Updated 25 October 2026 · Published by Zoorik
An Azure ephemeral OS disk is an operating system disk on the local storage of its virtual machine (VM). What you write to it isn’t saved to remote Azure Storage.
It suits stateless apps, which can lose a VM and carry on. Reads and writes have less delay.
It must fit in the VM’s cache, temp disk or NVMe disk. Azure Kubernetes Service (AKS) uses one by default when it fits.
An AKS ephemeral OS disk that doesn’t fit
In Microsoft’s AKS example, Standard_E2bds_v5 has 75 GiB of temporary storage. If you leave the OS disk type unset:
- A 100 GiB OS disk doesn’t fit, so AKS uses a managed OS disk.
- A 60 GiB OS disk fits.
- Standard_E4bds_v5 has 150 GiB, so a 100 GiB OS disk fits there.
Check the fit before you change an AKS node pool VM size.
What you give up
- A VM resize, a redeploy or a host failure wipes the OS disk.
- You can’t stop and deallocate the VM.
- No disk snapshots or Azure Backup.
Why it matters for your bill
- Microsoft says the VM price includes ephemeral OS disks. An SSD base disk, which serves some reads, costs extra.
- A managed OS disk is billed on its own. Each node has one. AKS picks a bigger default one on nodes with more virtual CPUs.
- Coral drains spare nodes so your autoscaler can remove them. It rebalances CPU and memory requests on AKS and EKS to match real use.
- The free assessment works out your own figure.
Read next: Kubernetes node size best practices: large or small nodes
Sources: Microsoft Learn, “Ephemeral OS disks” (opens in a new tab); Microsoft Learn, “Concepts - Storage in Azure Kubernetes Service (AKS)” (opens in a new tab); Microsoft Learn, “Cost Management for Kubernetes” (opens in a new tab); Microsoft Learn, “Overview of Azure Disk Storage” (opens in a new tab).