VMSnapshot extension
Application-consistent restore points use the Volume Shadow Copy Service (VSS) service (or pre-/postscripts for Linux) to verify application data consistency prior to creating a restore point. Achieving an application-consistent restore point involves the VM's running application providing a VSS service (for Windows) or pre- and postscripts (for Linux).
For Windows images, the VMSnapshot Windows extension, and for Linux images, the VMSnapshot Linux extension, are used for taking application consistent restore points. When there's a create application consistent restore point request issued from a VM, Azure installs the VM snapshot extension if it's not already present. The extension is updated automatically.
Important
Azure begins creating a restore point only after all extensions (including but not limited to VMSnapshot) provisioning state are complete.
Extension logs
You can view logs for the VMSnapshot extension on the VM under
C:\WindowsAzure\Logs\Plugins\Microsoft.Azure.RecoveryServices.VMSnapshot for Windows and under /var/log/azure/Microsoft.Azure.RecoveryServices.VMSnapshotLinux/extension.log for Linux.
Troubleshooting
Most common restore point failures are attributed to the communication with the VM agent and extension. To resolve failures, follow the steps in Troubleshoot restore point failures.
During certain VSS writer failure, Azure takes file system-consistent restore points for the next three times (irrespective of the frequency at which the restore point creation is scheduled) upon failing the initial creation request. From the fourth time onward, an application-consistent restore point is attempted.
Follow these steps to troubleshoot VSS writer issues.
Note
Avoid manually deleting the extension because it leads to failure of the subsequent creation of an application-consistent restore point.