compressed HDD image using dd...clearing unused blocks

[Paul Schenkeveld]

On Sat, Jul 07, 2007 at 10:28:01PM -0700, Michael Eubanks wrote:
> 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.

What about:

  # dd < /dev/zero > BIG_EMPTY_FILE bs=128k
  # rm BIG_EMPTY_FILE

Comes close to what you want, only a couple of indirect blocks are
not zeroed this way but the majority of unused blocks will be.

> -Michael S. Eubanks
>  mse0206 at yahoo.com

-- Paul Schenkeveld