45682: Acronis Backup for VMware: Slow Incremental Backup Caused by CBT

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Last update: 06-04-2016

The most probable reason for slow incremental backup is CBT malfunction which needs to be reset

This article applies to:

  • Acronis Backup for VMware (Acronis vmProtect)


  • You create a full backup of some VM;
  • You run the same backup task for the 2nd time and it takes approximately the same time as the full one;
  • The size of the data transferred during incremental backup (2nd run time) is small and corresponds the normal size of incremental backup.

For example: you have a 2TB+ VM which full backup takes 10h to complete. Incremental backup takes 5h to complete, while the size of the archive is increased only by 50-100 GB (which corresponds the actual data change rate). This is not normal since the time to complete the backup should be around 15 minutes in this case.



The problem is typically caused by Changed Block Tracking (CBT) VMware system which improperly detects the changed sectors range on the backed up VM virtual disks. In the above example as the result instead of 50GB of reported changes CBT returns the range of sectors equal to nearly the size of the original disk (2TB). What happens next is that Acronis Backup for VMware has to read through all the reported range of sectors and perform analysis. This therefore causes increased time of backup, while the amount of the backup traffic is normal.



The issue can be resolved by manually reseting the CBT:

  1. Power off the problematic VM.
  2. Make sure the VM does not have any snapshots.
  3. Right-click the VM, click Edit settings, in the Options tab select the General section, then click Configuration Parameters.
  4. Set the ctkEnabled value to false.
  5. Set all of the scsi0:x.ctkEnabled values to false.
  6. In Datastore browser, open the VM's source folder and delete any -CTK.VMDK files.
  7. Power on the VM.
  8. Start backup.

On a ESX(i) 6.0 host you can also apply this patch.


More information

The information about the range of changed sectors is stored in .ctk files located in the folder of the virtual disks on the corresponding datastores. Any VMware snapshot created on this VM triggers the .ctk files change, however they are flushed to the datastore only after the snapshot removal and until then they are stored in memory.

See also Acronis Backup Software Creates Large Incremental or Differential Backup Archives.