Behavior of VMware CBT when creating and deleting files immediately

[Robert G.]

Will VMware CBT mark blocks as changed if you create a large file (with random data) and delete it immediately afterwards?

Scenario:
1. Veeam full backup
2. Application backup to vmdisk
3. Copy the app backup to a share
4. Removal of the app backup
5. Veeam incremental backup
6. Profit

[ronnmartin61]

It depends. In a nutshell if the disk block consists entirely of deleted file data and "Exclude deleted file blocks" is enabled in the advanced job settings then the block is not processed. For a full description with affiliated caveats see - https://helpcenter.veeam.com/docs/backu ... ck&ver=120

[Robert G.]

Thank you for your quick reply.
What is the situation with Linux systems?

[ronnmartin61]

I think the "Limitations" section of the helpcenter document is pretty clear that this applies only to Windows VM's + NTFS filesystems. BitLooker for Linux, other Windows filesystem types, etc. is not supported

[Robert G.]

Of course is BiLocker just an Windows topic. I tested the scenario with a Linux VM.
VMware CBT will just mark every block as changed if something happend with it and Veeam will backup these blocks, so even (large) short-term temporary files.
I was hoping that Veeam would check if an operating system marked these blocks as "free" in its (free-space) block bitmap. So that these blocks can then be skipped (even if the block content has changed).



1. inc backup
2. head -c 30G < /dev/urandom > testfile
3. rm testfile
4. wait some minutes
5. inc backup -> 30gb change

[Andreas Neufert]

Yes, this is expected. With Windows we compare the file system table and the blocks and remove them.
For most of the files, the files would have existed in the previous backup and when you remove them before the next backup, then based on CBT we would read them again, but our inline deduplication would remove these blocks likely (as they are the same) before they go to the repository. So only for those files that are created and removed between backup sessions we would back them up based on VMware CBT.