Backup differencing disk in Hyper-v VMs

[jgiroux]

Hi all,

I am currently testing our internals backup restore procedures and found something that i'd like to check with you, forum members.

I have a VM, we'll call it VM01. VM01 has a parent disk containing Windows Server 2012 R2 (or Linux, it doesn't matter since I get the same result) and a differencing disk.

The VM is in a Clustered VM in a Hyper-v Failover Cluster, managed by VMM. When i check the disk setup of the VM, i have a 40 GB Parent disk located in a CSV, volume 1 (Many VMs refers to that parent disk).
The differencing Child disk has a total maximum size of 40 GB, but weights 4.5 GB, and is located in another CSV, volume 3.
Both failover cluster manager console and VMM correctly reports the VMM as having a differencing disk.

I do a full backup of VM01 using Veeam backup and replication 7. the full backup takes 17 GB at 1.0x compression and 2.4x dedup, data size is 40 GB.

I then do a full VM restore by creating another VM, different UUID, on a different Hyper-V host, on a different CSV. Total size of the restore is reported by Veeam as 44.5 GB

I complete the restore and when i check the restored VM01, i get fully standalone VM, with both VMM and Failover Cluster manager reporting the VM VHDX size to 40 GB and as fixed disk.
No reference to the parent disk.

Does Veeam merge parent and child differencing disk during backup or at some point ? Is there something i'm missing ?

Best regards,

jgiroux.

[Vitaliy S.]

Hello jgiroux,

I'm pretty sure Veeam restore job restores these disks as fixed size and merges all the content to a single file, but I can double-check with our dev team on Mon.

Thank you!

[jgiroux]

Hi Vitaliy,

Thank you, it is much appreciated.
I've conducted many restores and i always get that result, regardless of if i create the restored VM in the same cluster (which contains the parents disk) or in a completely different cluster with different CSVs.

It is in fact the behavior we wish to obtain, but i could not find any information related to this result.

Best regards,

jgiroux

[Vitaliy S.]

In this case you have confirmed that it works this way There is no "hidden" option to make it work differently. I was about to test it myself and consult with our QA team once I get to the office.