ssh/id_rsa.pub

Availability Zones provide a 99.99% SLA — the highest for Azure VMs. Always prefer Zones over Availability Sets when available in your region.

Virtual Machine Scale Sets

Virtual Machine Scale Sets (VMSS) let you create and manage a group of identical, load-balanced VMs. The number of VM instances can automatically increase or decrease based on demand.

Create a scale set


# Create a VMSS with autoscaling
az vmss create \
  --resource-group rg-webapp \
  --name vmss-web \
  --image Ubuntu2204 \
  --instance-count 2 \
  --vm-sku Standard_D2s_v3 \
  --admin-username azureuser \
  --ssh-key-values ~/.ssh/id_rsa.pub \
  --upgrade-policy-mode Automatic \
  --autoscale \
  --min-count 2 \
  --max-count 10 \
  --scale-out-cpu-threshold 75

The autoscale configuration creates a default scale-out rule (when average CPU > 75%) and a scale-in rule (when average CPU < 25%). The instance count stays between --min-count and --max-count.

VM Extensions and Custom Script

VM Extensions are small applications that run post-deployment tasks on VMs. They can install software, configure settings, or collect diagnostics. The Custom Script Extension lets you run arbitrary scripts.

Run a custom script on a VM


# Upload a script to a storage account
az storage blob upload \
  --account-name stscriptsprod \
  --container-name scripts \
  --name setup-web.sh \
  --file ./scripts/setup-web.sh

# Execute the script on a VM
az vm extension set \
  --resource-group rg-webapp \
  --vm-name vm-web-01 \
  --name CustomScript \
  --publisher Microsoft.Azure.Extensions \
  --settings '{"commandToExecute":"bash /tmp/setup-web.sh", "fileUris":["https://stscriptsprod.blob.core.windows.net/scripts/setup-web.sh"]}'

The Custom Script Extension downloads files from the fileUris and executes the command. It is commonly used for initial VM configuration that is not handled by cloud-init.

VM extensions run as root

Custom Script Extensions execute with root privileges on Linux and SYSTEM on Windows. Ensure scripts are tested and do not expose secrets or execute untrusted code.

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