Azure Virtual Machines Interview Questions

Core Concepts

Q: Azure VM series — how do you choose the right size?

Series	Use Case	Examples

|---|---|---|

B (Burstable)	Dev/test, low steady-state CPU	B2s, B4ms
D (General Purpose)	Most workloads, balanced CPU/RAM	D4s_v5, D8s_v5
E (Memory Optimised)	Databases, in-memory analytics	E8s_v5, E32s_v5
F (Compute Optimised)	CPU-heavy batch, gaming	F8s_v2
L (Storage Optimised)	High disk throughput, Cassandra	L8s_v3
N (GPU)	ML training, graphics rendering	NC6s_v3, ND96asr
H (HPC)	MPI workloads, simulations	HB120rs_v3

Naming convention: D4s_v5 = D series, 4 vCPUs, s=premium SSD capable, v5=version 5.

Cost optimisation:

•Spot VMs: 60-90% discount, can be evicted (use for batch/stateless)

•Reserved Instances: 1 or 3 year commitment, 40-72% savings

•Hybrid Benefit: Bring your own Windows Server/SQL license

Q: Azure VM availability options.

Availability Sets: Group VMs across fault domains (different racks) and update domains (patching groups).

•Protects from hardware failure (rack-level)

•Protects from planned maintenance (rolling updates)

•Single datacenter — no protection from datacenter failure

Availability Zones: Deploy VMs to physically separate datacenters within a region.

•Each zone has independent power, cooling, networking

•99.99% SLA (vs 99.95% for Availability Sets)

•Best practice for production workloads

VM Scale Sets (VMSS): Identical VMs with auto-scaling. Load balancer in front.

•Manual, scheduled, or metric-based scaling (CPU %, custom metrics)

•Supports Spot VMs (mixed instances)

•Use for: web tiers, batch processing, microservices

Q: Azure VM disk types.

Disk Type	Latency	IOPS	Use Case

|---|---|---|---|

Ultra Disk	<1ms	Up to 160K	Databases, latency-sensitive
Premium SSD v2	~2ms	Up to 80K	Production databases
Premium SSD (P-series)	<10ms	Up to 20K	Most production workloads
Standard SSD	~10ms	Moderate	Dev/test, web servers
Standard HDD	Higher	Low	Backups, archival

bash

# Attach premium SSD to VM
az vm disk attach --resource-group myrg --vm-name myvm   --name myDisk --new --size-gb 128 --sku Premium_LRS

# Enable accelerated networking (reduces latency, required for high-perf)
az network nic update --name myNIC --resource-group myrg   --accelerated-networking true

Q: VM extensions and custom script extension.

VM Extensions add post-deployment configuration:

•Custom Script Extension: Run scripts on VM after deployment

•DSC: PowerShell Desired State Configuration

•Azure Monitor Agent: Metrics and log collection

•Key Vault VM Extension: Automatically refresh certificates from Key Vault

•Disk Encryption: Enable Azure Disk Encryption (ADE with BitLocker/dm-crypt)

json

// Custom Script Extension via ARM template
{
  "type": "extensions",
  "name": "CustomScriptExtension",
  "properties": {
    "publisher": "Microsoft.Compute",
    "type": "CustomScriptExtension",
    "settings": {
      "fileUris": ["https://mystorageaccount.blob.core.windows.net/scripts/setup.ps1"],
      "commandToExecute": "powershell -ExecutionPolicy Unrestricted -File setup.ps1"
    }
  }
}

Revision Notes

VM SERIES: B(burstable) D(general) E(memory) F(compute) L(storage) N(GPU)
D4s_v5: D series, 4 vCPU, s=premium SSD, v5=generation

AVAILABILITY:
Availability Sets: same DC, different fault+update domains (99.95% SLA)
Availability Zones: separate DCs in region (99.99% SLA) — use for production
VM Scale Sets: auto-scaling group of identical VMs

DISKS: Ultra(<1ms) > Premium SSD v2 > Premium SSD > Standard SSD > HDD
Premium SSD = P-series (required for most production workloads)

COST: Spot (60-90% off, evictable) | Reserved (40-72% off, 1-3yr commit) | Hybrid Benefit (BYOL)

EXTENSIONS: Custom Script (run scripts) | AMA (monitoring) | Key Vault (cert rotation) | ADE (encryption)

Virtual Machines — Interview Q&A

Azure Virtual Machines Interview Questions

Core Concepts

Revision Notes