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**Applies to:**:heavy_check_mark: Linux VMs :heavy_check_mark: Windows VMs :heavy_check_mark: Flexible scale sets :heavy_check_mark: Uniform scale sets
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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 the individual VM instances. With Ephemeral OS disk, you get lower read/write latency to the OS disk and faster VM reimage.
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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.
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The key features of ephemeral disks are:
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- Ideal for stateless applications.
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- They can be used with both Marketplace and custom images.
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- Ability to fast reset or reimage VMs and scale set instances to the original boot state.
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- Lower latency, similar to a temporary disk.
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- Ephemeral OS disks are free, you incur no storage cost for OS disk.
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-They are available in all Azure regions.
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-Available in all Azure regions.
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- Ephemeral OS Disk is supported by [Shared Image Gallery](./shared-image-galleries.md).
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@@ -32,37 +32,44 @@ Key differences between persistent and ephemeral OS disks:
|**Size limit for OS disk**| 2 TiB | Cache size for the VM size or 2TiB, whichever is smaller. For the **cache size in GiB**, see [DS](sizes-general.md), [ES](sizes-memory.md), [M](sizes-memory.md), [FS](sizes-compute.md), and [GS](sizes-previous-gen.md#gs-series)|
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|**VM sizes supported**| All | VM sizes that supports Premium storage such as DSv1, DSv2, DSv3, Esv3, Fs, FsV2, GS, M |
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|**Size limit for OS disk**| 2 TiB | Cache size for the VM size or 2 TiB, whichever is smaller. For the **cache size in GiB**, see [DS](sizes-general.md), [ES](sizes-memory.md), [M](sizes-memory.md), [FS](sizes-compute.md), and [GS](sizes-previous-gen.md#gs-series)|
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|**VM sizes supported**| All | VM sizes that support Premium storage such as DSv1, DSv2, DSv3, Esv3, Fs, FsV2, GS, M |
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|**Disk type support**| Managed and unmanaged OS disk | Managed OS disk only |
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|**Region support**| All regions | All regions |
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|**Data persistence**| OS disk data written to OS disk are stored in Azure Storage | Data written to OS disk is stored to the local VM storage and is not persisted to Azure Storage. |
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|**Data persistence**| OS disk data written to OS disk are stored in Azure Storage | Data written to OS disk is stored on local VM storage and isn't persisted to Azure Storage. |
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|**Stop-deallocated state**| VMs and scale set instances can be stop-deallocated and restarted from the stop-deallocated state | VMs and scale set instances cannot be stop-deallocated |
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|**Specialized OS disk support**| Yes | No |
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|**OS disk resize**| Supported during VM creation and after VM is stop-deallocated | Supported during VM creation only |
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| **Resizing to a new VM size** | OS disk data is preserved | Data on the OS disk is deleted, OS is re-provisioned
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|**Page file placement**| For Windows, page file is stored on the resource disk | For Windows, page file is stored on the OS disk |
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| **Resizing to a new VM size** | OS disk data is preserved | Data on the OS disk is deleted, OS is reprovisioned
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|**Page file placement**| For Windows, page file is stored on the resource disk | For Windows, page file is stored on the OS disk (for both OS cache placement and Temp disk placement). |
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## Size requirements
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You can deploy VM and instance images up to the size of the VM cache. For example, Standard Windows Server images from the marketplace are about 127 GiB, which means that you need a VM size that has a cache larger than 127 GiB. In this case, the [Standard_DS2_v2](dv2-dsv2-series.md) has a cache size of 86 GiB, which is not large enough. The Standard_DS3_v2 has a cache size of 172 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. Basic Linux images in the Marketplace and Windows Server images that are denoted by `[smallsize]` tend to be around 30 GiB and can use most of the available VM sizes.
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You can choose to deploy Ephemeral OS Disk on VM cache or VM temp disk.
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The image OS disk’s size should be equal to or greater than the temp/cache size of the disk chosen.
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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](sizes.md) for details around which sizes support Premium storage.
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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.
