Dissecting that conversion

nbdkit -U - --filter=log --filter=nozero --filter=blocksize \
  --filter=delay --filter=stats --filter=noextents memory 100m \
  logfile=>(sed -n '/Zero.*\.\./ s/.*\(fast=.\).*/\1/p' |sort|uniq -c) \
  statsfile=/dev/stderr delay-write=20ms delay-zero=5ms maxdata=256k \
  --run 'qemu-img convert -n -f qcow2 -O raw src.qcow2 $nbd' "$@"
st=$?; sleep .2; exit $st