Monthly Archives: February 2019

Thoughts on standard Image SKU vs.’Smalldisk’ Image SKU

For a long time, organizations using Windows VM instances in Microsoft Azure didn’t have options regarding the OS disk for the instance. The default value is 127 GB and this hasn’t changed. Quite a while ago, Microsoft announced Windows VM instances with a smaller OS disk of only 32 GB as was announced in

Yes, I admit this may be old news but I haven’t given it much thought on how approach it when these Windows VM images became available, until recently…

More and more I’m involved into providing ARM templates for my customers and my main focus is on Azure IaaS deployments.

Together with Managed disks, it has become pretty easy to determine sizing for Azure VM Instances and having both Image SKUs available provide options.

However, while I was creating these ARM templates I noticed that I prefer to use the ‘Smalldisk’ Image SKU’s more over the standard one and the explanation for it is actually pretty simple.

For this post, I will use the following ARM template as a reference:

Looking at the “Properties” section of the Virtual Machine resource, you can see the relevant part of the OS Disk configuration:

“osDisk”: {
                 “createOption”: “FromImage”

In this configuration, the default size will be used which should be great in most scenarios. If a different size is required, then the notation may look like this:

“osDisk”: {
                 “createOption”: “FromImage”,
                 “diskSizeGB”: “[variables(‘OSDiskSizeinGB’)]”

You can specify the value either as a variable or a parameter to determine the size. In this example I use a variable and it must have a supported value for managed disks. In my case I used the following value:

“OSDiskSizeinGB”: “64”
OK, so nothing new here so far. However, to maintain maximum flexibility, you need to use the ‘Smalldisk’ Image SKU only which has the smallest possible size of 32 GB. From there, the only was is up.
To optimize Azure consumption by only paying for what you use and what you REALLY need, it may make sense that organizations create some governance and policies to determine sizing for their Azure VM instances. Not only for compute, but for storage as well. Managed Disks provide some guidance for that.
So for me, I’d focus on using the ‘Smalldisk’ Image SKU only and enlarge it when needed. It’s pretty easy to do by just adding one line in your ARM template for that VM, and an additional one for your variable…


Here’s my set of variables I use to select the ‘Smalldisk’ Image SKU:

“ImagePublisher”: “MicrosoftWindowsServer”,
“ImageOffer”: “WindowsServer”,
“ImageSKU”: “2019-Datacenter-smalldisk”,
“ImageVersion”: “latest”,

And here’s the relevant part of the Image reference:

“imageReference”: {
                                   “publisher”: “[variables(‘ImagePublisher’)]”,
                                   “offer”: “[variables(‘ImageOffer’)]”,
                                   “sku”: “[variables(‘ImageSKU’)]”,
                                   “version”: “[variables(‘ImageVersion’)]”
Hope this helps!



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