Hyper-V host-level snapshots

I follow and participate in several Hyper-V Virtualization forums, and thought I’d share the contents of a recent thread about Hyper-V host-level protection and how Unitrends Hyper-V backups work. The primary question raised was surrounding snapshot and guest VM management on the Hyper-V host. My comments are preceded by Maria, and the other forum participant’s comments are preceded by User.

Maria •  I can speak to how the Unitrends solution works (I think several of the products being discussed here behave similarly, at least on snapshot management, but I’ll speak to the one I know best). It’s pretty flexible, but first I’ll explain how it works with snapshots:
– Full and Full/Incremental backup strategies are available
– It creates snapshots prior to backup, then deletes them immediately after the backup completes
– It has granular recovery (file-level recovery) from the image-based snapshot backups coming out this quarter (we already have it for VMware snapshot backups)

So the snaps are not left in place after the backup, and there is no requirement on powering off the VM. We see minimal performance impact since we leverage MSFT API’s and the snaps are taken at a block level. Of course, if the host OS is already overloaded, you could notice an impact. It really just depends on your environment.

Additionally, if you for some reason don’t want snapshot backups, you can install our agent on your guest OS (we support about 100 different OSs and releases) and perform bare metal, file level and application-level backups.

I hope this info helps.

User • Thanks for the detailed explanation Maria, very helpful. I am particularly interested in the deletion of snapshots, I have more of a VMware background so need to clarify the details with HyperV. My understanding is that HyperV can’t delete a snapshot when online. You can delete from the console, but the consolidation of VHD and snapshots is only done when the machine is powered off. This can be seen at the file level. How does Unitrends accomplish this if HyperV cannot do it natively?

I want to be able to implement Vm level backups on HyperV but can’t leave snapshots hanging around

Maria • Got it. So Unitrends creates a VSS snapshot of the volume(s), not a snapshot of the VM/Guest OS. From the VSS-snapped volume(s), Unitrends accesses a consistent view of the VM hard drives and backs those up. No Hyper-V VM snapshots left around. VSS just caches writes during the backup, then flushes the cache. This is initiated from the Hyper-V host.

I may have confused you with my terminology. I apologize for that.

User • Lovely, that’s what I was questioning. Thanks. So my next question, as The VM is not snapshotted, and Microsoft do not support vm snapshots for SQL Server and Exchange, is this backup method supported by Microsoft for these server types? I’ve had corrupt SQL servers in the past when creating and deleting VMware snapshots at the VM level.

Maria • Unitrends Hyper-V host-level backups (VSS snapshots) leverage the VSS framework, including the VSS application integration for MS SQL and Exchange. Unitrends talks to the Hyper-V writer, which in turn talks to the (Hyper-V) Integration Services which are installed on the guest OS. This methodology is completely supported by MSFT.

So you restore to a point-in-time, and the applications are also consistent to the point-in-time you selected for restore. Unitrends synthesizes the Full’s and incremental’s to achieve the desired point-in-time.


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