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## Preview - Ephemeral OS Disks can now be stored on temp disks
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Ephemeral OS Disks can now be stored on VM temp/resource disk in addition to the VM cache. So, now you can use Ephemeral OS Disks with VM which don’t have a cache or has insufficient cache but has a temp/resource disk to store the Ephemeral OS disk such as Dav3, Dav4, Eav4 and Eav3. If a VM has sufficient cache and temp space, you will now also be able to specify where you want to store the ephemeral OS Disk by using a new property called [DiffDiskPlacement](/rest/api/compute/virtualmachines/list#diffdiskplacement). With this feature, when a Windows VM is provisioned, we configure the pagefile to be located on the OS Disk. This feature is currently in preview. This preview version is provided without a service level agreement, and it's not recommended for production workloads. To get started, [request access](https://forms.office.com/Pages/ResponsePage.aspx?id=v4j5cvGGr0GRqy180BHbR6cQw0fZJzdIsnbfbI13601URTBCRUZPMkQwWFlCOTRIMFBSNkM1NVpQQS4u).
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If you want to opt for **Temp disk placement**: Standard Ubuntu server image from marketplace comes with 30-GiB OS disk space. To enable Ephemeral OS disk, the 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. But Standard_B2ms has a temp size of 16 GiB, which wouldn't be able to fit 30 GiB of OS disk.
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## PowerShell
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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.
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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](sizes.md) for details around which sizes support Premium storage.
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To use an ephemeral disk for a PowerShell VM deployment, use [Set-AzVMOSDisk](/powershell/module/az.compute/set-azvmosdisk) in your VM configuration. Set the `-DiffDiskSetting` to `Local` and `-Caching` to `ReadOnly`.
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## Ephemeral OS Disks can now be stored on temp/Resource disks
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Ephemeral OS disk can now be stored either in VM cache disk or VM temp/resource disk.
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This feature enables Ephemeral OS disks to be created for all the VMs, which don't have a cache or insufficient cache (such as Dav3, Dav4, Eav4, and Eav3) but has temp disk to store the Ephemeral OS disk.
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[DiffDiskPlacement](/rest/api/compute/virtualmachines/list#diffdiskplacement) is the new property that can be used to specify where you want to place the Ephemeral OS disk.
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With this feature, when a Windows VM is provisioned, we configure the pagefile to be located on the OS Disk.
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## PowerShell
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To use an ephemeral disk for a PowerShell VM deployment, use [Set-AzVMOSDisk](/powershell/module/az.compute/set-azvmosdisk) in your VM configuration. Set the `-DiffDiskSetting` to `Local` and `-Caching` to `ReadOnly`.
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```powershell
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Set-AzVMOSDisk -DiffDiskSetting Local -Caching ReadOnly
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```
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For scale set deployments, use the [Set-AzVmssStorageProfile](/powershell/module/az.compute/set-azvmssstorageprofile) cmdlet in your configuration. Set the `-DiffDiskSetting` to `Local` and `-Caching` to `ReadOnly`.
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```powershell
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Set-AzVmssStorageProfile -DiffDiskSetting Local -OsDiskCaching ReadOnly
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```
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## Portal
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In the Azure portal, you can choose to use ephemeral disks when deploying a VM by opening the **Advanced** section of the **Disks** tab. For **Use ephemeral OS disk** select **Yes**.
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In the Azure portal, you can choose to use ephemeral disks when deploying a virtual machine or virtual machine scale sets by opening the **Advanced** section of the **Disks** tab. For choosing placement of Ephemeral OS disk, select **OS cache placement** or **Temp disk placement**.
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If the option for using an ephemeral disk is greyed out, you might have selected a VM size that does not have a cache size larger than the OS image or that doesn't support Premium storage. Go back to the **Basics** page and try choosing another VM size.
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You can also create scale-sets with ephemeral OS disks using the portal. Just make sure you select a VM size with a large enough cache size and then in **Use ephemeral OS disk** select **Yes**.
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If the option for using an ephemeral disk or OS cache placement or Temp disk placement is greyed out, you might have selected a VM size that doesn't have a cache/temp size larger than the OS image or that doesn't support Premium storage. Go back to the **Basics** page and try choosing another VM size.
