Ephemeral OS disks for Azure VMs

Applies to: ✔️ Linux VMs ✔️ Windows VMs ✔️ Flexible scale sets ✔️ Uniform scale sets

Ephemeral OS disks are created on the local virtual machine (VM) storage and not saved to the remote Azure Storage. Ephemeral OS disks work well for stateless workloads, where applications are tolerant of individual VM failures but are more affected by VM deployment time or reimaging of individual VM instances. With Ephemeral OS disk, you get lower read/write latency to the OS disk and faster VM reimage.

The key features of ephemeral disks are:

• Ideal for stateless applications.
• Supported by Marketplace, custom images, and by Azure Compute Gallery (formerly known as Shared Image Gallery).
• Ability to fast reset or reimage VMs and scale set instances to the original boot state.
• Lower latency, similar to a temporary disk.
• Ephemeral OS disks are free, you incur no storage cost for OS disks.
• Available in all Azure regions.

Key differences between persistent and ephemeral OS disks:

* 4 TiB is the maximum supported OS disk size for managed (persistent) disks. However, many OS disks are partitioned with master boot record (MBR) by default and because of this are limited to 2 TiB. For details, see OS disk.

Placement options for Ephemeral OS disks

Ephemeral OS disk can be stored either on VM's OS cache disk or VM's temp/resource disk. DiffDiskPlacement is the new property that can be used to specify where you want to place the Ephemeral OS disk. With this feature, when a Windows VM is provisioned, we configure the pagefile to be located on the OS Disk.

Size requirements

You can choose to deploy Ephemeral OS Disk on VM cache or VM temp disk. The image OS disk’s size should be less than or equal to the temp/cache size of the VM size chosen.

For example, if you want to opt for OS cache placement: Standard Windows Server images from the marketplace are about 127 GiB, which means that you need a VM size that has a cache equal to or larger than 127 GiB. The Standard_DS3_v2 has a cache size of 127 GiB, which is large enough. In this case, the Standard_DS3_v2 is the smallest size in the DSv2 series that you can use with this image.

For example, if you want to opt for Temp disk placement: Standard Ubuntu server image from marketplace is about 30 GiB. To enable Ephemeral OS disk on temp, the temp disk size must be equal to or larger than 30 GiB. Standard_B4ms has a temp size of 32 GiB, which can fit the 30 GiB OS disk. Upon creation of the VM, the temp disk space would be 2 GiB.

In the case of Temp disk placement, as Ephemeral OS disk is placed on temp disk it will share the IOPS with temp disk as per the VM size chosen by you.

Basic Linux and Windows Server images in the Marketplace that are denoted by [smallsize] tend to be around 30 GiB and can use most of the available VM sizes. Ephemeral disks also require that the VM size supports Premium storage. The sizes usually (but not always) have an s in the name, like DSv2 and EsV3. For more information, see Azure VM sizes for details around which sizes support Premium storage.

Unsupported features

• Capturing VM images
• Disk snapshots
• Azure Disk Encryption
• Azure Backup
• Azure Site Recovery
• OS Disk Swap

Trusted Launch for Ephemeral OS disks

Ephemeral OS disks can be created with Trusted launch. All regions are supported for Trusted Launch; not all virtual machines sizes are supported. Check Virtual machines sizes supported for supported sizes. VM guest state (VMGS) is specific to trusted launch VMs. It is a blob that is managed by Azure and contains the unified extensible firmware interface (UEFI) secure boot signature databases and other security information. When using trusted launch by default 1 GiB from the OS cache or temp storage based on the chosen placement option is reserved for VMGS.The lifecycle of the VMGS blob is tied to that of the OS Disk.

For example, If you try to create a Trusted launch Ephemeral OS disk VM using OS image of size 56 GiB with VM size Standard_DS4_v2 using temp disk placement you would get an error as "OS disk of Ephemeral VM with size greater than 55 GB is not allowed for VM size Standard_DS4_v2 when the DiffDiskPlacement is ResourceDisk." This is because the temp storage for Standard_DS4_v2 is 56 GiB, and 1 GiB is reserved for VMGS when using trusted launch. For the same example above, if you create a standard Ephemeral OS disk VM you would not get any errors and it would be a successful operation.

For more information on how to deploy a trusted launch VM

Confidential VMs using Ephemeral OS disks

AMD-based Confidential VMs cater to high security and confidentiality requirements of customers. These VMs provide a strong, hardware-enforced boundary to help meet your security needs. There are limitations to use Confidential VMs. Check the region, size and OS supported limitations for confidential VMs. Virtual machine guest state (VMGS) blob contains the security information of the confidential VM. Confidential VMs using Ephemeral OS disks by default 1 GiB from the OS cache or temp storage based on the chosen placement option is reserved for VMGS.The lifecycle of the VMGS blob is tied to that of the OS Disk.

For more information on confidential VM

Customer Managed key

You can choose to use customer managed keys or platform managed keys when you enable end-to-end encryption for VMs using Ephemeral OS disk. Currently this option is available only via PowerShell, CLI and SDK in all regions.

For more information on Encryption at host

Next steps

Create a VM with ephemeral OS disk using Azure Portal/CLI/PowerShell/ARM template. Check out the frequently asked questions on ephemeral os disk.