BRC - Image Backup Scan Modes Explained

Written By Tami Sutcliffe (Super Administrator)

Updated at August 31st, 2023

Image backup scan modes

There are three image backup scan modes available: 



Full scan (legacy) mode is based on a single VSS snapshot.

This mode compares the current snapshot with data stored on the Axcient appliance.

Legacy backup can take time because (a) a large quantity of data is being transferred via the network and (b) files are being compared in two different locations. At the same time, this mode creates a lower load to the protected device's CPU/RAM.

IR (Intelligent Replicator), also called Optimized scan, is based on two VSS snapshots: one from a previous backup cycle and the current snapshot. 

The comparison and incremental change calculations are performed on the protected device.

This mode is much faster than the legacy mode because the change calculation does not involve a network at all. Everything is done on the protected server. Obviously, this utilizes additional CPU time and RAM on the server (but not very much.)

Block level scan or FD (Fast Delta) is an evolution of the IR mode, with some additional optimization for a huge numbers of files.

Fast delta mode works in much the same way as the IR mode. The IR mode has a long history (since v6.4.x) while the FD mode was implemented with v6.6.x of AxOS (configurable via the backend only.) Starting with v6.7.2, settings can be configured via the UMC.


These image backup scan modes apply only to image backups on Windows machines. (These modes are not available for file level backups or Linux/Unix machines.) 

When selecting between these modes, please be aware of the following: 

  • IR/FD is preferred since it works much faster than the legacy mode. 
  • For IR/FD, a previous VSS snapshot must be stored on the protected server. If, for some reason, the previous snapshot is deleted, the backup will automatically switch to the transitional Legacy mode. This occurs because the required second snapshot, necessary for data comparison, is not available.
  • IR/FD will create a higher load and require more RAM on the protected device. Backups might fail on some older servers with 2GB of RAM and a slower CPU.  On the other hand, the backup process is assigned the lowest priority in Windows, so it shouldn't interrupt any standard activities.
  • In general, Legacy mode is not used widely but tends to be for troubleshooting purposes or for unsupported configurations.


[For internal references, see notes.]