SIRIS, ALTO, and NAS: Understanding offsite synchronization modes



This article describes the different offsite synchronization modes used by Datto appliances.

If you are experiencing issues with offsite synchronization falling behind, see the Error: Offsite Replication - Not Completely Synchronized Knowledge Base article. Cloud management options are not available on appliances with local-only billing plans.


  • Datto SIRIS
  • Datto ALTO
  • Datto NAS


Datto devices use two different methods to send backups to offsite severs: Streaming Mode, and Build and Send Mode. These modes are automatically selected by the Datto appliance based on need, and cannot be configured by devices operators through the GUI.

Build and Send Mode

Build and Send mode is a two-stage process. Before the Datto device sends a backup to an offsite server, it initiates the build stage. In this stage, the appliance creates a zip file of the backup, and stores it on its array drives. Some SIRIS Professional devices, and all SIRIS Enterprise devices, have dedicated transfer drives for this purpose.

When the build stage completes, the Datto device enters the send stage. In this stage, the appliance opens a secure connection to the offsite server, and uploads the compressed file to it. If the device loses connection to the offsite server in the middle of the sync, it can resume the upload at a later time, because it is sending a zip file.

When the file finishes uploading, the offsite server decompresses the backup, and imports it into the cloud dataset. At this point, you can use the backup for offsite virtualizations and file restores.

Streaming Mode

In streaming mode, the Datto appliance streams data to the offsite server in native ZFS format. This mode can be faster for users with very stable Internet connections. But, if network connectivity becomes unstable, and the connection to the offsite server drops, the sync will restart from the beginning. This is because streaming mode cannot pause and later resume.

By default, Datto devices will only go into this mode if there is a configuration issue, or if there is not enough hard drive space left on the appliance to build a send file.

Determining the Backup Mode

To determine which offsite synchronization mode the Datto device is using for a particular server, navigate to the home page of the GUI. Scroll down to the Replication Status section of the page, as shown in Figure 1.

Figure 1: Replication Status


In Build Mode, a gear icon will appear next to the server's name. The replication status will say "Preparing backup," and the Total Percent and ETA fields will show "N/A" values. The Rate field will track the speed at which the Datto appliance is compressing the backup, as shown in Figure 2.

Figure 2: Build Mode

Figure 3 illustrates Send Mode. When Send Mode begins, a cloud icon will replace the gear icon next to the server's name. The replication status will say "Syncing backup," and all reporting fields will show values.

Figure 3: Send Mode

Figure 4 shows Streaming Mode. In this mode, a cloud icon will appear next to the server's name. The replication status will say "Syncing backup," but the Total, Percent, and ETA fields will not report values.

stream.PNGFigure 4: Streaming Mode


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