This article describes how to create application-consistent backups for Azure-deployed Linux VMs using Azure Backup. It covers configuring the script framework and troubleshooting.
When Azure Backup takes snapshot of a VM, application consistency ensures that applications start correctly after VM restoration. To achieve this, use the Linux prescript and post-script framework, which supports Azure Resource Manager-deployed Linux VMs. However, these scripts don't work for Service Manager-deployed or Windows VMs.
How the framework works?The framework provides an option to run custom prescripts and post-scripts while you're taking VM snapshots. Prescripts run just before you take the VM snapshot, and post-scripts run immediately after you take the VM snapshot. Prescripts and post-scripts provide the flexibility to control your application and environment, while you're taking VM snapshots.
Prescripts invoke native application APIs, which quiesce the IOs, and flush in-memory content to the disk. These actions ensure the snapshot is application consistent. Post-scripts use native application APIs to thaw the IOs, which enable the application to resume normal operations after the VM snapshot.
Configure prescript and post-script for Azure Linux VMTo configure Prescript and post-script, follow these steps:
Sign in as the root user to the Linux VM that you want to back up.
From GitHub, download VMSnapshotScriptPluginConfig.json and copy it to the /etc/azure folder for all VMs you want to back up. If the /etc/azure folder doesn't exist, create it.
Copy the prescript and post-script for your application on all VMs 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.
To use the following files, ensure that you have the corresponding permissions:
VMSnapshotScriptPluginConfig.json: Permission 600. For example, only root user should have read and write permissions to this file, and no user should have execute permissions.
Pre-script file: Permission 700. For example, only 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.
Post-script Permission 700. For example, only 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 power. Secure the framework, and ensure only root user has access to critical JSON and script files. If the requirements aren't met, the script won't run, which results in a file system crash and inconsistent backup.
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 that's going to be backed up.
postScriptLocation: Provide the full path of the post-script on the VM that's going to be backed up.
preScriptParams: Provide the optional parameters that need to be passed to the prescript. All parameters should be in quotes. If you use multiple parameters, separate the parameters with a comma.
postScriptParams: Provide the optional parameters that need to be passed to the post-script. All parameters should be in quotes. If you use multiple parameters, separate the parameters with a comma.
preScriptNoOfRetries: Set the number of times the prescript should be retried if there's any error before terminating. Zero means only one try and no retry if there's a failure.
postScriptNoOfRetries: Set the number of times the post-script should be retried if there's any error before terminating. Zero means only one try and no retry if there's a failure.
timeoutInSeconds: Specify individual time-outs for the prescript and the post-script (maximum value can be 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 post-script fails. Setting this to false fails the backup if there's a script failure (except 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 will be 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 keeping this setting set to true unless your application has a dependency on disabling fsfreeze.
ScriptsExecutionPollTimeSeconds: Set the time the extension has to sleep between each poll to the script execution. For example, if the value is 2, the extension checks whether the pre/post script 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.
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 using Back up Azure virtual machines to Recovery Services vaults.
Make sure you add appropriate logging while writing your prescript and post-script, and review your script logs to fix any script issues. If you still have problems running scripts, refer to the following table for more information.
Error Error message Recommended action Pre-ScriptExecutionFailed The prescript returned an error, so backup might not be application-consistent. Look at the failure logs for your script to fix the issue. Post-ScriptExecutionFailed The post-script returned an error that might impact application state. Look at the failure logs for your script to fix the issue and check the application state. Pre-ScriptNotFound The prescript wasn't found at the location that's specified in the VMSnapshotScriptPluginConfig.json config file. Make sure that prescript is present at the path that's specified in the config file to ensure application-consistent backup. Post-ScriptNotFound The post-script wasn't found at the location that's specified in the VMSnapshotScriptPluginConfig.json config file. Make sure that post-script is present at the path that's specified in the config file to ensure application-consistent backup. IncorrectPluginhostFile ThePluginhost file, which comes with the VmSnapshotLinux extension, is corrupted, so prescript and post-script can't run and the backup won't be application-consistent. Uninstall the VmSnapshotLinux extension, and it will automatically be reinstalled with the next backup to fix the problem. IncorrectJSONConfigFile The VMSnapshotScriptPluginConfig.json file is incorrect, so prescript and post-script can't run and the backup won't be application-consistent. Download the copy from GitHub and configure it again. InsufficientPermissionforPre-Script For running scripts, root user should be the owner of the file and the file should have 700 permissions (that is, only owner should have read, write, and execute permissions). Make sure root user is the owner of the script file and that only owner has read, write and execute permissions. InsufficientPermissionforPost-Script For running scripts, root user should be the owner of the file and the file should have 700 permissions (that is, only owner should have read, write, and execute permissions). Make sure root user is the owner of the script file and that only owner has read, write and execute permissions. Pre-ScriptTimeout The execution of the application-consistent backup pre-script timed-out. Check the script and increase the time-out in the VMSnapshotScriptPluginConfig.json file that's located at /etc/azure. Post-ScriptTimeout The execution of the application-consistent backup post-scripts timed out. Check the script and increase the time-out in the VMSnapshotScriptPluginConfig.json file that's located at /etc/azure. Next steps
Configure VM backup to a Recovery Services vault
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