Try doing: $ nbdsh nbd> h.connect_command (["nbdkit", "-s", "null"]) At this point you may observe your laptop fan starts to spin and nbdkit is consuming 100% of CPU. In all other respects everything works fine, you can send commands etc. Anyway I tracked the issue down. nbdkit sits in a loop continuously reading stdin, each read(2) call returning EAGAIN. The reason for that is because libnbd opens a socketpair with the SOCK_NONBLOCK option and passes one half directly to the forked subprocess. It's obviously a bug, but I'm not sure if libnbd should be unsetting the SOCK_NONBLOCK option or if nbdkit should be doing it. Maybe both! Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-p2v converts physical machines to virtual machines. Boot with a live CD or over the network (PXE) and turn machines into KVM guests. http://libguestfs.org/virt-v2v