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VM Creation: Custom Scrips vs Custom Images

When we need custom applications or configuration to be done on the VM we can do this on Azure in two ways:
  • Custom ISO
  • Custom scripts extensions (known also as Formula in DevLabs context)
 I noticed that a recurrent questions appears in discussions with different people:
When I should use custom ISO vs custom scripts extensions?

Before jumping to a discussion where we would compare this two options and what are the advantages/disadvantages of each option, let's see what are the steps involved to create a script of an ISO.

Custom ISO 
We can create a custom ISO on our local machine, with all our applications installed on it. Once we have the ISO created we just need to take our VHD and prepared it for Azure. More about this steps can be found on Microsoft documentation (Capture a managed image of a generalized VM in Azure and Create custom VM images).

Custom scripts extensions
Custom scripts are executed after the VHD is deployed on the VM. Using this scripts we can push or install any kind of application or change OS configuration. I would compare custom scripts with post-deployment scripts, that are executed after the VM deployment finish.

Custom ISO vs Custom scripts extensions
Don't expect to have a winner from this fight. Each option has advantages and disadvantageous. The context of your project will define what option best suites your needs.
In the below table I tried to catch the most important things that you need to consider before selecting the provisioning mechanism that you want to use.
Custom Image Custom scripts
Pros: Fast deployment Pros: Environment/App updates can be pushed on the fly (artifacts)
Pros: No extra steps after deployment Pros: Last version of VM image, including updates is used
Pros: VMs from same image are identical Pros: Default settings can be specified like VM Size, VNET configuration
Cons: Image needs to be recreated when something change Pros: Default settings are used as default values, but during provisioning other values can be specified
Cons: No dynamic update of the image Cons: Deployment using formulas can take longer time (provisioning+running formula/scripts)
Cons: Windows and other Updates are pushed only after provisioning

What should I use? 
I would say that in most of the cases, custom scrips should be a good option. They are flexible enough to allow you to change or update the artifacts without having to recreate the ISO, offering you a deployment that already contains last updated of OS also. No time wasted for OS updates.
There are some narrow cases when you want to use custom images. The case that I see the most common one is when you want a fast provisioning of the VM. For example when you want to increase the number of VMs in a cluster, where scalability is extremely important.

The winner is...
As you already notice, custom scrips are my favorites and I see them a better solution in most of the cases.


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