unchanged data blocks filter - CBT veeam flavour

[raphael@schitz.net]

Hi, i read in the admin guide that veeam has it's own CBT mechanism (developed for hyper-v i guess) in v6 where CBT can't be enabled or is disabled.

Can we have more information on :
- how it works
- when it's used and when it's not
- if it's leverage on vm with snapshots

Thanks a lot

[Gostev]

Hi,

The way it works is whole virtual disk image is read, and during this process each disk block is hashed and compared with its previous state saved during the previous backup.

It is used if:
1. The use of CBT is disabled in the job settings.
2. CBT query fails for the VM for any reason.
3. In case of known border cases when certain sequence of state changes on VM makes CBT provide incorrect information.
4. When VM does not support CBT (virtual hardware version is less than 7).

Snapshot presense does not affect neither CBT nor proprietary changed block tracking mechanism, since vStorage API always process the actual VM state (as hypervisor sees VM virtual disk at this moment).

Thanks!

[raphael@schitz.net]

Thanks a lot fot the details Gostev.
Snapshot does not affect CBT but i was talking about this statement "Due to existing VMware limitations, Veeam Backup & Replication does not leverage VMware vSphere CBT for VMs that have one or more snapshots created manually" and the ability of your alternate mechanism to act as a better-than-nothing workaround when backup vm with some snapshots.
Is it possible to track this mechanism in the job log ?

[Gostev]

When CBT data is used, you will see [CBT] symbols next to each processed disk.

[raphael@schitz.net]

When CBT data is used, you will see [CBT] symbols next to each processed disk.

you mean the veeam's CBT ?

[Vitaliy S.]

Hi Raphael,

No, Anton's is referring to VMware CBT mechanism. As to Veeam's engine that you should be looking at "Default tracker will be used" line in the job logs.

Thanks!

[raphael@schitz.net]

Thanks Vitaliy, can you explain how this is different from previous version since an incremental backup/replication obviously means you only store what's changed ?

[Vitaliy S.]

Our own VMware change tracking mechanism hasn't been changed since version 1, so I didn't quite catch what you're referring to...

[raphael@schitz.net]

Ok, since it was not explained like this in the user guide since v6 i was thinking it's a new or enhanced feature. Am i missing something ?

[Vitaliy S.]

I guess you're confusing it with Hyper-V change tracking mechanism, which is new and unique to the market.

[raphael@schitz.net]

No, i'm talking about this part of the VMware user guide :

Whenever Veeam Backup & Replication cannot leverage VMware vSphere CBT – for example, if your VMs run an earlier version of virtual hardware or Changed Block Tracking is disabled at the ESX host level – it fails over to Veeam’s proprietary filtering mechanism. Instead of tracking changed blocks of data, Veeam Backup & Replication filters out unchanged data blocks.

Thanks

[Gostev]

Yes, this talks about good old proprietary changed block tracking we had since v1. Same stuff as I explained in my first response above.

[raphael@schitz.net]

ok, too bad it sounds new in the context

[Gostev]

We have a saying "everything new is just well-forgotten old"

[raphael@schitz.net]

We have a saying "everything new is just well-forgotten old"

Thanks for the details anyway