Agentless backup using Resilient Change Tracking (RCT)

Details

When performing agentless backup on a Hyper-V host with HyperBack, the backup task can use either RCT or non-RCT methods.

By default, RCT is enabled and provided as an option. However, it's only supported for second-generation Windows 2016 or later virtual machines. For other VMs, RCT is automatically disabled.

To enable RCT, open the AIP console, navigate to [Preferences] > [HyperBack Setting] > [Options] tab, and select "Use Microsoft Resilient Change Tracking (RCT)" under Tracking Method.

How to create RCT or non-RCT check point:

When RCT is enabled, Hyper-V checkpoints have a SnapshotType of Recovery. These checkpoints are created temporarily during backup tasks. Files with .mct or .rct extensions are generated in the same folder as the VHDX file, capturing incremental changes since the last backup.

When RCT is disabled, the SnapshotType for Hyper-V checkpoints is Standard. Either VG_Snapshot1 or VG_Snapshot2 is always present to track file changes and store them for incremental backups.

You can monitor these checkpoints using Hyper-V Manager or PowerShell.

PS >Get-VMSnapshot -vmname "inventory name"

RCT and non-RCT each have their advantages and disadvantages.

RCT

Advantage
・The backup processing time is short. The temporary file size is smaller for monitoring incremental backups.

Disadvantage

Phenomenon: RCT creates temporary checkpoints in Hyper-V Manager, which should be automatically removed. However, some checkpoints might persist, like those named VG_Recovery_XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX.

To resolve this, follow the instructions in the knowledge base or delete the mentioned checkpoints by highlighting them in Hyper-V Manager and pressing the Del key. After applying the described solutions, restart by performing a full backup. If the issue persists, switch to the non-RCT tracking method since AIP may not offer a solution.

Non-RCT

Advantage

・The RCT phenomenon mentioned above hasn't been verified.

Disadvantage

・Backups are slower compared to RCT, and the temporary tracking files (.vhdx/.avhdx) are larger.

Enable / Disable RCT 
You can toggle RCT settings, but they apply to all guest machines on registered Hyper-V hosts in AIP. After changing settings, unaffected guest machines (where RCT isn't supported) can still be used as normal. 

Change from RCT to non-RCT

・Start by creating a full backup. If you encounter a "profile cannot be found" error when running manually from HyperAgent task, right-click on an agentless task in Scheduled Task and run the full backup task.

・Before switching to non-RCT, the RCT tracking files (.avhdx.rct and .avhdx.mct) are held by the system and can't be instantly deleted. After the backup process, the folder structure will look like this.

The guest machine has VG_Snapshot1 or VG_Snapshot2 checkpoints.

You can proceed without deleting the mentioned files; but, if you need to delete them, follow these steps:

・When AIP is not running, and you delete VG_Snapshot1 or VG_Snapshot2 from Hyper-V Manager, .avhdx.rct, .avhdx.mct.mct, and .rct files are automatically removed.

・Then, shut down the system and delete vhdx.mrt and .vhdx.rct. 

・Since tracking is reset, it's advisable to create a full backup and begin a new incremental chain. Starting the chain with an incremental file results in them being the same size as a full backup, increasing storage needs. 

・After the first backup task, only .avhdx files are created with non-RCT tracking.

 

When changed from RCT to non-RCT

・To remove VG_Snapshot1 or VG_Snapshot2 checkpoints for the guest machine, simply right-click on those files in Hyper-V Manager and choose [Delete].

・Afterward, begin a fresh incremental chain starting from a new full backup. If you encounter a "Profile not found" error when manually running the HyperAgent task, causing the task to fail, simply right-click on the agentless task in Scheduled Task and select to run the full backup task. 

Backup task is running

After backup task execution

The above check points are automatically deleted.

＊This article is created by running AIP2022 7.0.3.8919.

Target Product

- ActiveImage Protector 2022

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