# Virtual Machines and Scale Sets — Azure

Source: https://www.skillbyai.com/en/azure/c-vms

> Size, place and scale Azure VMs for availability.

## Infrastructure as a service

An **Azure Virtual Machine** gives you a full operating system, so you control patches, runtimes and agents: maximum flexibility, maximum responsibility. A VM size (for example `Standard_D2s_v5`) fixes vCPU, memory and disk throughput; the **B-series** is burstable and cheap for low average load. Each VM has an OS disk and optional data disks backed by **managed disks** (Standard HDD, Standard SSD, Premium SSD, Premium SSD v2, Ultra). For availability, place VMs across **availability zones**, or at least in an **availability set**, which spreads them over fault and update domains inside one datacentre. **Virtual Machine Scale Sets** manage a group of identical VMs and add or remove instances with autoscale rules. **Spot VMs** use spare capacity at a large discount but can be evicted, which suits batch jobs, not your login server.

## Scale out across zones

A scale set places identical instances in different zones behind a load balancer, and autoscale changes the count.

![Three zone columns, each holding identical server blocks, with a balancer above distributing arrows to all of them.](assets/figures/azure/section-3-map.svg) — Figure 3.1 — A zone-spanning scale set behind a load balancer.

## Creating a zone-redundant scale set

A small Linux scale set across three zones with a CPU-based autoscale rule.

```bash
az vmss create -g rg-shop-dev-cin -n vmss-web \
  --image Ubuntu2204 --vm-sku Standard_B2s \
  --instance-count 2 --zones 1 2 3 \
  --admin-username azureuser --generate-ssh-keys

az monitor autoscale create -g rg-shop-dev-cin \
  --resource vmss-web --resource-type Microsoft.Compute/virtualMachineScaleSets \
  --name web-autoscale --min-count 2 --max-count 6 --count 2

az monitor autoscale rule create -g rg-shop-dev-cin --autoscale-name web-autoscale \
  --condition "Percentage CPU > 70 avg 5m" --scale out 1
```

## Stopped is not deallocated

Shutting a VM down from inside the OS leaves it *Stopped* and you still pay for compute. Stop it from Azure (`az vm deallocate`) to release the hardware. Disks and public IPs are billed either way.

**Quiz:** Which option is cheapest for an interruptible overnight batch job?

- [x] Spot VMs
- [ ] Premium SSD v2 disks
- [ ] A single VM in an availability set
- [ ] Ultra disks

*Answer:* Spot VMs. Spot VMs are heavily discounted but can be evicted, which a restartable batch job tolerates.