AnyBackup-VMware backup job Execution Output displays Information message: Failed to get partially changed block of the virtual machine disk



VMware CBT


Applied Products

· AnyBackup 6.0.x


In AnyBackup, while executing VMware backup job, you may find the message below under Execution Output window.

Failed to get partially changed block of the virtual machine disk [StorageName] CentOS7.2/CentOS7.2.vmdk, all the disk blocks are to be backed up, please bring the CBT feature into effect by VMware best practice. 

And in vmware.log file under /vmfs/volumes/Datastore/VMfolder, you see an entry similar to:

vcpu-0| I120: DISKLIB-CBT : Creating cbt node 92b78c-cbt failed with error Cannot allocate memory (0xbad0014, Out of memory)


AnyBackup VMware backup and recovery performance decreases. Because when Changed Block Tracking (CBT) feature fails, VMware backup and recovery jobs transfer the amount of data in the virtual disk provisioning size. 



This is a known issue of VMware ESXi 6.0 . It is caused by heap exhaustion of VMware ESXi. It might occurs in either of these two situations:

·     When attempting to enable Changed Block Tracking (CBT), if a virtual machine with a large number of virtual disks reached an upper threshold, it may cause heap exhaustion.

·     With multiple virtual machines with CBT enabled, and in case of Windows virtual machines with VSS enabled, taking a quiesced snapshot creates double amount of memory overhead. 

Note: The virtual disks might be spread across virtual machines or might be in a single virtual machine.



Upgrade to VMware ESXi 6.0 Build 2715440.

You can download the patch on VMware Downloads, or contact VMware for technical support.


More information

For more information, please refer to VMware Knowledge Base article: Backing up the virtual machine fails when CBT is enabled (2114076).


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