Differences Between a Differential Merge and a Full Backup



This article will discuss how to distinguish between between a full backup and a differential merge backup in the SIRIS/ALTO device web interface.


Initially, the ShadowSnap agent takes a full backup of each protected volume. During normal operation, subsequent backups will be incremental backups reflecting the gradual change on the server (typically a few hundred MB to a couple GB depending on the server's role and backup frequency).

If a backup job fails unexpectedly, the ShadowSnap agent might perform a differential merge to reconcile the information in the most recent backup with the current state of the server. During a differential merge, the agent scans the entirety of each protected volume, but only stores the changes between the state of the protected machine and the most recent backup.

Since the Datto SIRIS and ALTO web interfaces report the amount of data transferred to the Datto for each recovery point, a differential merge backup displayed in the recovery points page may be misleading since the value will be the same for both differential merge and full backups.

Distinguishing between full and differential merge backups

The most obvious indicator of a full backup in the middle of a chain is an inflated amount of space used for incremental backups. This is represented by Total Incremental Use as shown in Figure 1.

Figure 1 - Storage Overview

If a backup is running, a differential merge can be distinguished from a full by examining the following:

  • The amount of locally-used space (Local Storage Used or Used Incremental Space in the web interface) will always gradually increase during a full backup.
  • A differential merge will appear as a large data transfer across the network from the Datto device to the protected system.

If you are unable to determine if a full backup was taken, please contact Datto Technical Support for assistance.

Preventing fulls in the middle of a backup chain

If a server backing up with the Shadowsnap agent experiences a dirty shutdown due to power loss or corruption, a new full backup may be executed by the Shadowsnap agent in the middle of a backup chain.

Failure to address power or system issues may result in repeated full or failed backups until the issue is properly addressed.

To minimize the risk of dirty shutdowns or corruption causing backup issues, it is imperative that systems undergo a full filesystem and disk health check before deploying the agent. Please see the Installing The ShadowSnap Agent for a comprehensive list of considerations before deployment.

If you are experiencing full backups even though the server appears to be healthy and configured properly, check for security or backup programs that may interfere or prevent the Shadowsnap agent from storing, accessing, or modifying the "VSM000.IDX" incremental tracker file stored at the root of each protected volume's filesystem on the server (hidden as a protected operating system file). This file is used by ShadowSnap to track modified sectors on disk. It is loaded into kernel memory on boot and saved when the kernel shuts down.

If you are unable to resolve a backup issue after a thorough examination of the server and environment, please contact Datto Technical Support for assistance.

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