Why are Hyper-v backups larger than the actual data change inside the VM?

Question

Hyper-V

Good afternoon.

Colleagues, we use Windows Server 2012 r2 as hypervisors in our infrastructure.
We use Arcserve UDP for backup.

We noticed that with agentless backup every 15 minutes, the size of each backup is very large (more than 15GB). Although the zabix metrics do not show such changes inside the machine (about 500 mb including data overwriting).

Swap in the machine (vm) is not used due to the sufficient amount of RAM.

As part of the test machine, the mail system will be taken, but within the framework of other services, the result is similar:


You can see that a day has passed since the full backup and 53 GB of changes were found within the VHD file.

But on the actual wear that day there were almost no changes: