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Full Name: Jake

Backup method when not using CBT

Post by indyjake »

If I'm doing an incremental backup and decide to NOT use CBT, what method does Veeam use for determining what to backup within a VM? The reason I ask is that I have a particular VM that has an application installed which touches multiple files every day and modifies the last access time stamp on them. I believe this causes very large incremental backups when using CBT. The vendor of this particular application does not recommend disabling the last access time stamp in the Windows registry (Windows 2003 Server). Thanks for your input.


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Re: Backup method when not using CBT

Post by foggy »

Jake, Veeam B&R is a block-level solution, so in any case, whether VMware CBT is enabled or not, only changed blocks are copied. When VMware CBT cannot be used, we are using our proprietary changed block tracking engine, that basically scans source VM image, calculates hashes for all blocks, and then verifies what blocks have been changed on your VM compared to the VM image stored in the backup file. This process requires processing of the entire VM (which is longer compared to VMware CBT), but only changed blocks are transferred to the target repository.

In your case, only blocks storing last access time attributes will be changed and, hence, copied during job run (not the whole unchanged files). Disabling VMware CBT will not result in any traffic gain, but, instead, will cause longer backups.

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[MERGED] Question about CBT

Post by cstemaly »

I am running the latest Veeam Backup and Recovery Standard with VMware VMs. I'm backing up some VMs on a daily basis, and all VMs on a weekly basis. These two group overlap somewhat. For example, I have one application server where the data changes daily, so it's in my daily job, but it's also in my weekly job.

I performed some storage vMotions and had to reset the CBT from within vCenter (set CBT to false in the properties of the VM, then delete the *-cbt.vmdk files and reboot a couple of times). It got me to thinking - is there a mechanism whereby Veeam know which blocks have changed since the last daily backup versus which blocks have changed since the last weekly backup if there's only one *-cbt.vmdk file?

Vitaliy S.
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Re: Backup method when not using CBT

Post by Vitaliy S. »

If you reset VMware CBT Veeam will use its own proprietary change tracking engine to detect changed blocks, see foggy's description above for more details. Thanks!

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