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upgrade from stretch to buster killed my KVM snapshots

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Apparently internal snapshots of a qcow2-based vm are problematic when 
the vm boots from UEFI. I found that out when my nightly snapshots 
stopped working after I upgraded a server from stretch to buster.

Testing the effective line of the script with actual values, I got this 
result:

   #virsh snapshot-create-as --domain server --name serverS0
   error: Operation not supported: internal snapshots of a VM with 
pflash based firmware are not supported

An internet search of the error message revealed that this problem has 
been going on for a long time. The new behaviour is to simply not allow 
the snapshot to take place rather than risk creating a corrupted one.

Fortunately there is a simple workaround. I simply replaced the virsh 
snapshots with qemu-img snapshots. In my script, this meant changing a 
few lines like this (VM is the name of the VM while DOW is the day of 
the week number (0-6) - I put an 'S' between them in naming the snapshot):

from: virsh snapshot-create-as --domain $VM --name ${VM}S$DOW
to: qemu-img snapshot -c ${VM}S$DOW $VM.qcow2

While virsh works with the xml "domain" definition, qemu-img works 
directly with the qcow2 file. This means the script also has to know 
where the qcow2 file is. This can be accomplished either by adding the 
full path to the VM name (which would prevent it from being used in the 
virsh command to shut down the VM) or cd'ing to the folder. I chose the 
latter as the simplest.

There's always more than one to do something. In this case, I'm glad 
qemu-img was there to overcome a bug in libvirt. The other option would 
be to switch to external snapshots which, apparently, are different 
enough to not be affected by the internal snapshot bug. However they 
operate a lot differently and I really don't want to get into them for 
this application.

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upgrade from stretch to buster killed my KVM snapshots Gary Dale <gary@extremeground.com> - 2019-07-30 06:20 +0200

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