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We are excited to announce the Public Preview of using Premium or Standard SSD as base disk for creating VM/VMSS with Ephemeral OS Disk. By utilizing the SSD as Base Disk feature, customers can benefit from the following enhancements:

  • Enhanced SLA: VMs created with Premium SSD provide the maximum 99.9% SLA, while VMs created with Standard HDD provide 95% SLA. Customers can enhance SLA for their VMs by choosing Premium SSD as base disk.
  • Improved performance: While most writes occur on the local temporary disk, some reads will be performed from the base disk. Premium SSD disks provide 8-10 times higher IOPS than Standard HDD. (For details, refer Select a disk type for Azure IaaS VMs - managed disks - Azure Virtual Machines | Microsoft Learn). By choosing Premium SSD as the base disk, customers can enhance the disk read performance of their VMs.

Ephemeral OS disks are split into 2 disks - a diff disk on local storage and a base disk in managed disks. All writes to existing and new files occur on the diff disk, while the original files are read from the base disk. Both the diff disk and the base disk are crucial for the VM's operation.

SSD as Base Disk Feature is a new option that allows customers to choose the type of base disk that is used for the ephemeral OS disk. Previously, the base disk could only be Standard HDD. Now, customers can choose between the three types of disks: Standard HDD(Standard_LRS), Standard SSD (StandardSSD_LRS) or Premium SSD (Premium_LRS). By using SSD as the base disk, customers will achieve higher SLA and performance compared to Standard HDD. Standard HDD as base disk will continue to be free to use with Ephemeral OS disk. However, customers will incur additional costs for provisioning SSD as the base disk.

How to Use SSD as Base Disk Feature?

To utilize the SSD as Base Disk Feature, customers need to specify the disk type and size when creating or updating a VM with an ephemeral OS disk enabled. Customers can use the Azure CLI, Azure PowerShell, or Azure Resource Manager templates to achieve this.

For example, to create a VM with Ephemeral OS disk & Premium SSD base disk, customers can use the following command:

AzCLI

az vm create --name MyVM --resource-group MyRG --image UbuntuLTS --ephemeral-os-disk true --storage-sku Premium_LRS

PowerShell

Set-AzVMOSDisk -VM $VirtualMachine -DiffDiskSetting Local -DiffDiskPlacement ResourceDisk -CreateOption FromImage -Caching ReadOnly -StorageAccountType "Premium_LRS"


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