Reason for difference in Azure VM Scale Set applying (a.k.a. why the upgrade button isn't needed)
Just wanted to share some info on something that came up yesterday.
When updating a VM Scale Set (vmss) definition I found a situation where I needed to press the "Upgrade" button to apply the model in one situation and DID NOT need to press the button in another situation.
While looking into scripting this operation I stumbled across this medium article Updating Azure VM Scale Set without downtime with Rolling Updates
In the article it outlines how to get info on the "Upgrade Policy", this is something that isn't readily obvious IN the portal, so you really need to use PowerShell commands or the Azure CLI commands to see the info.
I'm using PowerShell AZ commands (NOT CLI).
Using the command Get-AzVmss I found that the Upgrade Policy for the scale set which I HAD to press the upgrade was set to manual, which makes sense. Manual Upgrade would mean a user would need to take an action.
Using the same command for the other scale set the Policy was Automatic, so again makes sense why I DID NOT need to press the button.
So as is often the case with technology pays to read the docs.