Today, we’re excited to announce the public preview of Ephemeral OS disk with full caching, a new feature designed to significantly enhance performance and reliability by utilizing local storage. This feature is ideal for IO-sensitive stateless workloads, as it eliminates dependency on remote storage by caching the entire OS image on local storage.
Key Advantages:
- High Performance: Provides extremely high-performance OS disks with consistently fast response times.
- Reliability: Ensures high availability, making it suitable for critical workloads.
Why Full OS Caching?
Currently, Ephemeral OS disks store OS writes locally but still rely on a remote base OS image for reads. With Ephemeral OS Disk with full caching, the entire OS disk image is cached on local storage, removing the dependency on remote storage for OS disk reads. Once caching is complete, all OS disk IO is served locally. This results in:
- Consistently fast OS disk performance with low‑millisecond latency
- Improved resilience during remote storage disruptions
- No impact to VM create times, as caching happens asynchronously after boot
This capability is well suited for IO-sensitive stateless workloads that need fast OS disk access, including:
- AI workloads
- Quorum‑based databases
- Data analytics and real‑time processing systems
- Large‑scale stateless services on General Purpose VM families
These workloads benefit directly from lower OS disk latency and reduced exposure to remote storage outages.
How It Works?
When full OS caching is enabled:
- VM’s Local storage (cache disk, resource disk, or NVMe disk) is used to host the full OS disk
- Local storage capacity is reduced by 2× the OS disk size to accommodate OS caching
- The OS disk is cached in the background after VM boot, ensuring fast provisioning
- All OS disk IOs happen on the local storage, thus providing 10X better IO performance and resiliency to storage interruptions
Public Preview Availability
During public preview, Ephemeral OS disk with full caching is available for most general purpose VM SKUs (excluding 2‑vCPUs and 4‑vCPUs VMs) in 29 regions - AustraliaCentral, AustraliaCentral2, AustraliaSouthEast, BrazilSoutheast, CanadaCentral, CanadaEast, CentralIndia, CentralUSEUAP, EastAsia, GermanyWestCentral, JapanEast, JioIndiaCentral, JioIndiaWest, KoreaCentral, KoreaSouth, MalaysiaSouth, MexicoCentral, NorthEurope, NorwayWest, QatarCentral, SouthAfricaNorth, SwedenCentral, SwitzerlandWest, TaiwanNorth, UAECentral, UKSouth, UKWest, WestCentralUS, and WestIndia.
We’re continuing to expand support across regions, and tooling as we move toward general availability.
Getting Started
Customers can enable Ephemeral OS disk with full caching when creating new VMs or VMSS by updating their ARM templates or REST API definitions and setting the enableFullCaching flag for Ephemeral OS disks.
ARM template to create VMs with full caching:
"resources": [
"name": "[parameters('virtualMachineName')]",
"type": "Microsoft.Compute/virtualMachines",
"apiVersion": "2025-04-01",
..
..
"osDisk":
{
"diffDiskSettings":
{
"option": "Local",
"placement": "ResourceDisk",
"enableFullCaching": true
},
"caching": "ReadOnly",
"createOption": "FromImage",
"managedDisk":
{
"storageAccountType": "StandardSSD_LRS"
}
}
ARM template to create VMSS with full caching:
"resources": [
"name": "[parameters('vmssName')]",
"type": "Microsoft.Compute/virtualMachineScaleSets",
"apiVersion": "2025-04-01",
..
..
"osDisk":
{
"diffDiskSettings":
{
"option": "Local",
"placement": "ResourceDisk",
"enableFullCaching": true
},
"caching": "ReadOnly",
"createOption": "FromImage",
"managedDisk":
{
"storageAccountType": "StandardSSD_LRS"
}
}
Your feedback during public preview will help shape the final experience.
Microsoft