Application consistent VM snapshots in Windows Client VMs not possible

  • 30 November 2021
  • 5 replies
  • 551 views

Userlevel 1
Badge +2

Hello everybody,

 

I noticed that application consistent VM snapshots of e.g. Windows 10 Pro VMs with NGT enabled and installed in the VM and verified with ‘ncli ngt list’ doesn’t work and results in the following alert…

 

VSS snapshot is not supported for the VM 'TEST-VM', because VSS software is not installed.

 

and …

 

VSS is enabled but VSS software or pre_freeze/post_thaw scripts are not installed on the guest VM(s) TEST-VM protected by TEST-VM.

 

Using a Windows Server 2016 VM with the respective Protection Domain generates application consistent Nutanix VM snapshots. With Windows 10 VMs it won’t work. Verified in 2 different Nutanix clusters.

 

Clusters are equipped with AOS 5.15.4 LTS version.

 

Nowhere I could find a hint, that Windows 10 VMs are not supported.

 

Regards,

Didi7

 


5 replies

Userlevel 4
Badge +5

The following article has the answer:

https://portal.nutanix.com/page/documents/details?targetId=Prism-Element-Data-Protection-Guide-v6_0:wc-dr-application-consistent-snapshots-wc-r.html

Basically, on Windows Server app consistent snapshots will work in most cases when the NGT is active. On Windows 10 you need both NGT and pre_freeze/post_thaw scripts for app consistent snapshots.

Userlevel 1
Badge +2

Hello Sergei,

 

app consistent snapshot backups with Windows Servers work perfectly, as long as NGT is installed and activated on the VM and the relevant Protection Domain exist.

 

In other words, to have the same effect on e.g. Windows 10 you need to create a pre_freeze.bat and post_thaw.bat script like the following …

 

@echo off

ver

 

Is it that simple? We don’t need to shutdown any services on those specific VMs.

 

Strange that this does work on Windows Server without those scripts but not on Windows 10.

 

Regards,

Didi7

 

Userlevel 1
Badge +2

Ok, it works now for Windows 10 using pre_freeze.bat and post_thaw.bat in e.g.

 

C:\Program Files\Nutanix\scripts

 

with contents …

 

@echo off

ver

 

Funny!

 

Userlevel 4
Badge +5

The difference here between the Windows Server and Client versions is in the VSS components. For the application consistent snapshots the VSS is handling the consistency. and on the server versions it is enough. As the client Windows versions are lacking that capability, the Nutanix cluster has to rely on the freeze/thaw scripts to quesce the IO and it has no way to check if the IO is actually quesced, so even if you put empty script files, the snapshot will be counted as app consistent, but if your scripts do not do the proper job to quesce the applications, those snapshots will not in fact be application consistent and it is only a way to trick the system. Those snapshots will be in fact crash-consistent.

So, to make your snapshots truly app-consistent on Windows 10 you need to ensure that your freeze/thaw scripts are well-written and that they actually quesce the application IO.

Userlevel 1
Badge +2

Of course those pre_freeze/post_thaw scripts are fake, but as long as the Windows 10 VM just acts as let’s say a file-server, it is still application or OS consistent.

In VMware vSphere you just need VMware Tools and no scripts to create application consistent snapshots (of course as long as there is no specific application) for Windows client VMs, like Windows 10, depending on the backup software.

Veeam B&R has its own proprietary AAIP integration to create app consistent snapshots being independent from VMware Tools, when using vSphere as Hypervisor.

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