An update domain is a logical group of virtual machines that Azure updates or reboots together during planned platform maintenance. Only one update domain is handled at a time, helping keep the wider service available. section 3.2.1 section 3.5.2
Think of it as scheduling maintenance by small teams instead of sending every VM on vacation simultaneously. Azure assigns VMs to update domains when they are placed in an availability set. section 3.5.2
Key points:
- Groups VMs for coordinated updates or reboots while avoiding downtime to the greater infrastructure. section 3.5.2
- Processes only one update domain at a time. section 3.2.1 section 3.5.2
- Supports up to 20 update domains in an availability set. section 3.5.2
- Works alongside fault domains: update domains address planned maintenance, while fault domains separate shared power, cooling, and networking risks. section 3.2.1 section 3.5.2
A nine-VM availability set
An availability set can contain nine VMs distributed across six update domains and three fault domains. During platform maintenance, Azure reboots one update domain while the others continue serving users.
Figure 3.30 An Azure availability set with three fault domains and five update domains
Use the diagram to see how update domains and fault domains distribute VMs inside an availability set. section 3.5.2