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## Scale set template deployment
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The process to create a scale set that uses an ephemeral OS disk is to add the `diffDiskSettings` property to the
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`Microsoft.Compute/virtualMachineScaleSets/virtualMachineProfile` resource type in the template. Also, the caching policy must be set to `ReadOnly` for the ephemeral OS disk.
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`Microsoft.Compute/virtualMachineScaleSets/virtualMachineProfile` resource type in the template. Also, the caching policy must be set to `ReadOnly` for the ephemeral OS disk. placement can be changed to `CacheDisk` for OS cache disk placement.
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"storageProfile": {
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"osDisk": {
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"diffDiskSettings": {
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"option": "Local"
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"option": "Local" ,
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"placement": "ResourceDisk"
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},
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"caching": "ReadOnly",
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"createOption": "FromImage"
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```
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## VM template deployment
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You can deploy a VM with an ephemeral OS disk using a template. The process to create a VM that uses ephemeral OS disks is to add the `diffDiskSettings` property to the Microsoft.Compute/virtualMachines resource type in the template. Also, the caching policy must be set to `ReadOnly` for the ephemeral OS disk.
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You can deploy a VM with an ephemeral OS disk using a template. The process to create a VM that uses ephemeral OS disks is to add the `diffDiskSettings` property to Microsoft.Compute/virtualMachines resource type in the template. Also, the caching policy must be set to `ReadOnly` for the ephemeral OS disk. placement option can be changed to `CacheDisk` for OS cache disk placement.
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```json
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{
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"type": "Microsoft.Compute/virtualMachines",
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"name": "myVirtualMachine",
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"location": "East US 2",
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"apiVersion": "2018-06-01",
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"apiVersion": "2019-12-01",
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"properties": {
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"storageProfile": {
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"osDisk": {
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"diffDiskSettings": {
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"option": "Local"
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"option": "Local" ,
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"placement": "ResourceDisk"
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},
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"caching": "ReadOnly",
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"createOption": "FromImage"
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## Reimage a VM using REST
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You can reimage a Virtual Machine instance with ephemeral OS disk using REST API as described below and via Azure Portal by going to Overview pane of the VM. For scale sets, reimaging is already available through Powershell, CLI, and the portal.
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You can reimage a Virtual Machine instance with ephemeral OS disk using REST API as described below and via Azure portal by going to Overview pane of the VM. For scale sets, reimaging is already available through PowerShell, CLI, and the portal.
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```
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POST https://management.azure.com/subscriptions/{sub-
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A: No, once the ephemeral OS disk is provisioned, the OS disk cannot be resized.
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**Q: Can the ephemeral OS disk placement be modified after creation of VM?**
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A: No, once the ephemeral OS disk is provisioned, the OS disk placement cannot be changed. But the VM can be recreated via ARM template deployment/PowerShell/CLI by updating the OS disk placement of choosing. Resulting in the recreation of the VM with Data on the OS disk deleted and OS is reprovisioned.
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**Q: Can I attach a Managed Disks to an Ephemeral VM?**
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A: Yes, you can attach a managed data disk to a VM that uses an ephemeral OS disk.
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**Q: Can the ephemeral OS disk be created using Powershell or CLI?**
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A: Yes, you can create VMs with Ephemeral OS Disk using REST, Templates, PowerShell and CLI.
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A: Yes, you can create VMs with Ephemeral OS Disk using REST, Templates, PowerShell, and CLI.
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**Q: What features are not supported with ephemeral OS disk?**
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@@ -246,8 +256,8 @@ A: Ephemeral disks do not support:
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> [!NOTE]
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>
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> Ephemeral disk will not be accessible through portal. You will receive "Resource not Found" or "404" error when accessing the ephemeral disk which is expected.
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> Ephemeral disk will not be accessible through the portal. You will receive a "Resource not Found" or "404" error when accessing the ephemeral disk which is expected.
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>
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## Next steps
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You can create a VM with an ephemeral OS disk using the [Azure CLI](/cli/azure/vm#az_vm_create).
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You can create a VM with an ephemeral OS disk using the [Azure CLI](/cli/azure/vm#az_vm_create).
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