How do I verify that an Azure VM scale set instance is using SSD for ephemeral OS disk?

Question

I'm using a VM scale set for Azure DevOps build agents, successfully. I have it configured for ephemeral OS disk, but it's hard to confirm that the instances are actually set up the way I expect.

Here's how the scale set is configured:

az vmss create `
    --name test-scaleset `
    --resource-group test-scaleset-rg `
    --image /subscriptions/<mysub>/resourceGroups/<sharedimagegalleryresourcegroup>/providers/Microsoft.Compute/galleries/<sharedimagegallery>/images/devops-build/versions/latest `
    --vm-sku Standard_D8s_v3 `
    --ephemeral-os-disk true `
    --os-disk-caching ReadOnly `
    --instance-count 1 `
    --disable-overprovision `
    --upgrade-policy-mode manual `
    --single-placement-group false `
    --zones 1 2 3 `
    --platform-fault-domain-count 1 `
    --load-balancer '""' `
    --authentication-type password `
    --admin-username devops-admin `
    --admin-password "somethingrandomenough" `
    --storage-sku Standard_LRS `
    --assign-identity [system] `
    --scope /subscriptions/<mysub>/resourceGroups/test-scaleset-rg `
    --vnet-name test-scaleset-vnet `
    --subnet test-vms `
    --no-wait

This creates a VM scale set, and I can see on the properties that it is configured for ephemeral OS disk:

VMSS properties

When I look at the Disks panel for a created instance, I see this:

OS Disk properties

So, how do I interpret this? is the OS Disk ephemeral and running on a standard HDD? Or is Standard HDD just referring to the image source location?

The instance storage profile looks like this:

        "storageProfile": {
            "imageReference": {
                "id": "/subscriptions/<mysub>/resourceGroups/<sharedimagegalleryresourcegroup>/providers/Microsoft.Compute/galleries/<sharedimagegallery>/images/devops-build/versions/latest",
                "exactVersion": "1.0.4"
            },
            "osDisk": {
                "osType": "Windows",
                "diffDiskSettings": {
                    "option": "Local"
                },
                "name": "my-vmss-instance-OS__1_c560735c41c74691a53efaac977b6c66",
                "createOption": "FromImage",
                "caching": "ReadOnly",
                "managedDisk": {
                    "storageAccountType": "Standard_LRS",
                    "id": "/subscriptions/<mysub>/resourceGroups/<vmss-rg>/providers/Microsoft.Compute/disks/my-vmss-instance-OS__1_c560735c41c74691a53efaac977b6c66"
                },
                "diskSizeGB": 192
            },
            "dataDisks": []
        }

I've tried to create the VMSS (as above), but specifying Premium_LRS as the storage account type, and the deployment fails with the message that Ephemeral OS disks require Standard_LRS. That last point is interesting because the VMSS has to use a VM SKU that supports Premium storage? Quite confusing.

Charles Xu Charles Xu · Accepted Answer · 2021-02-15T02:09:34

For the Ephemeral OS disk, first, only the VM size supports Premium storage, then it supports the Ephemeral OS disk. Second, it seems the Ephemeral OS disk currently only supports the Standard HDD disk type. You can see this limit in the Azure Portal when you select Use ephemeral OS disk:

So you can't set the OS disk with premium type when you choose Use ephemeral OS disk, at least currently.

How do I verify that an Azure VM scale set instance is using SSD for ephemeral OS disk?

1 Answers