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Differential Merge



This article explains the nature of a differential merge backup, and describes the "Force a Differential Merge" feature on Datto appliances.


  • Datto SIRIS
  • Datto ALTO


What is a Differential Merge?

A differential merge allows a backup agent to traverse the live backup dataset of the protected server, compare it to the protected system volumes, and back up new changes.

A differential merge should not be confused with a Differential Backup Chain. The process applies the same concept as a VSS differential backup.  Since each new incremental is compiled into a 'full' live dataset to accomplish the Inverse Chain, the difference from the 'last full' is usually just the size of a new incremental.

This process can sometimes resolve issues with backup virtualization by including changes that a normal incremental might not. A Differential Merge can also allow an agent to proceed with incremental backups in a situation that might otherwise require an incremental chain to restart with a new full backup. In the Protect tab of the appliance GUI, a backup running as a Differential Merge will appear as a transfer the size of a full backup. However, this should only consume the space of an incremental backup on the Datto.

Distinguishing a Differential Merge from a Full Backup

  • A differential merge will appear as a 'Differential' backup on the Manage Recovery Points page of the Datto appliance GUI.
  • It can also be identified by monitoring used space on the Datto appliance or by network traffic.
  • Regarding a full backup, the used space will be notably increasing on a 1-to-1 ratio to the displayed transfer size as a full backup in progress.
  • A differential merge will appear as a large data transfer across the network from the Datto device to the protected system.
  • A full backup will show a large transfer moving from the protected system to the Datto.

Force a Differential Merge

To force a differential merge, follow these steps:

  1. Open the GUI for the Datto device.
  2. Click on the Protect tab.
  3. Find the agent that you wish to configure.
  4. Click Configure Agent Settings for that agent.
  5. Go to the Advanced section, and scroll to Force a Differential Merge, as shown in Figure 1.
  6. Click the Force Differential Merge button. The next scheduled backup will be a differential merge.

Figure 1: Force a Differential Merge (click to enlarge)

Additional Resources

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