availability-sets

name	availability-sets
description	Availability Sets — FDs, UDs, when AS still applies

Availability Sets

When to use this skill

• Region without AZs
• Legacy SKU requiring AS

Key concepts

• AS provides intra-region anti-affinity across fault domains (separate racks) and update domains (separate planned-maintenance batches).
• Default: 2 FDs, 5 UDs (some regions support 3 FDs).
• AS gives 99.95% single-region VM SLA when ≥2 VMs are in one AS.
• Prefer AZ over AS in any region that supports zones.
• AS cannot be changed after VM create — destroy/recreate to move.

Azure CLI examples

az vm availability-set create -g <rg> -n as-app \
  --platform-fault-domain-count 2 --platform-update-domain-count 5

az vm create -g <rg> -n vm-app-1 --availability-set as-app ...

Common pitfalls

• Putting VMs in AS in an AZ region — you lose the AZ resiliency story.
• Trying to use AS and AZ on the same VM — not allowed.
• Forgetting AS doesn't protect against unplanned region-wide events.

References

• Microsoft Learn: search for "Availability Sets Azure VM" on https://learn.microsoft.com

---

Validation-first: verify every Azure fact against the Microsoft Learn MCP server (cite the Learn URL); Learn overrides built-in knowledge. If no Learn MCP server is configured, flag the answer ⚠️ unverified and mark specs as indicative. Analysis only — verify against Microsoft documentation before applying.