Budget local storage explicitly before choosing full caching for an ephemeral OS disk

Source facts

Ephemeral OS full caching stores the entire OS disk locally rather than using the default split between a local difference disk and a remote base disk. Microsoft’s prerequisites require a stateless OS, an eligible VM with at least eight vCPUs, API version 2025-04-01 or later, and local capacity greater than twice the OS disk size plus 1 GiB. Full caching consumes twice the OS disk size locally. Microsoft Learn.

Applicability

The documented full-caching availability is limited to Azure public regions. Evaluate the current supported VM-series list and the exact image size, not only a processor count. Identify the local storage also expected by temporary files or application work. Keep durable application state outside the proposed ephemeral OS design and review the source’s lifecycle and encryption restrictions before approval.

DSE recommendation

Check full-caching eligibility and calculate remaining local storage before selecting the VM size. Put the image size, required caching allocation, and application temporary-space requirement in the same sizing record. Ask the application owner to approve the resulting headroom. Do not assume that a VM capable of hosting a default ephemeral OS disk automatically satisfies full-caching requirements.

Verification

Inspect the requested full-caching setting and deployed VM size against the approved design. Measure available local space after deployment and exercise the application’s temporary-storage workload. Repeat the capacity calculation when the image grows or the VM size changes. Record insufficient space as a failed acceptance condition rather than compensating by placing durable data on an unreviewed local path.

Official references

Microsoft Learn: Ephemeral OS disks for Azure VMs. Source reviewed September 9, 2026.