VMware Snapshots, CBT and Revert

[lp@albersdruck.de]

Hello all!

Whenever I snapshot a VM, do some things and then revert to the snapshot, afterwards delete this snapshot of the VM, CBT no longer works and I have to manually reset CBT (or have even more fun, see case ID #00519931). This snapshot is the only snapshot on said VM.

ESXi 5.5
vCenter 5.5b
V B&R 7.0.0.833

Is this behavior normal as I could not find anything about CBT and snapshot/revert?
Do you experience similar behavior with snapshots and reverting to them?
BTW is there a simple and fast test I can do to see if a backup using CBT would work on a VM? The VM is quite large and a "full" backup run to test takes around 10 hours out of my backup window.

[Vitaliy S.]

Hello Lars,

Since CBT is basically the block IDs that point to your virtual disk, the snapshot revert operation might easily mess up these pointers, making CBT reporting wrong data.

This is what I have found in VMware knowledge base:

In some cases, such as a power failure or hard shutdown while virtual machines are powered on, CBT might reset and lose track of incremental changes. In vSphere 4.1 and prior, Cold Migration (but not Storage vMotion) could reset but not disable CBT. In vSphere 5.x, Storage vMotion resets CBT.

Can you please tell me if you've considered using SureBackup/On-demand Sandbox functionality to test your patches/run tests as opposed to doing it on production VMs?

Thank you!

[tsightler]

Reverting a snapshot does break the CBT chain so it is normal behavior for the next backup run to issue "Cannot use CBT" warning messages and for Veeam to revert to "snap and scan", however, that should only be an issue for the backup immediately following the snapshot revert. Future runs of the job should not generate further CBT errors unless there is additional snapshot activity.

[lp@albersdruck.de]

Reverting a snapshot does break the CBT chain so it is normal behavior for the next backup run to issue "Cannot use CBT" warning messages and for Veeam to revert to "snap and scan", however, that should only be an issue for the backup immediately following the snapshot revert. Future runs of the job should not generate further CBT errors unless there is additional snapshot activity.

I can confirm this behavior now.
It did not work a week ago though, but that might be because there were snapshots 3 levels deep and a lot of reverting going on.

Thanks for the explanation!