Enable Changed Block Tracking (CBT) on virtual machines

[bolsen]

We've recently upgraded to ESXi 4.1 and we'd like to start taking incremental backups of our virtual machines.  I understand CBT must be enabled on all virtual machines before incremental backups will work (HW v7).

I have a couple questions about this:

• Is there any downside to enabling this feature?  Does it cause extra overhead on the host or disk aray?
  
• Is CBT enabled by default on all new VMs?
  

[a_p_]

To answer your questions first. There shouldn't be any issues with CBT enabled. And "No", it's not enabled by default.

Usually your backup software should be able to enable CBT for the VM's, if it is able to use CBT. It does not help to enable it manually if the backup SW doesn't use it. (see http://kb.vmware.com/kb/1020128)

André

[DSTAVERT]

There is some overhead since the changed blocks are written twice. Once to the VMDK and once to the cbt file.

[a_p_]

There's a good article about CBT at http://itknowledgeexchange.techtarget.com/virtualization-pro/what-is-changed-block-tracking-in-vsphe... Absolutely worth reading!

André