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VMWare Backups on NBU 7.1.x - Clarification about BLIB

Created: 22 Mar 2012 • Updated: 11 Jun 2012 | 7 comments
ddm2's picture
This issue has been solved. See solution.


I would clarify a point about Block Level Incremental Backups.

I know that BLIB requires the VMware's Changed Block Tracking feature and apply only to vmx-7 and later virtual machines.

But I don't know if this feature is enabled by default and how to check.

Best regards,


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Mark_Solutions's picture

BLIB is a setting on the main attributes tab of the Policy and is not enabled by default

"Perform Block Level Incremental Backups"

Hope this helps

Authorised Symantec Consultant

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Obi Wan's picture

Once BLIB is enabled and the first full backup is taken, NetBackup then knows the "state" of that VM. From that known state, each subsequent incremental, from a point-in-time perspective, can be selected to perform a full restore of either a file or full VMDK which is rather unique in the industry - doing a full restore from an incremental point in time backup.

This is much different than change block tracking and is another reason why Symantec can drive innovation into the backup market. We can take technologies from across our entire product line and give it a backup personality.

Remember the key is the "state" of the VM. If a developer swaps out a production VM with a new image, the "state" will have changed and the BLIB timeline must be re-started with a new full backup.

Hope this helps!

ddm2's picture

Sorry, my question wasn't clear.

Is the Changed Block Feature enabled by default on the VMware side? How to check this?



Mark_Solutions's picture

I dont believe it is enabled by default as you have to add it in to a clients configuration.

This may give you an idea of where changed block tracking is located / added:

Authorised Symantec Consultant

Don't forget to "Mark as Solution" if someones advice has solved your issue - and please bring back the Thumbs Up!!.

ddm2's picture

I've already read this howto, but I'm not sure I understand what it says:

"The Enable block-level incremental backup option (BLIB) in the NetBackup VMware policy applies to full backups and incremental backups. BLIB skips blocks on the virtual disk that are unmodified since the last backup. If you want to skip unmodified blocks during full backups but not incrementals, do not configure the policy for BLIB. Instead, enable changed block tracking by means of the vSphere Client interface."


"To enable changed block tracking for full backups only (vSphere Client interface):


I understand that I should follow this howto only if I want "to skip unmodified blocks during full backups but not incrementals".


ddm2's picture

I tried to configure BLIB without touching VM configuration.

I was able to restore single file from incrementals without touching VM configuration.

Mick Scott's picture

There's plenty to be confused about.

documentation says "NetBackup automatically enables VMware’s Changed Block Tracking feature on the virtual machine when BLIB is selected within the NetBackup policy. "

havent been able to verify that CBT is enabled automatically.

Page 66 in the Admin guide "Recovery options related to policy selections" there is a table that shows which combination of VM backup type (Mapped Full, Full etc) and full or incremental schedules enable you to do individual file restores .. or not.