SIRIS, ALTO, and NAS: 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 is not to 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 functions as a form of consistency checking and can resolve problems with backups by auditing for and correcting errors, such as corruption. 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 to be the size of a full backup. However, it should only consume the space of an incremental backup on the storage array, and the Manage Recovery Points page will record the transfer type accurately.

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.
  • When it is running, a differential merge will appear as a large data transfer across the network between the Datto device and the protected system.
  • If the transfer is a full backup, you will see the available storage space on your Datto appliance decreasing at a 1-to-1 ratio to the amount of data reported as transferred in the backup progress bar of the Datto appliance's Protect page.

Force a Differential Merge

Manually initiating a differential merge can sometimes help resolve backup issues such as failed screenshots or file system verification failures. They can be a good alternative to creating a new full backup as they offer the benefit of continuing the existing backup chain and require less space than a full backup.

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)

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