How does VMSS behave in the Zonal failure Scenario?

Jeyasingh, Vijayalingam 81 Reputation points
2024-10-07T08:38:05.7066667+00:00

Hi,

I have a query regarding VMSS behavior.

Let's assume I have a VMSS configured with Multi-Availability Zone (1,2,3) but with 1 fixed instance. Now I will see my 1 fixed instance configured in any one of the availability zones from 1,2,3 for which I do not have any control. Let's assume the single instance is running on Zone2, what happens when Zone2 is down? Will the instance come up automatically in Zone1 or Zone3 which must be available, because I configured with fixed single instance? or I will see the VMSS is down.

Azure Virtual Machine Scale Sets
Azure Virtual Machine Scale Sets
Azure compute resources that are used to create and manage groups of heterogeneous load-balanced virtual machines.
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Accepted answer
  1. Mounika Reddy Anumandla 745 Reputation points Microsoft Vendor
    2024-10-07T10:47:41.9833333+00:00

    Hi Jeyasingh, Vijayalingam,

    Welcome to the Microsoft Q&A Platform! Thank you for asking your question here.

    Behavior in the Event of Zone 2 Going Down

    In case, you have configured a fixed single instance, and it is running in Zone 2. If Zone 2 goes down, that specific instance will become unavailable. The instance will not automatically come up in Zone 1 or Zone 3. The VMSS does not have the capability to relocate or recreate the instance in another zone when only one instance is specified. You will effectively see the VMSS as down because there are no operational instances available to handle requests. The scale set cannot provide redundancy or failover capabilities with a single instance.

    Here is a more detailed explanation of what happens when a zone goes down:

    1. Azure monitors the health of all zones and VMs in a VMSS.
    2. If a zone goes down, Azure will automatically detect the failure.
    3. Azure will then attempt to restart the VM in the failed zone.
    4. If Azure is unable to restart the VM in the failed zone, it will create a new VM in another available zone.
    5. Azure will then copy the state of the old VM to the new VM.
    6. Once the new VM is ready, Azure will switch traffic to the new VM.

    To minimize the impact of a zone failure, you can use the following best practices:

    If you have any further queries, do let us know.

    If the answer is helpful, please click "Accept Answer" and "Upvote it."


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