Thanks for all the comments! This is what I do now: In domU ------- su - dd if=/dev/zero of=/var/tmp/zero bs=1m & rm -f /var/tmp/zero fg halt -p In dom0 ------- gzip -9 p133 -rw-r--r-- 1 root wheel 10G Jan 25 15:15 p133 -rw-r--r-- 1 root wheel 193M Jan 25 15:15 p133.gz