Configure application-consistent backup for Azure Linux VMs by using Azure Backup

This article describes how to create application-consistent backups for Azure-deployed Linux virtual machines (VMs) by using Azure Backup. It covers how to configure the script framework and troubleshooting.

When Azure Backup takes a snapshot of a VM, application consistency ensures that applications start correctly after VM restoration. To achieve this behavior, use the Linux prescript and postscript framework, which supports Linux VMs deployed with Azure Resource Manager. These scripts don't work for VMs deployed with System Center Service Manager or Windows.

How does the framework work?

The framework provides an option to run custom prescripts and postscripts while you're taking VM snapshots. Prescripts run before you take the VM snapshot. Postscripts run immediately after you take the VM snapshot. Prescripts and postscripts provide the flexibility to control your application and environment while you're taking VM snapshots.

Prescripts invoke native application APIs, which quiet the I/Os, and flush in-memory content to the disk. These actions ensure that the snapshot is application consistent. Postscripts use native-application APIs to thaw the I/Os, which enables the application to resume normal operations after the VM snapshot.

Configure prescript and postscript for Azure Linux VMs

To configure prescript and postscript, follow these steps:

1. Sign in as the root user to the Linux VM that you want to back up.

2. From GitHub, download VMSnapshotScriptPluginConfig.json and copy it to the /etc/azure folder for all the VMs that you want to back up. If the /etc/azure folder doesn't exist, create it.

3. Copy the prescript and postscript for your application on all VMs that you plan to back up. You can copy the scripts to any location on the VM. Be sure to update the full path of the script files in the VMSnapshotScriptPluginConfig.json file.

4. To use the following files, ensure that you have the corresponding permissions:

   • VMSnapshotScriptPluginConfig.json: Permission is 600. For example, only the root user should have read and write permissions to this file, and no user should have execute permissions.
   • Prescript file: Permission is 700. For example, only the root user should have read, write, and execute permissions to this file. The file is expected to be a shell script, but theoretically, this script can internally spawn or refer to other scripts like a Python script.
   • Postscript: Permission is 700. For example, only the root user should have read, write, and execute permissions to this file. The file is expected to be a shell script, but theoretically, this script can internally spawn or refer to other scripts like a Python script.

   Important

   The framework gives users numerous powers. Secure the framework, and ensure that only the root user has access to critical JSON and script files.

   If the requirements aren't met, the script can't run, which results in a file system crash and inconsistent backup.

5. Configure VMSnapshotScriptPluginConfig.json as described here:

   • pluginName: Leave this field as is, or your scripts might not work as expected.
   • preScriptLocation: Provide the full path of the prescript on the VM to be backed up.
   • postScriptLocation: Provide the full path of the postscript on the VM to be backed up.
   • preScriptParams: Provide the optional parameters that must be passed to the prescript. All parameters must be in quotation marks. If you use multiple parameters, separate the parameters with a comma.
   • postScriptParams: Provide the optional parameters that must be passed to the postscript. All parameters must be in quotation marks. If you use multiple parameters, separate the parameters with a comma.
   • preScriptNoOfRetries: Set the number of times that the prescript should be retried if an error occurs before terminating. Zero means only one try, and no retry if there's a failure.
   • postScriptNoOfRetries: Set the number of times that the postscript should be retried if an error occurs before terminating. Zero means only one try, and no retry if there's a failure.
   • timeoutInSeconds: Specify individual timeouts for the prescript and the postscript (maximum value is 1800).
   • continueBackupOnFailure: Set this value to true if you want Azure Backup to fall back to a file system-consistent/crash-consistent backup if prescript or postscript fails. Setting this value to false fails the backup if a script failure occurs. (An exception is when you have a single-disk VM that falls back to crash-consistent backup regardless of this setting.) When the continueBackupOnFailure value is set to false, if the backup fails, the backup operation is attempted again based on a retry logic in service (for the stipulated number of attempts).
   • fsFreezeEnabled: Specify whether Linux fsfreeze should be called while you're taking the VM snapshot to ensure file system consistency. We recommend that you keep this setting set to true unless your application has a dependency on disabling fsfreeze.
   • ScriptsExecutionPollTimeSeconds: Set the time that the extension has to sleep between each poll to the script execution. For example, if the value is 2, the extension checks whether the prescript or postscript execution completed every 2 seconds. The minimum and maximum value it can take is 1 and 5, respectively. The value should be strictly an integer.

6. The script framework is now configured. If the VM backup is already configured, the next backup invokes the scripts and triggers application-consistent backup. If the VM backup isn't configured, configure it by following the steps in Back up Azure virtual machines to Recovery Services vaults.

Troubleshoot Azure Linux VM application-consistent backup errors

Make sure that you add appropriate logging while you write the prescript and postscript. Review your script logs to fix any script issues. If you still have problems running scripts, refer to the following table.

Related content

• Configure VM backup to a Recovery Services vault