Question

VMware CBT and Full backups

  • 1 June 2023
  • 3 replies
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Userlevel 2
Badge +11

Hello!

 

I know CBT is a VMware functionality that Commvault and other backup softwares use. Netbackup leverages the advantages of CBT on both full and incre/diff jobs. Does the same apply to Commvault?

 

Check this doc from NBU: https://www.veritas.com/content/support/en_US/doc/21902280-127283730-0/v27791754-127283730

  • Backup of entire virtual machine, with full schedule: Backs up only the blocks that have changed since the .vmdk was created. Note that the blocks that are not initialized are excluded from the backup.
  • Backup of entire virtual machine, with incremental schedule: Backs up only the blocks that have changed since the last backup, as follows: For cumulative incrementals, BLIB backs up only the blocks that changed since the last full backup. / For differential incrementals, BLIB backs up only the blocks that changed since the previous backup of any kind.

 

So, will Commvault full backups also get the advantages of CBT?

Regards,

Pedro


3 replies

Userlevel 2
Badge +7

@PedroRocha - Commvault does take advantage of VMware’s CBT features. Here is some documentation around CBT.

 

https://documentation.commvault.com/2022e/expert/32270_changed_block_tracking_for_vmware.html
https://documentation.commvault.com/11.25/expert/36581_protection_methods.html

 

 

Userlevel 2
Badge +11

Hi, thanks for the commente. I know it takes the advantage, but in the commvault docs it always states it is with incrementals, different than the Netbackup docs that states that both (full and incrementals) takes it.

 

So, would commvault FULL backups take the advantage, or only incrementals?

Userlevel 7
Badge +16

Hi @PedroRocha 

VMware states:

  • Changed Block Tracking (CBT) is a feature that identifies blocks of data that have changed or in use. It enables incremental backups to identify changes from the last previous backup, writing only changed or in-use blocks.

As far as I know the *-ctk.vmdk file only tracks blocks modified since the last backup.

Based on the definition given by VMware and seeing what is kept in the ctk file, I assume Veritas validates if the block being backed up already exists in the backup and then continues to process or drop the block. Based on this assumption it would basically be the same looking at what deduplication does.

So is veritas actually leveraging CBT for full backups? I'm not sure they are.. unless they maintain their own file with changed blocks..

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