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