Libvirt 6.0 now requires that every disk in the backing chain has an
explicit backing format. For example this will be rejected by
libvirt:
qemu-img create -f qcow2 -b backing-disk disk.qcow2
with the error:
Original error from libvirt: Requested operation is not valid:
format of backing image 'backing-disk' of image 'disk.qcow2' was not
specified in the image metadata (See
https://libvirt.org/kbase/backing_chains.html for troubleshooting)
[code=55 int1=-1]
Instead you have to use the -F option to specify the format, eg:
qemu-img create -f qcow2 -b backing-disk -F raw disk.qcow2
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