One use for this is to use nbdkit’s xz plugin as a source, to easily create new images from highly compressed templates. In the example below nbdkit transparently (and in RAM) uncompresses the disk image, serving it up over
/tmp/sock (a socket):
$ nbdkit -r -f -U /tmp/sock \ /usr/lib64/nbdkit/plugins/nbdkit-xz-plugin.so \ file=f17x64.img.xz
Virt-resize fetches the image from the socket† and writes & resizes it to the destination file:
$ truncate -s 20G /tmp/output.img # adjust target size $ virt-resize 'nbd://?socket=/tmp/sock' /tmp/output.img $ killall nbdkit; rm /tmp/sock
You can then use virt-sysprep to set the hostname and run configuration scripts in the output image.
† Unfortunately you also have to temporarily disable SELinux for this to work, because SELinux prevents qemu from connecting to the NBD socket. Grrrr.