growfs on file-backed fs

[Richard P. Williamson]

Hello all,

according to the handbook, I can create a file-backed fs
(Example 12-5. Creating a New File-Backed Disk with vnconfig)

# dd if=/dev/zero of=newimage bs=1k count=5k
..
# vnconfig -s labels -c vn0 newimage
# disklabel -r -w vn0 auto
# newfs vn0c
...
# mount /dev/vn0c /mnt


I've been trying to do that, and end up with a file that I
can turn into an mfsroot.gz (or mdroot.gz for that matter).
But I can't get my device to boot from a flash device using
a 'home-built' mfsroot.gz.  I've tried various combinations
of switches for vnconfig, disklabel and newfs, but nothing
wants to allow it to play the way I need.

When I replace the mfsroot.gz with mfsroot.flp from the floppy
startup images, then it boots.

I have, in the past done a growfs on the mfsroot.flp, in order
to make it big enough to hold the files I need.  

But I'd rather just do the commands as described in example 12-5
above.  What do I need to add to that to make it a bootable mfsroot?

Regards,
rip