AzCLI support for Ephemeral OS disk with full caching during VM/VMSS create/update #32351
Description
Preconditions
- No need to upgrade Python SDK or the Python SDK is ready.
Related command
We are launching a new feature Ephemeral OS disk with full caching wiht GA target Mar'26. There are many 1P customers like AKS Underlay, PostGreSQL, App Gateway etc. who are interested in the offering.
Change required - To add additional field in AzCLI/PS
AzCLI
az vm create
--resource-group vivek-rg
--name myVM
--image Win2022AzureEditionCore
--ephemeral-os-disk true
--ephemeral-os-disk-placement ResourceDisk
--ephemeral-os-disk-enable-full-caching true/false <- New field introduced
--admin-username XXX
--admin-password XXX
--generate-ssh-keys
--size Standard_B16ms
--storage-sku StandardSSD_LRS
az vmss update
--ephemeral-os-disk true
--ephemeral-os-disk-placement ResourceDisk
--ephemeral-os-disk-enable-full-caching true/false <- New field introduced
Corresponding Arm template:
"osDisk":
{
"diffDiskSettings": {
"option": "Local",
"placement": "ResourceDisk",
“enableFullCaching ":“True“ ß New field introduced
},
"caching": "ReadOnly",
"createOption": "FromImage",
"managedDisk": {
"storageAccountType": "StandardSSD_LRS"
}
}ar'26
Resource Provider
Microsoft.Compute/virtualmachines
Description of Feature or Work Requested
Microsoft announces the enhanced ephemeral OS disk feature for Azure Virtual Machines (VMs), designed to optimize performance and deployment efficiency for stateless workloads. Ephemeral OS Disks are stored directly on the VM’s local storage, eliminating reliance on remote Azure Storage. This approach delivers lower read/write latency, faster VM reimaging, and accelerated deployment times, making it ideal for applications that prioritize agility and responsiveness.
Ephemeral OS Disk is available in two caching modes, catering to diverse workload needs:
-
Partial caching (Default): Partial caching splits OS disk between a diff disk on local storage and a base disk in managed disks. All writes occur on the diff disk, while the base disk serves read operations for original files. Optimized for cloud-native and stateless applications, partial ephemeral OS disk balances performance with efficiency. All existing ephemeral VMs have been created in partial mode.
-
Full caching (new feature): Full caching caches the entire OS disk on local storage, completely removing dependency on remote storage in steady state. Ideal for IO-sensitive stateless workloads, full caching enhances both performance and reliability by eliminating remote read/write latency. Workloads such as quorum-based databases, data analytics, and real-time processing, benefit from this feature. However, full caching requires 2x the OS disk space to store the complete image locally.
Key Advantages of ephemeral OS disk with full caching mode:
- High Performance: Provides extremely high-performance OS disks with consistently fast response times.
- Reliability: Ensures high availability, making it suitable for critical workloads.
By leveraging ephemeral OS disks, Azure customers can achieve faster deployments, reduced latency, and enhanced performance, ensuring their applications remain highly responsive and scalable.
Minimum API Version Required
2025-11-01
Swagger PR link / SDK link
Azure/azure-rest-api-specs#37861
Request Example
No response
Target Date
2026-03-31
PM Contact
viveksingla
Engineer Contact
aygaur, sgundu;
Additional context
No response