arcserve-KB : UDP Agentless on Microsoft Hyper-V and Incremental backups

Last Update: 2016-11-23 11:44:09 UTC


Changed Block Tracking (CBT) is a VMware feature that helps perform incremental backups. VMware Data Recovery uses this technology and so can developers of backup and recovery software.

UDP Agentless on VMware Environment leverages on the CBT (Changed Block Tracking) feature provided by VMware.

Microsoft Hyper-V does not have this feature nor a driver to facilitate incremental backups.


UDP Agentless on MS Hyper-V will install a UDP CBT driver on the MS Hyper-V host during the very first backup. This UDP CBT driver will be installed as a service and can be seen as an installed program as seen below:

How are the changes tracked by the CBT driver and will the actual changes be stored on the MS Hyper-V host ?

  • The changes are saved in the CBT service working path ->  C:\Windows\ARCserve UDP Host-Based VM Backup\cbt_service\cbttemp\spool, and then merged together as a bitmap file when backup job requests for the changes.
  • A bitmap is stored for the changes. Each bit in the bitmap indicates whether a specific location of the virtual disk file is changed or not. The actual changes are not stored, so the bitmap file’s size is quite small.
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