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Use ephemeral OS disk nodes for Azure Batch pools

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Some Azure virtual machine (VM) series support the use of ephemeral OS disks, which create the OS disk on the node virtual machine local storage. The default Batch pool configuration uses Azure managed disks for the node OS disk, where the managed disk is like a physical disk, but virtualized and persisted in remote Azure Storage.

For Batch workloads, the main benefits of using ephemeral OS disks are reduced costs associated with pools, the potential for faster node start time, and improved application performance due to better OS disk performance. When choosing whether ephemeral OS disks should be used for your workload, consider the following impacts:

  • There's lower read/write latency to ephemeral OS disks, which may lead to improved application performance.
  • There's no storage cost for ephemeral OS disks, whereas there's a cost for each managed OS disk.
  • Reimage for compute nodes is faster for ephemeral disks compared to managed disks, when supported by Batch.
  • Node start time may be slightly faster when ephemeral OS disks are used.
  • Ephemeral OS disks aren't highly durable and available; when a VM is removed for any reason, the OS disk is lost. Since Batch workloads are inherently stateless, and don't normally rely on changes to the OS disk being persisted, ephemeral OS disks are appropriate to use for most Batch workloads.
  • Ephemeral OS disks aren't currently supported by all Azure VM series. If a VM size doesn't support an ephemeral OS disk, a managed OS disk must be used.

Note

Ephemeral OS disk configuration is only applicable to 'virtualMachineConfiguration' pools, and aren't supported by 'cloudServiceConfiguration’ pools. We recommend using 'virtualMachineConfiguration for your Batch pools, as 'cloudServiceConfiguration' pools do not support all features and no new capabilities are planned. You won't be able to create new 'cloudServiceConfiguration' pools or add new nodes to existing pools after February 29, 2024. For more information, see Migrate Batch pool configuration from Cloud Services to Virtual Machine.

VM series support

To determine whether a VM series supports ephemeral OS disks, check the documentation for each VM instance. For example, the Ddv4 and Ddsv4-series supports ephemeral OS disks.

Alternately, you can programmatically query to check the 'EphemeralOSDiskSupported' capability. An example PowerShell cmdlet to query this capability is provided in the ephemeral OS disk frequently asked questions.

Create a pool that uses ephemeral OS disks

The EphemeralOSDiskSettings property isn't set by default. You must set this property in order to configure ephemeral OS disk use on the pool nodes.

Tip

Ephemeral OS disks cannot be used in conjunction with Spot VMs in Batch pools due to the service managed eviction policy.

The following example shows how to create a Batch pool where the nodes use ephemeral OS disks and not managed disks.

Code examples

This code snippet shows how to create a pool with ephemeral OS disks using Azure Batch Python SDK with the Ephemeral OS disk using the temporary disk (cache).

virtual_machine_configuration=batch.models.VirtualMachineConfiguration(
    image_reference=image_ref_to_use,
    node_agent_sku_id=node_sku_id,
    os_disk=batch.models.OSDisk(
        ephemeral_os_disk_settings=batch.models.DiffDiskSettings(
            placement=batch.models.DiffDiskPlacement.cache_disk
        )
    )
)

This is the same code snippet but for creating a pool with ephemeral OS disks using the Azure Batch .NET SDK and C#.

VirtualMachineConfiguration virtualMachineConfiguration = new VirtualMachineConfiguration(
        imageReference: imageReference,
        nodeAgentSkuId: nodeAgentSku
        );
virtualMachineConfiguration.OSDisk = new OSDisk();
virtualMachineConfiguration.OSDisk.EphemeralOSDiskSettings = new DiffDiskSettings();
virtualMachineConfiguration.OSDisk.EphemeralOSDiskSettings.Placement = DiffDiskPlacement.CacheDisk;

Next steps