What is the difference between OS Upgrade and Reimage for VMSS

Joel Forsyth 26 Reputation points

We are using a scale set for Service Fabric. We have a custom image deployed to our scale set using the 'latest' image version from a Shared Image Gallery. Per the documentation the VMs in the scale set should update to the latest version within 3hrs. On our scale set there is a message that says, "Automatic OS upgrades are not available for the image used by this scale set." but no information is given as to why this is the case. There's also no documentation on this message and I've triple checked all of the requirements and we meet them all.

If this is not going to work, then we can manually trigger the upgrades but I want to know which function to use that best fits our use case. OS Upgrade and Reimage seem to do the same thing, but I'm sure there are some differences. Can anyone clear this up for me?

Azure Virtual Machines
Azure Virtual Machines
An Azure service that is used to provision Windows and Linux virtual machines.
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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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  1. KarishmaTiwari-MSFT 18,337 Reputation points Microsoft Employee

    We are sorry that you are seeing issues with Automatic OS Upgrade for your image.
    Only certain OS platform images are currently supported. Custom images are supported if the scale set uses custom images through Shared Image Gallery.


    You can find more details on Requirements for configuring automatic OS image upgrade as well as Service Fabric requirements in this document: https://learn.microsoft.com/en-us/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-automatic-upgrade#supported-os-images

    Please double check if you meet all the above requirements and if you are still seeing issues, let me know.

    Difference between Upgrade and Reimage:
    Upgrading will apply any changes that were applied to the scaleset as a whole. So for example, if you apply a custom script extension to VMSS1 you need to update the VMSS instances in order for that custom script to actually be applied.

    If you select reimage, it will remove the VMSS instance and replace it with a brand new one. This can be used if you are having issues with a single instance and want to essentially delete it and redeploy it.

    Source: https://social.msdn.microsoft.com/Forums/en-US/0186975f-aa64-4edc-9a9e-2195b705e6fb/reimaging-vs-upgrade-options?forum=WAVirtualMachinesforWindows

    Few more articles on this which you might find helpful: