Max limit on the number of virtual machines that can be present in a fault domain

Question

Is there a max limit on the number of virtual machines that can be present in a fault domain or an update domain in an availability set? I am assuming fault domains are of the similar concept of racks in datacenter and I believe those racks generally have some limits, hence the question

Answer 1

There is a limit of 200 VM's per availability set, see here for more details.

Answer 2

Hello @Siddhant Jain

To your question, "max limit on the number of virtual machines that can be present in a fault domain or an update domain in an availability set?"

Each virtual machine in your availability set is assigned an update domain and a fault domain by the underlying Azure platform. This is controlled by the backend services.

Yes, you’re correct in your understanding that fault domains are similar to racks in a datacenter, designed to provide fault isolation. In Azure, each availability set can be configured with up to 3 fault domains and 20 update domain

• Fault Domains: These are essentially groups of virtual machines that share a common power source and network switch. By spreading VMs across multiple fault domains, Azure ensures that a failure in one domain doesn’t affect all VMs in the availability set.
• Update Domains: These are groups of VMs and underlying physical hardware that can be rebooted at the same time. This helps ensure that not all VMs are down during maintenance. Within an availability set, individual VMs are spread across up to 20 update domains. During scheduled maintenance, only one update domain is updated at any given time. Update domains aren't necessarily updated sequentially.

As mentioned in Sam Cogan's comment, you can see the max number of VM's per each availability set is 200.

It is important to note that the limits on the number of virtual machines in a fault domain or an update domain are in place to ensure high availability and reliability

Hope this helps! Feel free to tag me in your comments, if you have any further queries.

If I have answered your query, please click "Accept as answer" as a token of appreciation