Last night I was "optimizing" a qcow2 virtual image (after zero filling the free space), running (as root):
qemu-img convert -f qcow2 -O qcow2 old.img new.img
The copying and scanning for zero blocks pretty much took over the disk. Anything else I tried to run which needed access to something on that disk would hang for minutes at a time before getting through.
Is that normal behavior? I thought folks were supposed to take turns and play nice together :-).