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SINC_dmack

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Windows Server Backup fails after expanding HyperV child's fixed VHDx

We have a server running Windows Server 2012 R2 with four HyperV children.  (One is running Server '08 R2 with Exchange; one is running Server '12 R2 with Exchange; one is running Server '12 R2 with Sharepoint; one is running Server '12 R2 and acting as a domain controller / DNS; all are using fixed VHDxes.)  It is set to backup nightly at 1AM using a VSS Full Backup of the parent and the children.  About a month ago, the backups started failing with "The mounted backup volume is inaccessible.  Please retry the operation."  The only way I could get the backups working again was to delete the backup set (wbadmin delete catalog) and format the backup destination, and then recreate the backup.  

A couple days ago, the backups started failing again with the same error message.  I had expanded the VHDx of one of the HyperV children earlier that day, so I suspect that's what caused the backups to fail.  (The VM is set up with the VHDx as a SCSI Controller so I was able to expand the VHDx without shutting down the VM.)  Then it occurred to me that I had expanded a child's VHDx around the time that the backups had started failing a month before, reinforcing my assumption.

I "modified" the backup schedule by clicking on Backup Schedule and hitting Next through all of the screens, and I shut down all of the HyperV children and rebooted the parent server.  None of that had any effect.

My questions:  
--Why does expanding a VHDx cause the HyperV parent backups to fail?
--Can I get the backups running again without having to delete the existing backups?  If so, how?

(I have not tried specifically shutting down a VM and expanding its VHDx while it is offline to see if that breaks the backups, and don't really want to try that yet.)
Avatar of SINC_dmack
SINC_dmack

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If I modify the backup set to exclude the VHDx in question, the changes do not save--it still does a "full" backup.  Not a full backup as in backing up all 1TB of data, but rather an incremental backup of all of the items in the backup set.  And it fails.

If I delete the backup set (but not modify the backup destination, leaving the pre-existing backups intact), and then recreate it, specifically excluding the recently-expanded VM and its VHDx, the backup will complete successfully.

WAIT, I may have misspoken.  The backup is still running.  Yeah, it was actually doing a full backup of the VHDs, which wasn't helpful, so I cancelled it.
Avatar of kevinhsieh
I have never used the Windows backup. It probably needs to do a full backup when you resume a VHDX.

Your terminology is wrong. You dont have parent and children. You have a host that runs and manages Hyper-V.  You then have four VMs which are also called guests.

Parent child relationship woul. d refer to linked clones, which is completely different.
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Patrick Bogers
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Thanks for the input, everyone.  I don't really want to award points based on "sorry, there is no workaround in Windows Backup", so I'll solicit recommendations for an alternative to Windows Backup.  I'd like something that is simple, lightweight, can do bare metal backups that can be restored by booting from Windows Server media, and inexpensive.  Points will go to whoever makes the suggestion that I feel would be the best candidate.
Sometimes the answer is not what expected or hoped upon but you have gotten the right answer.
May i suggest posting a new question about what backup solution is cheap and lightweight?

Cheers