# Azure Stateless VM Could Use Ephemeral OS Disk

> VMs tagged stateless=true that still boot from a billed persistent OS disk are recommended to switch to a free ephemeral OS disk. Savings equal the managed-disk tier rate times 730 hours for the disk's size and SKU, subject to a $5 monthly floor; missing disk metadata or rates abstain.

Source: https://zop.dev/integrations/azure/recommendations/azure-stateless-vm-could-use-ephemeral-os-disk
Updated: 2026-08-19

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## Why the stateless tag has to be set by hand

Fires only when Metadata ephemeral_os_disk is present-and-not-"true" (discoverer-authoritative osDisk.diffDiskSettings.option; fail-closed when absent) AND the customer tag stateless=true (workload intent, no cloud equivalent) AND the OS-disk cost producer is present (os_disk_size_gb + os_disk_sku Metadata + a resolvable managed-disk tier rate). Abstains (returns nil) on any missing input, and never emits SavingsUSD=0 under this cost-driven category.

## The OS disk line item ephemeral removes

concrete-or-abstain. The lever switches a persistent managed OS disk to a free ephemeral OS disk; the real saving = the managed OS-disk line item it removes = the flat monthly Managed Disks tier price for the OS disk's size + storageAccountType (tierRate × 730), where tierRate is the pricing_cache row keyed Premium_SSD_Managed_Disk_P\{n} / Standard_SSD_Managed_Disk_E\{n} / Standard_HDD_Managed_Disk_S\{n} (same flat-monthly-tier basis as RC-240/RC-1386). This is the separate OS-disk charge and not a fraction of VM compute: CurrentCostUSD = SavingsUSD = tierRate×730, OptimizedCostUSD = 0. Producer (wired): (1) discoverer enrichAzureVM stamps os_disk_size_gb (osDisk.diskSizeGB) + os_disk_sku (osDisk.managedDisk.storageAccountType) from the same ARG projection (no extra SDK call); (2) the Managed Disks tier rate is already ingested by the aggregator pricing (Premium/Standard SSD/Standard HDD Managed Disks meters → azureDiskTierKeyFromMeter) and reaches the rule via the existing SKURatesAware/TargetSKUCandidatesAware bulk-fetch, with no new setter or pricing change. IOPS-priced families (UltraSSD/PremiumV2) have no flat tier rate → abstain. $5/month minimum-savings floor. Note: the stateless axis remains tag-gated (no authoritative cloud/API signal exists to de-tag it).
the quoted saving assumes the ephemeral OS disk keeps Azure's default free Standard HDD base disk. Azure also lets a customer opt into a Premium/Standard SSD base disk to preserve read performance/SLA, and that base disk is itself a billed managed disk, so if that non-default option is chosen the realized saving is only partial (possibly ~$0). The Description and Remediation now call this out explicitly; SavingsUSD is not discounted for SSD-tier source disks, since the full saving is genuinely correct in the default/common configuration this rec targets.

## Rebuilding the VM with a local OS disk

1. Confirm the workload is truly stateless (no data persisted to the OS disk)
2. Capture the VM configuration (size, NICs, data disks, extensions)
3. Recreate the VM with storageProfile.osDisk.diffDiskSettings.option=Local at the same size
4. Keep the ephemeral OS disk's base disk on the default free Standard HDD to realize the full quoted saving; only pick a Premium/Standard SSD base disk if the workload needs the extra read performance/SLA, and expect the saving to shrink (possibly to ~$0) in that case
5. Reattach data disks and re-apply extensions; validate boot and workload health