Explanation
Why the waste happens and who it affects.
A full snapshot stores a complete copy of the disk's used data every time and can be stored on either Standard HDD or Premium SSD storage. An incremental snapshot stores only the changes since the previous snapshot of the same disk and is always stored on Standard HDD storage, regardless of the source disk type. Snapshots are used for backup, rollback and cloning, not for live I/O, so paying Premium storage rates for them buys nothing.
The waste appears when scripts, older templates, runbooks or manual portal actions create full snapshots with the Premium_LRS SKU, sometimes copying the SKU of a Premium SSD source disk. Each additional full snapshot of the same disk stores the whole used size again, so a daily or pre-change snapshot routine multiplies the cost. Microsoft's FinOps best practices for Storage include a specific recommendation to downgrade premium snapshots, and the incremental snapshot documentation recommends custom images in Azure Compute Gallery on Standard HDD instead of full Premium snapshots for scaling out VM deployments.
Billing model
The pricing dimensions that drive this cost.
Managed disk snapshots are billed on stored data, with the rate set by snapshot type and storage.
- Used size billing
- Snapshots are billed per GB-month for the used data they hold, not for the provisioned size of the source disk
- Full snapshot
- Stores the complete used data of the disk each time and can be stored on Standard HDD or Premium SSD storage
- Incremental snapshot
- After the first full copy, stores only the delta since the last snapshot and is always stored on Standard HDD storage
- Snapshot redundancy
- LRS and ZRS snapshots are priced the same
How to detect
5 checks to find it in your estate.
- Run the Azure Resource Graph query from Microsoft's FinOps storage best practices: resources where type == 'microsoft.compute/snapshots' and tostring(sku.tier) == 'Premium'
- In the same query, filter properties.incremental == false to find full snapshots, and group by properties.creationData.sourceResourceId to find disks with several full snapshots
- Review the cost of the snapshot meters in Cost Management by meter subcategory to see how much snapshot spend is on Premium storage
- Check snapshot automation (scripts, runbooks, ARM or Bicep templates, Terraform) for a Premium_LRS SKU or a missing incremental: true property
- Identify full Premium snapshots used as golden images for scaling out VMs, which Microsoft recommends replacing with Azure Compute Gallery images
How to fix
5 ways to remove the waste.
- Change snapshot automation to create incremental snapshots (az snapshot create --incremental true, New-AzSnapshotConfig -Incremental, or incremental: true in templates); incremental snapshots are stored on Standard HDD automatically
- For existing full Premium snapshots that must be kept, change them to Standard storage (az snapshot update --sku Standard_LRS or Standard_ZRS) or replace them with a new incremental snapshot of the source disk, then delete the Premium copy once the replacement is verified
- Delete redundant full snapshots of the same disk that are no longer needed for recovery or compliance
- Replace full Premium snapshots used to clone VMs with custom images in Azure Compute Gallery stored on Standard HDD
- Account for incremental snapshot limits before switching: up to 500 incremental snapshots per disk at a time, and incremental snapshots cannot be moved between subscriptions or resource groups, only copied
Documentation
Vendor references for pricing and configuration.