Hi Rich---we were running qemu and nbdkit on the same machine, connecting over tcp. after updating to the latest development version of nbdkit (containing the commits you mentioned), everything works perfectly. running bonnie++ inside the guest indicates that connecting qemu to nbdkit has performance similar to or better than connecting qemu directly to the disk image.  thanks a lot for your work and advice--we just wanted to drop you this note to tell you about our experience, in case someone else ends up having similar questions.