compressed HDD image using dd...clearing unused blocks

Hello all,

I am updating a system that has been around for some
time now.  I would like to make a compressed disk
image after the final setup is complete, although, I'm
guessing that the unused blocks will not allow me to
compress the image as well as I could with a
previously clean disk (considering the disk has been
in use for some time now).  Is there a way to do this
- zero out unused blocks to optimize compression?  I
generally do this with Windows machines using the
cipher command (killing cipher  after it has finished
writing zeroes).  After running cipher I use dd to
create a compressed HDD image for later use.  I'd like
to be able to do the same with FreeBSD.

-Michael S. Eubanks
 mse0206@yahoo.com

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