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Application-consistent restore points capture Virtual Machine data in a state that guarantees application integrity at the time of the snapshot. To achieve this, Azure coordinates with applications running on the Virtual Machine before taking the snapshot:
- Windows: The VMSnapshot Windows extension triggers the Volume Shadow Copy Service (VSS) to quiesce application writes.
- Linux: The VMSnapshot Linux extension runs pre- and post-scripts to flush and resume application I/O.
When you request an application-consistent restore point, Azure automatically installs the VMSnapshot extension on the Virtual Machine if it isn't already present. The extension updates automatically no manual management is required.
Important
Azure begins creating a restore point only after all extensions on the Virtual Machine, including VMSnapshot, has finished provisioning. Monitor extension provisioning state before expecting restore point creation to succeed.
To confirm the extension is installed and in a Provisioned state, check Virtual Machine > Extensions + applications in the Azure portal, or run:
az vm extension show \
--resource-group <resourceGroupName> \
--vm-name <vmName> \
--name VMSnapshot
Extension logs
Use the following paths to access VMSnapshot extension logs directly on the Virtual Machine:
| OS | Log path |
|---|---|
| Windows | C:\WindowsAzure\Logs\Plugins\Microsoft.Azure.RecoveryServices.VMSnapshot |
| Linux | /var/log/azure/Microsoft.Azure.RecoveryServices.VMSnapshotLinux/extension.log |
Troubleshooting
Communication issues between the Virtual Machine agent and the VMSnapshot extension cause most restore point failures. Start troubleshooting with Troubleshoot restore point failures.
VSS writer failures (Windows)
When a VSS writer fails, Azure can't take an application-consistent snapshot and falls back to a file system-consistent restore point. This fallback applies for the next three attempts, regardless of the configured restore point schedule. From the fourth attempt onward, Azure resumes attempting application-consistent restore points.
To resolve VSS writer failures, see Troubleshoot VSS writer issues.
Warning
Don't manually delete the VMSnapshot extension. Deleting it breaks subsequent application-consistent restore point creation until the extension is reinstalled and provisioned again.