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Hyper-V VM Total backup size - Fixed VHDX with Snapshots

Level 1

Good afternoon everyone,

Backups have started exceeding the capacity of my backup media, so I'm just trying to tally up the expected amount of data that should be getting backed up.  Looking at the backup set sizes on the media is further confusing me as things don't seem to match up and raising a few questions:

Here is a quick example to illustrate of my query:


I am running a FULL backup of a Hyper-V VM, using the "Faster Method" which leaves the VM on an AVHDX.

The VHD has the following current disk configuration:

- FIxed VHDX1 - 100GB (57.6GB used)
- AVHDX1 - 12.6GB
- FIxed VHDX2 - 500GB (140GB used)
- AVHDX2 - 10.9GB
TOTAL - 622.6GB
TOTAL (if whitespace in VHD was not backed up) - 221.8GB

When I look at the media and check the size of the backup set for this VM, the size is actually 476GB, and the Byte count in the Job Log is 402GB.

I would like to try and understand how it gets to 476GB - Does it take 622GB and have internal disk sapce optimization mechanisms to reduce the size?

I would also like to understand how the size of the AVHDX affects the eveutnal total size of the backup - Let's say a backup didnt run for 3 days. These two AVHDX's would continue to grow.  Would the next full backup feel the effect of the increase in AVHDX size?

Hopefully I've explained that OK(!), and I very much appreciate any assistance you can give.

(Also, appreciate that v15 of End of Support !)


Many thanks,




Partner    VIP    Accredited


Backup Exec would use compression, and different types of data compress differently. A text file compresses better than a *.jpg which is already compressed. Check your job log to see what sort of compression you are getting.


Level 4

I've never figured either why the Job history size does not match the actual size of the backup. It's always been the same through the different versions of backup exec. There is no compression on our backups.