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Date: Sun, 08 Jul 2007 10:37:06 -0700
From: Darren Pilgrim <freebsd@bitfreak.org>
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To: Michael Eubanks <mse0206@yahoo.com>
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Cc: freebsd-chat@freebsd.org
Subject: Re: compressed HDD image using dd...clearing unused blocks
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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.

In my experience, using dd to create images of UFS/UFS2 volumes is 
problematic, though I can't qualify that.  It's better to use an 
archival program like dump or tar, IMO.  That approach also gets you a 
restored volume with no fragmentation.